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CUDA Fortran Programming Guide and Fortran Programming Guide and Reference Version 2017 | viii PREFACE This document describes CUDA Fortran, a small set of extensions to Fortran that
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CUDA FORTRAN PROGRAMMING GUIDE ANDREFERENCEVersion2017CUDA Fortran Programming Guide and Reference Version2017|iiTABLE OF CONTENTSPreface............................................................................................................viiiIntended Audience........................................................................................... viiiOrganization................................................................................................... viiiConventions....................................................................................................viiiTerminology..................................................................................................... ixRelated Publications........................................................................................... ixChapter1. Introduction.........................................................................................1Chapter2.Programming Guide............................................................................... 32.1.CUDA Fortran Host and Device Code..................................................................32.2.CUDA Fortran Kernels.................................................................................... 52.3.Thread Blocks............................................................................................. 62.4.Memory Hierarchy........................................................................................ 62.5.Subroutine / Function Qualifiers.......................................................................72.5.1.Attributes(host)...................................................................................... 72.5.2.Attributes(global)....................................................................................72.5.3.Attributes(device)................................................................................... 72.5.4.Restrictions........................................................................................... 82.6.Variable Qualifiers........................................................................................ 82.6.1.Attributes(device)................................................................................... 82.6.2.Attributes(managed)................................................................................ 82.6.3.Attributes(constant).................................................................................92.6.4.Attributes(shared)................................................................................... 92.6.5.Attributes(pinned)...................................................................................92.6.6.Attributes(texture).................................................................................. 92.7.Datatypes in Device Subprograms......................................................................92.8.Predefined Variables in Device Subprograms....................................................... 102.9.Execution Configuration................................................................................102.10.Asynchronous Concurrent Execution................................................................ 112.10.1.Concurrent Host and Device Execution....................................................... 112.10.2.Concurrent Stream Execution.................................................................. 112.11.Kernel Loop Directive................................................................................. 122.11.1. Syntax.............................................................................................. 122.11.2.Restrictions on the CUF kernel directive..................................................... 132.11.3.Summation Example..............................................................................142.12.Using Fortran Modules................................................................................ 142.12.1.Accessing Data from Other Modules...........................................................152.12.2.Call Routines from Other Modules............................................................. 152.12.3.Declaring Device Pointer and Target Arrays..................................................162.12.4.Declaring Textures................................................................................182.13.Building a CUDA Fortran Program...................................................................20CUDA Fortran Programming Guide and Reference Version2017|iii2.14.Emulation Mode........................................................................................ 21Chapter3.Reference..........................................................................................223.1.New Subroutine and Function Attributes............................................................223.1.1.Host Subroutines and Functions................................................................. 223.1.2.Global Subroutines.................................................................................223.1.3.Device Subroutines and Functions.............................................................. 233.1.4.Restrictions on Device Subprograms............................................................ 233.2.Variable Attributes...................................................................................... 233.2.1.Device data......................................................................................... 243.2.2.Managed data.......................................................................................243.2.3.Pinned arrays....................................................................................... 253.2.4.Constant data.......................................................................................263.2.5. Shared data......................................................................................... 273.2.6.Texture data........................................................................................ 283.2.7.Value dummy arguments..........................................................................283.3.Allocating Device Memory, Pinned Memory, and Managed Memory..............................293.3.1.Allocating Device Memory........................................................................ 293.3.2.Allocating Device Memory Using Runtime Routines...........................................293.3.3.Allocate Pinned Memory.......................................................................... 303.3.4.Allocating Managed Memory......................................................................303.3.5.Allocating Managed Memory Using Runtime Routines........................................ 303.4.Data transfer between host and device memory.................................................. 313.4.1.Data Transfer Using Assignment Statements.................................................. 313.4.2.Implicit Data Transfer in Expressions........................................................... 323.4.3.Data Transfer Using Runtime Routines......................................................... 323.5. Invoking a kernel subroutine.......................................................................... 333.6.Device code.............................................................................................. 333.6.1.Datatypes Allowed................................................................................. 343.6.2.Built-in variables................................................................................... 343.6.3.Fortran Intrinsics...................................................................................353.6.4.New Intrinsic Functions........................................................................... 363.6.5.Warp-Vote Operations............................................................................. 393.6.6.Atomic Functions...................................................................................403.6.7.Fortran I/O..........................................................................................4184.108.40.206.PRINT Example................................................................................ 423.6.8.Shuffle Functions...................................................................................423.6.9.Restrictions..........................................................................................443.7.Host code................................................................................................. 443.7.1.SIZEOF Intrinsic.....................................................................................443.7.2.Overloaded Reduction Intrinsics................................................................. 443.8.Fortran Modules......................................................................................... 453.8.1.Device Modules..................................................................................... 463.8.2.Host Modules........................................................................................48CUDA Fortran Programming Guide and Reference Version2017|ivChapter4.Runtime APIs...................................................................................... 534.1. Initialization.............................................................................................. 534.2.Device Management.....................................................................................534.2.1.cudaChooseDevice................................................................................. 534.2.2.cudaDeviceGetCacheConfig.......................................................................534.2.3.cudaDeviceGetLimit............................................................................... 544.2.4.cudaDeviceGetSharedMemConfig................................................................ 544.2.5.cudaDeviceGetStreamPriorityRange.............................................................544.2.6.cudaDeviceReset................................................................................... 544.2.7.cudaDeviceSetCacheConfig....................................................................... 544.2.8.cudaDeviceSetLimit................................................................................ 554.2.9.cudaDeviceSetSharedMemConfig.................................................................554.2.10.cudaDeviceSynchronize.......................................................................... 554.2.11.cudaGetDevice.................................................................................... 554.2.12.cudaGetDeviceCount............................................................................. 554.2.13.cudaGetDeviceProperties........................................................................ 554.2.14.cudaSetDevice.....................................................................................564.2.15.cudaSetDeviceFlags...............................................................................564.2.16.cudaSetValidDevices..............................................................................564.3.Thread Management.................................................................................... 564.3.1.cudaThreadExit..................................................................................... 564.3.2.cudaThreadSynchronize........................................................................... 564.4.Error Handling........................................................................................... 574.4.1.cudaGetErrorString.................................................................................574.4.2.cudaGetLastError...................................................................................574.4.3.cudaPeekAtLastError...............................................................................574.5.Stream Management.................................................................................... 574.5.1.cudaforGetDefaultStream.........................................................................574.5.2.cudaforSetDefaultStream......................................................................... 584.5.3.cudaStreamAttachMemAsync..................................................................... 584.5.4.cudaStreamCreate................................................................................. 584.5.5.cudaStreamCreateWithFlags......................................................................584.5.6.cudaStreamCreateWithPriority................................................................... 594.5.7.cudaStreamDestroy................................................................................ 594.5.8.cudaStreamGetPriority............................................................................ 594.5.9.cudaStreamQuery.................................................................................. 594.5.10.cudaStreamSynchronize..........................................................................594.5.11.cudaStreamWaitEvent............................................................................ 594.6.Event Management...................................................................................... 604.6.1.cudaEventCreate................................................................................... 604.6.2.cudaEventCreateWithFlags....................................................................... 604.6.3.cudaEventDestroy.................................................................................. 604.6.4.cudaEventElapsedTime............................................................................ 60CUDA Fortran Programming Guide and Reference Version2017|v4.6.5.cudaEventQuery.................................................................................... 604.6.6.cudaEventRecord................................................................................... 614.6.7.cudaEventSynchronize.............................................................................614.7.Execution Control....................................................................................... 614.7.1.cudaFuncGetAttributes............................................................................614.7.2.cudaFuncSetCacheConfig......................................................................... 614.7.3.cudaFuncSetSharedMemConfig................................................................... 614.7.4.cudaSetDoubleForDevice.......................................................................... 624.7.5.cudaSetDoubleForHost.............................................................................624.8.Memory Management................................................................................... 624.8.1. cudaFree.............................................................................................624.8.2.cudaFreeArray...................................................................................... 634.8.3.cudaFreeHost....................................................................................... 634.8.4.cudaGetSymbolAddress............................................................................634.8.5.cudaGetSymbolSize................................................................................ 634.8.6.cudaHostAlloc.......................................................................................634.8.7.cudaHostGetDevicePointer........................................................................644.8.8.cudaHostGetFlags.................................................................................. 644.8.9.cudaHostRegister................................................................................... 644.8.10.cudaHostUnregister...............................................................................644.8.11.cudaMalloc......................................................................................... 644.8.12.cudaMallocArray.................................................................................. 644.8.13.cudaMallocManaged.............................................................................. 654.8.14.cudaMallocPitch................................................................................... 654.8.15.cudaMalloc3D...................................................................................... 654.8.16.cudaMalloc3DArray............................................................................... 654.8.17.cudaMemAdvise................................................................................... 654.8.18.cudaMemcpy.......................................................................................664.8.19.cudaMemcpyArrayToArray....................................................................... 664.8.20.cudaMemcpyAsync................................................................................ 664.8.21.cudaMemcpyFromArray.......................................................................... 674.8.22.cudaMemcpyFromSymbol........................................................................ 674.8.23.cudaMemcpyFromSymbolAsync................................................................. 674.8.24.cudaMemcpyPeer................................................................................. 674.8.25.cudaMemcpyPeerAsync...........................................................................674.8.26.cudaMemcpyToArray..............................................................................684.8.27.cudaMemcpyToSymbol............................................................................684.8.28.cudaMemcpyToSymbolAsync.....................................................................684.8.29.cudaMemcpy2D....................................................................................684.8.30.cudaMemcpy2DArrayToArray.................................................................... 684.8.31.cudaMemcpy2DAsync............................................................................. 694.8.32.cudaMemcpy2DFromArray....................................................................... 694.8.33.cudaMemcpy2DToArray...........................................................................69CUDA Fortran Programming Guide and Reference Version2017|vi4.8.34.cudaMemcpy3D....................................................................................694.8.35.cudaMemcpy3DAsync............................................................................. 694.8.36.cudaMemGetInfo.................................................................................. 704.8.37.cudaMemPrefetchAsync.......................................................................... 704.8.38.cudaMemset....................................................................................... 704.8.39.cudaMemset2D.................................................................................... 704.8.40.cudaMemset3D.................................................................................... 704.9.Unified Addressing and Peer Device Memory Access.............................................. 714.9.1.cudaDeviceCanAccessPeer........................................................................ 714.9.2.cudaDeviceDisablePeerAccess.................................................................... 714.9.3.cudaDeviceEnablePeerAccess.....................................................................714.9.4.cudaPointerGetAttributes.........................................................................714.10.Version Management...................................................................................714.10.1.cudaDriverGetVersion............................................................................ 714.10.2.cudaRuntimeGetVersion..........................................................................72Chapter5.Examples........................................................................................... 735.1.Matrix Multiplication Example........................................................................ 735.1.1.Source Code Listing................................................................................745.1.2.Source Code Description..........................................................................755.2.Mapped Memory Example.............................................................................. 765.3.Cublas Module Example................................................................................ 785.4.CUDA Device Properties Example.....................................................................805.5.CUDA Asynchronous Memory Transfer Example.....................................................815.6.Managed Memory Example.............................................................................83Chapter6.Contact Information............................................................................. 84CUDA Fortran Programming Guide and Reference Version2017|viiLIST OF TABLESTable1 Intrinsic Datatypes ..................................................................................... 9Table2 Device Code Intrinsic Datatypes ................................................................... 34Table3 Fortran Numeric and Logical Intrinsics ............................................................35Table4 Fortran Mathematical Intrinsics .................................................................... 35Table5 Fortran Numeric Inquiry Intrinsics ................................................................. 36Table6 Fortran Bit Manipulation Intrinsics .................................................................36Table7 Fortran Reduction Intrinsics .........................................................................36Table8 Fortran Random Number Intrinsics ................................................................ 36Table9 Arithmetic and Bitwise Atomic Functions ........................................................ 40Table10 Counting Atomic Functions ........................................................................ 41Table11 Compare and Swap Atomic Function .............................................................41Table12 CUDA Built-in Routines ............................................................................. 46Table13 CUDA Device libm Routines ........................................................................47CUDA Fortran Programming Guide and Reference Version2017|viiiPREFACEThis document describes CUDA Fortran, a small set of extensions to Fortran thatsupports and is built upon the CUDA computing architecture.Intended AudienceThis guide is intended for application programmers, scientists and engineers proficientin programming with the Fortran, C, and/or C++ languages. The PGI tools are availableon a variety of operating systems for the X86, AMD64, Intel 64, and OpenPOWERhardware platforms. This guide assumes familiarity with basic operating system usage.OrganizationThe organization of this document is as follows:Introductioncontains a general introductionProgramming Guideserves as a programming guide for CUDA FortranReferencedescribes the CUDA Fortran language referenceRuntime APIsdescribes the interface between CUDA Fortran and the CUDA Runtime APIExamplesprovides sample code and an explanation of the simple example.ConventionsThis guide uses the following conventions:italicis used for emphasis.Constant Widthis used for filenames, directories, arguments, options, examples, and for languagestatements in the text, including assembly language statements.PrefaceCUDA Fortran Programming Guide and Reference Version2017|ixBoldis used for commands.[ item1 ]in general, square brackets indicate optional items. In this case item1 is optional. Inthe context of p/t-sets, square brackets are required to specify a p/t-set.{ item2 | item 3 }braces indicate that a selection is required. In this case, you must select either item2 oritem3.filename ...ellipsis indicate a repetition. Zero or more of the preceding item may occur. In thisexample, multiple filenames are allowed.FORTRANFortran language statements are shown in the text of this guide using a reduced fixedpoint size.C/C++C/C++ language statements are shown in the test of this guide using a reduced fixedpoint size.The PGI compilers and tools are supported on 64-bit variants of the Linux, ApplemacOS, and Windows operating systems on a variety of x86-compatible processors.There are a wide variety of releases and distributions of each of these types ofoperating systems. The PGI compilers and PGI profiler are also supported on Linux forOpenPOWER.TerminologyIf there are terms in this guide with which you are unfamiliar, PGI provides a glossary ofterms which you can access at www.pgroup.com/support/definitions.htmRelated PublicationsThe following documents contain additional information related to CUDA Fortranprogramming. ISO/IEC 1539-1:1997, Information Technology Programming Languages FORTRAN, Geneva, 1997 (Fortran 95). NVIDIA CUDA Programming Guide, NVIDIA, Version 3.1.1, 7/21/2010. Availableonline at http://www.nvidia.com/cuda. NVIDIA CUDA Compute Unified Device Architecture Reference Manual, NVIDIA,Version 3.1, June 2010. Available online at http://www.nvidia.com/cuda. PGI Compiler Users Guide, The Portland Group, Release 2017. Available online atwww.pgroup.com/doc/pgiug.pdf.www.pgroup.com/support/definitions.htmhttp://www.nvidia.com/cuda.http://www.nvidia.com/cuda.PrefaceCUDA Fortran Programming Guide and Reference Version2017|xCUDA Fortran Programming Guide and Reference Version2017|1Chapter1.INTRODUCTIONWelcome to Release 2017 of PGI CUDA Fortran, a small set of extensions to Fortran thatsupports and is built upon the CUDA computing architecture.Graphic processing units or GPUs have evolved into programmable, highly parallelcomputational units with very high memory bandwidth, and tremendous potential formany applications. GPU designs are optimized for the computations found in graphicsrendering, but are general enough to be useful in many data-parallel, compute-intensiveprograms.NVIDIA introduced CUDA, a general purpose parallel programming architecture,with compilers and libraries to support the programming of NVIDIA GPUs. CUDAcomes with an extended C compiler, here called CUDA C, allowing direct programmingof the GPU from a high level language. The programming model supports four keyabstractions: cooperating threads organized into thread groups, shared memory andbarrier synchronization within thread groups, and coordinated independent threadgroups organized into a grid. A CUDA programmer must partition the program intocoarse grain blocks that can be executed in parallel. Each block is partitioned into finegrain threads, which can cooperate using shared memory and barrier synchronization. Aproperly designed CUDA program will run on any CUDA-enabled GPU, regardless ofthe number of available processor cores.CUDA Fortran includes a Fortran 2003 compiler and tool chain for programmingNVIDIA GPUs using Fortran. PGI 2017 includes support for CUDA Fortran on Linux,Apple macOS and Windows. CUDA Fortran is an analog to NVIDIA's CUDA Ccompiler. Compared to the PGI Accelerator and OpenACC directives-based model andcompilers, CUDA Fortran is a lower-level explicit programming model with substantialruntime library components that give expert programmers direct control of all aspects ofGPGPU programming.The CUDA Fortran extensions described in this document allow the followingoperations in a Fortran program: Declaring variables that are allocated in the GPU device memory Allocating dynamic memory in the GPU device memory Copying data from the host memory to the GPU memory, and back Writing subroutines and functions to execute on the GPUIntroductionCUDA Fortran Programming Guide and Reference Version2017|2 Invoking GPU subroutines from the host Allocating pinned memory on the host Using asynchronous transfers between the host and GPU Using zero-copy and CUDA Unified Virtual Addressing features. Accessing read-only data through texture memory caches. Automatically generating GPU kernels using the kernel loop directive. Launching GPU kernels from other GPU subroutines running on the device usingCUDA 5.0 and above dynamic parallelism features. Relocatable device code: Creating and linking device libraries such as the cublas;and calling functions defined in other modules and files. Interfacing to CUDA C.CUDA Fortran Programming Guide and Reference Version2017|3Chapter2.PROGRAMMING GUIDEThis section introduces the CUDA programming model through examples written inCUDA Fortran. For a reference for CUDA Fortran, refer to Reference.2.1.CUDA Fortran Host and Device CodeAll CUDA programs, and in general any program which uses a GPU for computation,must perform the following steps: 1. Initialize and select the GPU to run on. Oftentimes this is implicit in the programand defaults to NVIDIA device 0. 2. Allocate space for data on the GPU. 3. Move data from the host to the GPU, or in some cases, initialize the data on theGPU. 4. Launch kernels from the host to run on the GPU. 5. Gather results back from the GPU for further analysis our output from the hostprogram. 6. Deallocate the data on the GPU allocated in step 2. This might be implicitlyperformed when the host program exits.Here is a simple CUDA Fortran example which performs the required steps:Programming GuideCUDA Fortran Programming Guide and Reference Version2017|4Explicit Device SelectionHost code Device Codeprogram t1use cudaforuse mytestsinteger, parameter :: n = 100integer, allocatable, device :: iarr(:)integer h(n)istat = cudaSetDevice(0)allocate(iarr(n))h = 0; iarr = hcall test1 (iarr)h = iarrprint *,&"Errors: ", count(h.ne.(/ (i,i=1,n) /))deallocate(iarr)end program t1module mytestscontainsattributes(global) &subroutine test1( a )integer, device :: a(*)i = threadIdx%xa(i) = ireturnend subroutine test1end module mytestsIn the CUDA Fortran host code on the left, device selection is explicit, performedby an API call on line 7. The provided cudafor module, used in line 2, containsinterfaces to the full CUDA host runtime library, and in this case exposes the interfaceto cudaSetDevice() and ensures it is called correctly. An array is allocated on thedevice at line 8. Line 9 of the host code initializes the data on the host and the device,and, in line 10, a device kernel is launched. The interface to the device kernel is explicit,in the Fortran sense, because the module containing the kernel is used in line 3. At line11 of the host code, the results from the kernel execution are moved back to a host array.Deallocation of the GPU array occurs on line 14.Implicit Device SelectionHere is a CUDA Fortran example which is slightly more complicated than the precedingone.Host code Device Codeprogram testrampuse cublasuse rampinteger, parameter :: N = 20000real, device :: x(N)twopi = atan(1.0)*8call buildramp(x,N)!$cuf kernel dodo i = 1, Nx(i) = 2.0 * x(i) * x(i)end doprint *,"float(N) = ",sasum(N,x,1)end programmodule rampreal, constant :: twopicontainsattributes(global) &subroutine buildramp(x, n)real, device :: x(n)integer, value :: nreal, shared :: termif (threadidx%x == 1) term = &twopi / float(n)call syncthreads()i = (blockidx%x-1)*blockdim%x &+ threadidx%xif (i Programming GuideCUDA Fortran Programming Guide and Reference Version2017|5In this case, the device selection is implicit, and defaults to NVIDIA device 0. The devicearray allocation in the host code at line 5 looks static, but actually occurs at program inittime. Larger array sizes are handled, both in the kernel launch at line 7 in the host code,and in the device code at line 10. The device code contains examples of constant andshared data, which are described in Reference. There are actually two kernels launchedfrom the host code: one explicitly provided and called from line 10, and a second,generated using the CUDA Fortran kernel loop directive, starting at line 11. Finally, thisexample demonstrates the use of the cublas module, used at line 2 in the host code,and called at line 12.As these two examples demonstrate, all the steps listed at the beginning of this sectionfor using a GPU are contained within the host code. It is possible to program GPUswithout writing any kernels and device code, through library calls and CUDA Fortrankernel loop directives as shown, or by using higher-level directive-based models;however, programming in a lower-level model like CUDA provides the programmercontrol over device resource utilization and kernel execution.2.2.CUDA Fortran KernelsCUDA Fortran allows the definition of Fortran subroutines that execute in parallel onthe GPU when called from the Fortran program which has been invoked and is runningon the host or, starting in CUDA 5.0, on the device. Such a subroutine is called a devicekernel or kernel.A call to a kernel specifies how many parallel instances of the kernel must be executed;each instance will be executed by a different CUDA thread. The CUDA threads areorganized into thread blocks, and each thread has a global thread block index, and alocal thread index within its thread block.A kernel is defined using the attributes(global) specifier on the subroutinestatement; a kernel is called using special chevron syntax to specify the number ofthread blocks and threads within each thread block:! Kernel definitionattributes(global) subroutine ksaxpy( n, a, x, y ) real, dimension(*) :: x,y real, value :: a integer, value :: n, i i = (blockidx%x-1) * blockdim%x + threadidx%x if( i Programming GuideCUDA Fortran Programming Guide and Reference Version2017|6In this case, the call to the kernel ksaxpy specifies n/64 thread blocks, each with 64threads. Each thread is assigned a thread block index accessed through the built-inblockidx variable, and a thread index accessed through threadidx. In this example,each thread performs one iteration of the common SAXPY loop operation.2.3.Thread BlocksEach thread is assigned a thread block index accessed through the built-in blockidxvariable, and a thread index accessed through threadidx. The thread index may bea one-, two-, or three-dimensional index. In CUDA Fortran, the thread index for eachdimension starts at one.Threads in the same thread block may cooperate by using shared memory, and bysynchronizing at a barrier using the SYNCTHREADS() intrinsic. Each thread in the blockwaits at the call to SYNCTHREADS() until all threads have reached that call. The sharedmemory acts like a low-latency, high bandwidth software managed cache memory.Currently, the maximum number of threads in a thread block is 1024.A kernel may be invoked with many thread blocks, each with the same thread blocksize. The thread blocks are organized into a one-, two-, or three-dimensional grid ofblocks, so each thread has a thread index within the block, and a block index within thegrid. When invoking a kernel, the first argument in the chevron syntax is the gridsize, and the second argument is the thread block size. Thread blocks must be able toexecute independently; two thread blocks may be executed in parallel or one after theother, by the same core or by different cores.The dim3 derived type, defined in the cudafor module, can be used to declarevariables in host code which can conveniently hold the launch configuration values ifthey are not scalars; for example:type(dim3) :: blocks, threads...blocks = dim3(n/256, n/16, 1)threads = dim3(16, 16, 1)call devkernel( ... )2.4.Memory HierarchyCUDA Fortran programs have access to several memory spaces. On the host side, thehost program can directly access data in the host main memory. It can also directly copydata to and from the device global memory; such data copies require DMA access to thedevice, so are slow relative to the host memory. The host can also set the values in thedevice constant memory, again implemented using DMA access.On the device side, data in global device memory can be read or written by all threads.Data in constant memory space is initialized by the host program; all threads can readdata in constant memory. Accesses to constant memory are typically faster than accessesto global memory, but it is read-only to the threads and limited in size. Threads in thesame thread block can access and share data in shared memory; data in shared memoryhas a lifetime of the thread block. Each thread can also have private local memory; dataProgramming GuideCUDA Fortran Programming Guide and Reference Version2017|7in thread local memory may be implemented as processor registers or may be allocatedin the global device memory; best performance will often be obtained when thread localdata is limited to a small number of scalars that can be allocated as processor registers.Through use of the CUDA API as exposed by the cudafor module, access to CUDAfeatures such as mapped memory, peer-to-peer memory access, and the unified virtualaddress space are supported. Users should check the relevant CUDA documentationfor compute capability restrictions for these features. For an example of device arraymapping, refer to Mapped Memory Example.Starting with CUDA 6.0, managed or unified memory programming is available oncertain platforms. For a complete description of unified memory programming, seeAppendix J. of the CUDA_C_Programming_Guide. Managed memory provides acommon address space, and migrates data between the host and device as it is used byeach set of processors. On the host side, the data is resident in host main memory. On thedevice side, it is accessed as resident in global device memory.2.5.Subroutine / Function QualifiersA subroutine or function in CUDA Fortran has an additional attribute, designatingwhether it is executed on the host or on the device, and if the latter, whether it is akernel, called from the host, or called from another device subprogram. A subprogram declared with attributes(host), or with the host attribute bydefault, is called a host subprogram. A subprogram declared with attributes(global) or attributes(device) iscalled a device subprogram. A subroutine declared with attributes(global) is also called a kernel subroutine.2.5.1.Attributes(host)The host attribute, specified on the subroutine or function statement, declares that thesubroutine or function is to be executed on the host. Such a subprogram can only becalled from another host subprogram. The default is attributes(host), if none of thehost, global, or device attributes is specified.2.5.2.Attributes(global)The global attribute may only be specified on a subroutine statement; it declaresthat the subroutine is a kernel subroutine, to be executed on the device, and may onlybe called using a kernel call containing the chevron syntax and runtime mappingparameters.2.5.3.Attributes(device)The device attribute, specified on the subroutine or function statement, declares thatthe subprogram is to be executed on the device; such a routine must be called from asubprogram with the global or device attribute.Programming GuideCUDA Fortran Programming Guide and Reference Version2017|82.5.4.RestrictionsThe following restrictions apply to subprograms. A device subprogram must not contain variables with the SAVE attribute, or withdata initialization. A kernel subroutine may not also have the device or host attribute. Calls to a kernel subroutine must specify the execution configuration, as describedin "Predefined Variables in Device Subprograms," on page 9. Such a call isasynchronous, that is, the calling routine making the call continues to execute beforethe device has completed its execution of the kernel subroutine. Device subprograms may not be contained in a host subroutine or function, andmay not contain any subroutines or functions.2.6.Variable QualifiersVariables in CUDA Fortran have a new attribute that declares in which memory the datais allocated. By default, variables declared in modules or host subprograms are allocatedin the host main memory. At most one of the device, managed, constant, shared, orpinned attributes may be specified for a variable.2.6.1.Attributes(device)A variable with the device attribute is called a device variable, and is allocated in thedevice global memory. If declared in a module, the variable may be accessed by any subprogram in thatmodule and by any subprogram that uses the module. If declared in a host subprogram, the variable may be accessed by that subprogramor subprograms contained in that subprogram.A device array may be an explicit-shape array, an allocatable array, or an assumed-shape dummy array. An allocatable device variable has a dynamic lifetime, from whenit is allocated until it is deallocated. Other device variables have a lifetime of the entireapplication.2.6.2.Attributes(managed)Starting with CUDA 6.0, on certain platforms, a variable with the managed attribute iscalled a managed variable. Managed variables may be used in both host and device code.Variables with the managed attribute migrate between the host and device, dependingon where the accesses to the memory originate. Managed variables may be read andwritten by the host, but there are access restrictions on the managed variables if kernelsare active on the device. On the device, managed variables have characteristics similar todevice variables, but managed variables cannot be allocated from the device, as devicevariables can be, starting in CUDA 5.0 in support of dynamic parallelism.Programming GuideCUDA Fortran Programming Guide and Reference Version2017|92.6.3.Attributes(constant)A variable with the constant attribute is called a device constant variable. Deviceconstant variables are allocated in the device constant memory space. When declaredin a module, the variable may be accessed by any subprogram in that module and byany subprogram that uses the module. Device constant data may not be assigned ormodified in any device subprogram, but may be modified in host subprograms. Deviceconstant variables may not be allocatable, and have a lifetime of the entire application.2.6.4.Attributes(shared)A variable with the shared attribute is called a device shared variable or a sharedvariable. A shared variable may only be declared in a device subprogram, and mayonly be accessed within that subprogram, or by other device subprograms to whichit is passed as an argument. A shared variable may not be data initialized. A sharedvariable is allocated in the device shared memory for a thread block, and has a lifetimeof the thread block. It can be read or written by all threads in the block, though a writein one thread is only guaranteed to be visible to other threads after the next call to theSYNCTHREADS() intrinsic.2.6.5.Attributes(pinned)A variable with the pinned attribute is called a pinned variable. A pinned variablemust be an allocatable array. When a pinned variable is allocated, it will be allocated inhost pagelocked memory. The advantage of using pinned variables is that copies frompage-locked memory to device memory are faster than copies from normal paged hostmemory. Some operating systems or installations may restrict the use, availability, or sizeof page-locked memory; if the allocation in page-locked memory fails, the variable willbe allocated in the normal host paged memory and required for asynchronous moves.2.6.6.Attributes(texture)A variable with the texture attribute is called a texture variable. A texture variable mustbe an F90 pointer, and can be of type real or integer. Texture variables may be accessedonly in device subprograms, and can only be read, not written. The advantage of usingtexture variables is that the accesses to texture data goes through a separate cache on thedevice, which may result in improved performance for many codes. Texture variablesare bound to underlying device arrays in host code using F90 pointer assignments.2.7.Datatypes in Device SubprogramsThe following intrinsic datatypes are allowed in device subprograms and device data:Table1 Intrinsic DatatypesType Type KindProgramming GuideCUDA Fortran Programming Guide and Reference Version2017|10integer 1,2,4,8logical 1,2,4,8real 4,8double precision equivalent to real(kind=8)complex 4,8character(len=1) 1Derived types may contain members with these intrinsic datatypes or other allowedderived types.2.8.Predefined Variables in Device SubprogramsDevice subprograms have access to block and grid indices and dimensions throughseveral built-in read-only variables. These variables are of type dim3; the modulecudafor defines the derived type dim3 as follows:type(dim3) integer(kind=4) :: x,y,zend typeThese predefined variables are not accessible in host subprograms. The variable threadidx contains the thread index within its thread block; forone- or two-dimensional thread blocks, the threadidx%y and/or threadidx%zcomponents have the value one. The variable blockdim contains the dimensions of the thread block; blockdim hasthe same value for all thread blocks in the same grid. The variable blockidx contains the block index within the grid; as with threadidx,for one-dimensional grids, blockidx%y and/or blockidx%z has the value one. The variable griddim contains the dimensions of the grid. The constant warpsize is declared to be type integer. Threads are executed ingroups of 32, called warps; warpsize contains the number of threads in a warp, andis currently 32.2.9.Execution ConfigurationA call to a kernel subroutine must specify an execution configuration. The executionconfiguration defines the dimensionality and extent of the grid and thread blocks thatexecute the subroutine. It may also specify a dynamic shared memory extent, in bytes,and a stream identifier, to support concurrent stream execution on the device.A kernel subroutine call looks like this:call kernel(arg1,arg2,...)whereProgramming GuideCUDA Fortran Programming Guide and Reference Version2017|11 grid and block are either integer expressions (for one-dimensional grids andthread blocks), or are type(dim3), for one- or two-dimensional grids and threadblocks. If grid is type(dim3), the value of each component must be equal to or greaterthan one, and the product is usually limited by the compute capability of the device. If block is type(dim3), the value of each component must be equal to or greaterthan one, and the product of the component values must be less than or equal to1024. The value of bytes must be an integer; it specifies the number of bytes of sharedmemory to be allocated for each thread block, in addition to the statically allocatedshared memory. This memory is used for the assumed-size shared variables in thethread block; refer to Shared data for more information. If the value of bytes is notspecified, its value is treated as zero. The value of streamid must be an integer greater than or equal to zero; itspecifies the stream to which this call is associated. Nonzero stream values can becreated with a call to cudaStreamCreate. Starting in CUDA 7.0, the constantcudaStreamPerThread can be specified to use a unique default stream for eachCPU thread.2.10.Asynchronous Concurrent ExecutionThere are two components to asynchronous concurrent execution with CUDA Fortran.2.10.1.Concurrent Host and Device ExecutionWhen a host subprogram calls a kernel subroutine, the call actually returns to the hostprogram before the kernel subroutine begins execution. The call can be treated as a kernellaunch operation, where the launch actually corresponds to placing the kernel on a queuefor execution by the device. In this way, the host can continue executing, includingcalling or queueing more kernels for execution on the device. By calling the runtimeroutine cudaDeviceSynchronize, the host program can synchronize and wait for allpreviously launched or queued kernels.Programmers must be careful when using concurrent host and device execution; in caseswhere the host program reads or modifies device or constant data, the host programshould synchronize with the device to avoid erroneous results.2.10.2.Concurrent Stream ExecutionOperations involving the device, including kernel execution and data copies to and fromdevice memory, are implemented using stream queues. An operation is placed at theend of the stream queue, and will only be initiated when all previous operations on thatqueue have been completed.An application can manage more concurrency by using multiple streams. Each user-created stream manages its own queue; operations on different stream queues mayexecute out-of-order with respect to when they were placed on the queues, and mayexecute concurrently with each other.Programming GuideCUDA Fortran Programming Guide and Reference Version2017|12The default stream, used when no stream identifier is specified, is stream zero; streamzero is special in that operations on the stream zero queue will begin only after allpreceding operations on all queues are complete, and no subsequent operations on anyqueue begin until the stream zero operation is complete.2.11.Kernel Loop DirectiveCUDA Fortran allows automatic kernel generation and invocation from a region of hostcode containing one or more tightly nested loops. Launch configuration and mapping ofthe loop iterations onto the hardware is controlled and specified as part of the directivebody using the familiar CUDA chevron syntax. As with any kernel, the launch isasynchronous. The program can use cudaDeviceSynchronize() or CUDA Events to waitfor the completion of the kernel.The work in the loops specified by the directive is executed in parallel, across thethread blocks and grid; it is the programmer's responsibility to ensure that parallelexecution is legal and produces the correct answer. The one exception to this rule is ascalar reduction operation, such as summing the values in a vector or matrix. For theseoperations, the compiler handles the generation of the final reduction kernel, insertingsynchronization into the kernel as appropriate.2.11.1.SyntaxThe general form of the kernel directive is: !$cuf kernel do[(n)] >The compiler maps the launch configuration specified by the grid and block values ontothe outermost n loops, starting at loop n and working out. The grid and block valuescan be an integer scalar or a parenthesized list. Alternatively, using asterisks tells thecompiler to choose a thread block shape and/or compute the grid shape from the threadblock shape and the loop limits. Loops which are not mapped onto the grid and blockvalues are run sequentially on each thread.There are two ways to specify the optional stream argument: !$cuf kernel do[(n)] >Or !$cuf kernel do[(n)] >Kernel Loop Directive Example 1 !$cuf kernel do(2) > do j = 1, m do i = 1, n a(i,j) = b(i,j) + c(i,j) end do end doIn this example, the directive defines a two-dimensional thread block of size 32x4.The body of the doubly-nested loop is turned into the kernel body: ThreadIdx%x runs from 1 to 32 and is mapped onto the inner i loop.Programming GuideCUDA Fortran Programming Guide and Reference Version2017|13 ThreadIdx%y runs from 1 to 4 and is mapped onto the outer j loop.The grid shape, specified as (*,*), is computed by the compiler and runtime by dividingthe loop trip counts n and m by the thread block size, so all iterations are computed.Kernel Loop Directive Example 2 !$cuf kernel do > do j = 1, m do i = 1, n a(i,j) = b(i,j) + c(i,j) end do end doWithout an explicit n on the do, the schedule applies just to the outermost loop, that is,the default value is 1. In this case, only the outer j loop is run in parallel with a threadblock size of 256. The inner i dimension is run sequentially on each thread.You might consider if the code in Kernel Loop Directive Example 2 would perform betterif the two loops were interchanged. Alternatively, you could specify a configuration likethe following in which the threads read and write the matrices in coalesced fashion. !$cuf kernel do(2) > do j = 1, m do i = 1, n a(i,j) = b(i,j) + c(i,j) end do end doKernel Loop Directive Example 3In Kernel Loop Directive Example 2, the 256 threads in each block each do one element ofthe matrix addition. Further expansion of the work along the i direction and all workacross the j dimension is handled by the mapping onto the grid dimensions.To "unroll" more work into each thread, specify non-asterisk values for the grid, asillustrated here: !$cuf kernel do(2) > do j = 1, m do i = 1, n a(i,j) = b(i,j) + c(i,j) end do end doNow the threads in a thread block handle all values in the i direction, in concert,incrementing by 256. One thread block is created for each j. Specifically, the j loop ismapped onto the grid x-dimension, because the compiler skips over the constant 1 in thei loop grid size. In CUDA built-in language, gridDim%x is equal to m.2.11.2.Restrictions on the CUF kernel directiveThe following restrictions apply to CUF kernel directives: If the directive specifies n dimensions, it must be followed by at least that manytightly-nested DO loops.Programming GuideCUDA Fortran Programming Guide and Reference Version2017|14 The tightly-nested DO loops must have invariant loop limits: the lower limit, upperlimit, and increment must be invariant with respect to any other loop in the kerneldo. There can be no GOTO or EXIT statements within or between any loops that havebeen mapped onto the grid and block configuration values. The body of the loops may contain assignment statements, IF statements, loops, andGOTO statements. Only CUDA Fortran data types are allowed within the loops. CUDA Fortran intrinsic functions are allowed, if they are allowed in device code,but the device-specific intrinsics such as syncthreads, atomic functions, etc. are not. Subroutine and function calls to attributes(device) subprograms are allowed if theyare in the same module as the code containing the directive. Arrays used or assigned in the loop must have the device attribute. Implicit loops and F90 array syntax are not allowed within the directive loops. Scalars used or assigned in the loop must either have the device attribute, or thecompiler will make a device copy of that variable live for the duration of the loops,one for each thread. Except in the case of reductions; when a reduction has a scalartarget, the compiler generates a correct sequence of synchronized operations toproduce one copy either in device global memory or on the host.2.11.3.Summation ExampleThe simplest directive form for performing a dot product on two device arrays takesadvantage of the properties for scalar use outlined previously. rsum = 0.0 !$cuf kernel do > do i = 1, n rsum = rsum + x(i)* y(i) end doFor reductions, the compiler recognizes the use of the scalar and generates just one finalresult.This CUF kernel can be followed by another CUF kernel in the same subprogram: !$cuf kernel do > do i = 1, n rsum= x(i) * y(i) z(i) = rsum end doIn this CUF kernel, the compiler recognizes rsum as a scalar temporary which should beallocated locally on every thread. However, use of rsum on the host following this loop isundefined.2.12.Using Fortran ModulesModern Fortran uses modules to package global data, definitions, derived types, andinterface blocks. In CUDA Fortran these modules can be used to easily communicatedata and definitions between host and device code. This section includes a few examplesof using Fortran Modules.Programming GuideCUDA Fortran Programming Guide and Reference Version2017|152.12.1.Accessing Data from Other Modulesin the following example, a set of modules are defined in one file which are accessed byanother module.Accessing data from other modules.In one file, moda.cuf, you could define a set of modules:module moda real, device, allocatable :: a(:)end modulemodule modb real, device, allocatable :: b(:)end moduleIn another module or file, modc.cuf, you could define another module which uses thetwo modules moda and modb:module modc use moda use modb integer, parameter :: n = 100 real, device, allocatable :: c(:) contains subroutine vadd() !$cuf kernel do do i = 1, n c(i) = a(i) + b(i) end do end subroutineend moduleIn the host program, you use the top-level module, and get the definition of n and theinterface to vadd. You can also rename the device arrays so they do not conflict with thehost naming conventions:program tuse modc, a_d => a, b_d => b, c_d => creal a,b,c(n)allocate(a_d(n),b_d(n),c_d(n))a_d = 1.0b_d = 2.0call vadd()c = c_dprint *,all(c.eq.3.0)end2.12.2.Call Routines from Other ModulesStarting with CUDA 5.0, in addition to being able to access data declared in anothermodule, you can also call device functions which are contained in another module. Inthe following example, the file ffill.cuf contains a device function to fill an array:Programming GuideCUDA Fortran Programming Guide and Reference Version2017|16Calling routines from other modules using relocatable device code.module ffill contains attributes(device) subroutine fill(a) integer, device :: a(*) i = (blockidx%x-1)*blockdim%x + threadidx%x a(i) = i end subroutineend moduleTo generate relocatable device code, compile this file with the Mcuda=rdc flag:% pgf90 -Mcuda=rdc -c ffill.cufNow write another module and test program that calls the subroutine in this module.Since you are calling an attributes(device) subroutine, you do not use the chevronsyntax. For convenience, an overloaded Fortran sum function is included in the filetfill.cuf which, in this case, takes 1-D integer device arrays.module testfill use ffill contains attributes(global) subroutine Kernel(arr) integer, device :: arr(*) call fill(arr) end subroutine Kernel integer function sum(arr) integer, device :: arr(:) sum = 0 !$cuf kernel do do i = 1, size(arr) sum = sum + arr(i) end do end function sumend module testfillprogram tfilluse testfillinteger, device :: iarr(100)iarr = 0call Kernel(iarr)print *,sum(iarr)==100*101/2end program tfillThis file also needs to be compiled with the Mcuda=rdc flag and then can be linkedwith the previous object file:% pgf90 -Mcuda=rdc tfill.cuf ffill.o2.12.3.Declaring Device Pointer and Target ArraysRecently, PGI added support for F90 pointers that point to device data. Currently,this is limited to pointers that are declared at module scope. The pointers can beaccessed through module association, or can be passed in to global subroutines. Theassociated() function is also supported in device code. The following code showsmany examples of using F90 pointers. These pointers can also be used in CUF kernels.Programming GuideCUDA Fortran Programming Guide and Reference Version2017|17Declaring device pointer and target arrays in CUDA Fortran modulesmodule devptr! currently, pointer declarations must be in a module real, device, pointer, dimension(:) :: mod_dev_ptr real, device, pointer, dimension(:) :: arg_dev_ptr real, device, target, dimension(4) :: mod_dev_arr real, device, dimension(4) :: mod_res_arrcontains attributes(global) subroutine test(arg_ptr) real, device, pointer, dimension(:) :: arg_ptr ! copy 4 elements from one of two spots if (associated(arg_ptr)) then mod_res_arr = arg_ptr else mod_res_arr = mod_dev_ptr end if end subroutine testend module devptrProgramming GuideCUDA Fortran Programming Guide and Reference Version2017|18program testuse devptrreal, device, target, dimension(4) :: a_devreal result(20)a_dev = (/ 1.0, 2.0, 3.0, 4.0 /)! Pointer assignment to device array declared on host,! passed as argument. First four result elements.arg_dev_ptr => a_devcall test(arg_dev_ptr)result(1:4) = mod_res_arr!$cuf kernel do do i = 1, 4 mod_dev_arr(i) = arg_dev_ptr(i) + 4.0 a_dev(i) = arg_dev_ptr(i) + 8.0end do! Pointer assignment to module array, argument nullified! Second four result elementsmod_dev_ptr => mod_dev_arrarg_dev_ptr => null()call test(arg_dev_ptr)result(5:8) = mod_res_arr! Pointer assignment to updated device array, now associated! Third four result elementsarg_dev_ptr => a_devcall test(arg_dev_ptr)result(9:12) = mod_res_arr!$cuf kernel do do i = 1, 4 mod_dev_arr(i) = 25.0 - mod_dev_ptr(i) a_dev(i) = 25.0 - arg_dev_ptr(i)end do! Non-contiguous pointer assignment to updated device array! Fourth four element elementsarg_dev_ptr => a_dev(4:1:-1)call test(arg_dev_ptr)result(13:16) = mod_res_arr! Non-contiguous pointer assignment to updated module array! Last four elements of the resultnullify(arg_dev_ptr)mod_dev_ptr => mod_dev_arr(4:1:-1)call test(arg_dev_ptr)result(17:20) = mod_res_arrprint *,all(result==(/(real(i),i=1,20)/))end2.12.4.Declaring TexturesIn 2012, PGI added support for CUDA texture memory fetches through a special textureattribute ascribed to F90 pointers that point to device data with the target attribute. InCUDA Fortran, textures are currently just for read-only data that travel through thetexture cache. Since there is separate hardware to support this cache, in many casesusing the texture attribute is a performance boost, especially in cases where the accessesare irregular and noncontiguous amongst threads. The following simple exampledemonstrates this capability:Programming GuideCUDA Fortran Programming Guide and Reference Version2017|19Declaring textures in CUDA Fortran modulesmodule memtests real(8), texture, pointer :: t(:) ! declare the texture contains attributes(device) integer function bitrev8(i) integer ix1, ix2, ix ix = i ix1 = ishft(iand(ix,z'0aa'),-1) ix2 = ishft(iand(ix,z'055'), 1) ix = ior(ix1,ix2) ix1 = ishft(iand(ix,z'0cc'),-2) ix2 = ishft(iand(ix,z'033'), 2) ix = ior(ix1,ix2) ix1 = ishft(ix,-4) ix2 = ishft(ix, 4) bitrev8 = iand(ior(ix1,ix2),z'0ff') end function bitrev8 attributes(global) subroutine without( a, b ) real(8), device :: a(*), b(*) i = blockDim%x*(blockIdx%x-1) + threadIdx%x j = bitrev8(threadIdx%x-1) + 1 b(i) = a(j) return end subroutine attributes(global) subroutine withtex( a, b ) real(8), device :: a(*), b(*) i = blockDim%x*(blockIdx%x-1) + threadIdx%x j = bitrev8(threadIdx%x-1) + 1 b(i) = t(j) ! This subroutine accesses a through the texture return end subroutineend module memtestsProgramming GuideCUDA Fortran Programming Guide and Reference Version2017|20program tuse cudaforuse memtestsreal(8), device, target, allocatable :: da(:)real(8), device, allocatable :: db(:)integer, parameter :: n = 1024*1024integer, parameter :: nthreads = 256integer, parameter :: ntimes = 1000type(cudaEvent) :: start, stopreal(8) b(n)allocate(da(nthreads))allocate(db(n))istat = cudaEventCreate(start)istat = cudaEventCreate(stop)db = 100.0d0da = (/ (dble(i),i=1,nthreads) /)call without (da, db)istat = cudaEventRecord(start,0)do j = 1, ntimes call without (da, db)end doistat = cudaEventRecord(stop,0)istat = cudaDeviceSynchronize()istat = cudaEventElapsedTime(time1, start, stop)time1 = time1 / (ntimes*1.0e3)b = dbprint *,sum(b)==(n*(nthreads+1)/2)db = 100.0d0t => da ! assign the texture to da using f90 pointer assignmentcall withtex (da, db)istat = cudaEventRecord(start,0)do j = 1, ntimes call withtex (da, db)end doistat = cudaEventRecord(stop,0)istat = cudaDeviceSynchronize()istat = cudaEventElapsedTime(time2, start, stop)time2 = time2 / (ntimes*1.0e3)b = dbprint *,sum(b)==(n*(nthreads+1)/2)print *,"Time with textures",time2print *,"Time without textures",time1print *,"Speedup with textures",time1 / time2deallocate(da)deallocate(db)end2.13.Building a CUDA Fortran ProgramCUDA Fortran is supported by the PGI Fortran compilers when the filename uses aCUDA Fortran extension. The .cuf extension specifies that the file is a free-formatCUDA Fortran program; the .CUF extension may also be used, in which case theprogram is processed by the preprocessor before being compiled. To compile a fixed-Programming GuideCUDA Fortran Programming Guide and Reference Version2017|21format program, add the command line option Mfixed. CUDA Fortran extensions canbe enabled in any Fortran source file by adding the Mcuda command line option.To enable CUDA 7.5 or 8.0 features, use Mcuda=cuda7.5 or Mcuda=cuda8.0. Startingin PGI 14.1, Mcuda=rdc is on by default to generate relocatable device code. You canoverride this option by specifying Mcuda=nordc.If you are using many instances of the CUDA kernel loop directives, that is, CUFkernels, you may want to add the Minfo switch to verify that CUDA kernels are beinggenerated where you expect and whether you have followed the restrictions outlined inthe preceding sections.Starting in PGI 14.1, a separate LLVM-based back-end is also supported. This back-end isutilized when -g is specified as a compiler option as it enables the generation of debuginformation. To always use the LLVM-based back-end, use the Mcuda=llvm commandline option. To enable debugging on the host without using the LLVM-based back-end,specify Mcuda=nodebug on the command line.2.14.Emulation ModePGI Fortran compilers support an emulation mode for program development onworkstations or systems without a CUDA-enabled GPU and for debugging. To builda program using emulation mode, compile and link with the Mcuda=emu commandline option. In emulation mode, the device code is compiled for and runs on the host,allowing the programmer to use a host debugger or full i/o capabilities.It is important to note that the emulation is far from exact. In particular, emulation modemay execute a single thread block at a time. This will not expose certain errors, such asmemory races. In emulation mode, the host floating point units and intrinsics are used,which may produce slightly different answers than the device units and intrinsics.CUDA Fortran Programming Guide and Reference Version2017|22Chapter3.REFERENCEThis section is the CUDA Fortran Language Reference.3.1.New Subroutine and Function AttributesCUDA Fortran adds new attributes to subroutines and functions. This section describeshow to specify the new attributes, their meaning and restrictions.A Subroutine may have the host, global, or device attribute, or may have both hostand device attribute. A Function may have the host or device attribute, or both. Theseattributes are specified using the attributes(attr) prefix on the Subroutine orFunction statement; if there is no attributes prefix on the subprogram statement, thendefault rules are used, as described in the following sections.3.1.1.Host Subroutines and FunctionsThe host attribute may be explicitly specified on the Subroutine or Function statement asfollows:attributes(host) subroutine sub(...) attributes(host) integer function func(...) integer attributes(host) function func(...) The host attributes prefix may be preceded or followed by any other allowablesubroutine or function prefix specifiers (recursive, pure, elemental, function returndatatype). A subroutine or function with the host attribute is called a host subroutine orfunction, or a host subprogram. A host subprogram is compiled for execution on the hostprocessor. A subprogram with no attributes prefix has the host attribute by default.3.1.2.Global SubroutinesThe global attribute may be explicitly specified on the Subroutine statement as follows:attributes(global) subroutine sub(...)Functions may not have the global attribute. A subroutine with the global attribute iscalled a kernel subroutine. A kernel subroutine may not be recursive, pure, or elemental,so no other subroutine prefixes are allowed. A kernel subroutine is compiled as aReferenceCUDA Fortran Programming Guide and Reference Version2017|23kernel for execution on the device, to be called from a host routine using an executionconfiguration. A kernel subroutine may not be contained in another subroutine orfunction, and may not contain any other subprogram.3.1.3.Device Subroutines and FunctionsThe device attribute may be explicitly specified on the Subroutine or Function statementas follows:attributes(device) subroutine sub(...)attributes(device) datatype function func(...)datatype attributes(device) function func(...)A subroutine or function with the device attribute may not be recursive, pure, orelemental, so no other subroutine or function prefixes are allowed, except for thefunction return datatype. A subroutine or function with the device or kernel attribute iscalled a device subprogram. A device subprogram is compiled for execution on the device.A subroutine or function with the device attribute must appear within a Fortran module,and may only be called from device subprograms in the same module.3.1.4.Restrictions on Device SubprogramsA subroutine or function with the device or global attribute must satisfy the followingrestrictions: It may not be recursive, nor have the recursive prefix on the subprogram statement. It may not be pure or elemental, nor have the pure or elemental prefix on thesubprogram statement. It may not contain another subprogram. It may not be contained in another subroutine or function.For more information, refer to Device Code.3.2.Variable AttributesCUDA Fortran adds new attributes for variables and arrays. This section describes howto specify the new attributes and their meaning and restrictions.Variables declared in a host subprogram may have one of three new attributes: they maybe declared to be in device global memory, in managed memory, or in pinned memory.Variables in modules may be declared to be in device global memory, in the managedmemory space, or in constant memory space. Additionally, the texture attribute can beadded to read-only data declared in modules which enables reading the data throughthe texture cache on the device.Variables declared in a device program units may have one of three new attributes:they may be declared to be in device global memory, in constant memory space, in thethread block shared memory, or without any additional attribute they will be allocatedin thread local memory. For performance and useability reasons, the value attribute canalso be used on scalar dummy arguments so they are passed by value, rather than theFortran default to pass arguments by reference.ReferenceCUDA Fortran Programming Guide and Reference Version2017|243.2.1.Device dataA variable or array with the device attribute is defined to reside in the device globalmemory. The device attribute can be specified with the attributes statement, or as anattribute on the type declaration statement. The following example declares two arrays,a and b, to be device arrays of size 100.real :: a(100)attributes(device) :: areal, device :: b(100)These rules apply to device data: An allocatable device array dynamically allocates device global memory. Device variables and arrays may appear in modules, but may not be in a Commonblock or an Equivalence statement. Members of a derived type may not have the device attribute unless they areallocatable. Device variables and arrays may be passed as actual arguments to host and devicesubprograms; in that case, the subprogram interface must be explicit (in the Fortransense), and the matching dummy argument must also have the device attribute. Device variables and arrays declared in a host subprogram cannot have the Saveattribute.In host subprograms, device data may only be used in the following manner: In declaration statements In Allocate and Deallocate statements As an argument to the Allocated intrinsic function As the source or destination in a data transfer assignment statement As an actual argument to a kernel subroutine As an actual argument to another host subprogram or runtime API call As a dummy argument in a host subprogramA device array may have the allocatable attribute, or may have adjustable extent.3.2.2.Managed dataA variable or array with the managed attribute is managed by the unified memorysystem and migrates between host main memory and device global memory. Themanaged attribute can be specified with the attributes statement, or as an attributeon the type declaration statement. Managed arrays can be automatic or allocatable. Thefollowing example declares two arrays, a and b, to be managed arrays of size 100, andallocates a third array, c with size 200.real :: a(100)attributes(managed) :: areal, managed :: b(100)real, allocatable, managed :: c(:). . .allocate(c(200))These rules apply to managed data on the host:ReferenceCUDA Fortran Programming Guide and Reference Version2017|25 Managed variables and arrays may appear in host subprograms and modules, butmay not be in a Common block or an Equivalence statement. Managed variables and arrays declared in a host subprogram cannot have the Saveattribute. Derived types may have the managed attribute. Members of a derived type may have the managed attribute. Managed derived types may also contain allocatable device arrays. Managed variables and arrays may be passed as actual arguments to other hostsubprograms; if the subprogram interface is overloaded, the generic matchingpriority is match another managed dummy argument first, match a dummy with thedevice attribute next, and match a dummy with no (or host) attribute last. Passing a non-managed actual argument to a managed dummy argument willresult in either a compilation error if the interface is explicit, or unexpected behaviorotherwise. Managed variables and arrays may be passed as actual arguments to globalsubroutines just as device variables and arrays are. By default, managed data is allocated with global scope, i.e. the flag passed tocudaMallocManaged is cudaMemAttachGlobal. The scope of a managed variable can be changed with a calltocudaStreamAttachMemAsync. Individual managed variables can be associated with a given stream bycallingcudaSetStreamDefault. All subsequently allocated managed variables can also be associated with a givenstream by callingcudaSetStreamDefault. Accessing managed data on the host while a running kernel is accessing manageddata within the same scope on the device will result in a segmentation fault.These rules apply to managed data on the device: The managed attribute may be used on dummy arguments. Managed data is treated as if it were device data. There is no support for allocating or deallocating managed data on the device.Even if your application only uses a single GPU, if you are running on systemswhich have multiple GPUs that are not peer-to-peer enabled, managed memorywill be allocated as zero-copy memory and performance will suffer accordingly.A workaround is to set the environment variable CUDA_VISIBLE_DEVICESso only one GPU is seen, or to force allocation on the GPU by settingCUDA_MANAGED_FORCE_DEVICE_ALLOC. The CUDA C Programming Guide hasmore details on this in Appendix J, section J.22.214.171.124.2.3.Pinned arraysAn allocatable array with the pinned attribute will be allocated in special page-locked host memory, when such memory is available. The advantage of using pinnedmemory is that transfers between the device and pinned memory are faster and canbe asynchronous. An array with the pinned attribute may be declared in a module orin a host subprogram. The pinned attribute can be specified with the attributesReferenceCUDA Fortran Programming Guide and Reference Version2017|26statement, or as an attribute on the type declaration statement. The following exampledeclares two arrays, p and q, to be pinned allocatable arrays.real :: p(:)allocatable :: pattributes(pinned) :: preal, allocatable, pinned :: q(:)Pinned arrays may be passed as arguments to host subprograms regardless of whetherthe interface is explicit, or whether the dummy argument has the pinned and allocatableattributes. Where the array is deallocated, the declaration for the array must still havethe pinned attribute, or the deallocation may fail.3.2.4.Constant dataA variable or array with the constant attribute is defined to reside in the deviceconstant memory space. The constant attribute can be specified with the attributesstatement, or as an attribute on the type declaration statement. The following exampledeclares two arrays, c and d, to be constant arrays of size 100.real :: c(100)attributes(constant) :: creal, constant :: d(100)These rules apply to constant data: Constant variables and arrays can appear in modules, but may not be in a Commonblock or an Equivalence statement. Constant variables appearing in modules maybe accessed via the use statement in both host and device subprograms. Constant data may not have the Pointer, Target, or Allocatable attributes. Members of a derived type may not have the constant attribute. Arrays with the constant attribute must have fixed size. Constant variables and arrays may be passed as actual arguments to host and devicesubprograms, as long as the subprogram interface is explicit, and the matchingdummy argument also has the constant attribute. Constant variables cannotbe passed as actual arguments between a host subprogram and a device globalsubprogram. Within device subprograms, variables and arrays with the constant attribute maynot be assigned or modified. Within host subprograms, variables and arrays with the constant attribute may beread and written.In host subprograms, data with the constant attribute may only be used in thefollowing manner: As a named entity within a USE statement. As the source or destination in a data transfer assignment statement As an actual argument to another host subprogram As a dummy argument in a host subprogramReferenceCUDA Fortran Programming Guide and Reference Version2017|273.2.5.Shared dataA variable or array with the shared attribute is defined to reside in the shared memoryspace of a thread block. A shared variable or array may only be declared and usedinside a device subprogram. The shared attribute can be specified with the attributesstatement, or as an attribute on the type declaration statement. The following exampledeclares two arrays, s and t, to be shared arrays of size 100.real :: c(100)attributes(shared) :: creal, shared :: d(100)These rules apply to shared data: Shared data may not have the Pointer, Target, or Allocatable attributes. Shared variables may not be in a Common block or Equivalence statement. Members of a derived type may not have the shared attribute. Shared variables and arrays may be passed as actual arguments to from a devicesubprogram to another device subprogram, as long as the interface is explicit andthe matching dummy argument has the shared attribute.Shared arrays that are not dummy arguments may be declared as assumed-size arrays;that is, the last dimension of a shared array may have an asterisk as its upper bound:real, shared :: x(*)Such an array has special significance. Its size is determined at run time by the callto the kernel. When the kernel is called, the value of the bytes argument in theexecution configuration is used to specify the number of bytes of shared memory thatis dynamically allocated for each thread block. This memory is used for the assumed-size shared memory arrays in that thread block; if there is more than one assumed-sizeshared memory array, they are all implicitly equivalenced, starting at the same sharedmemory address. Programmers must take this into account when coding.Shared arrays may be declared as Fortran automatic arrays. For automatic arrays, thebounds are declared as an expression containing constants, parameters, blockdimvariables, and integer arguments passed in by value. The allocation of automatic arraysalso comes from the dynamic area specified via the chevron launch configuration. Ifmore than one automatic array is declared, the compiler and runtime manage the offsetsinto the dynamic area. Programmers must provide a sufficient number of bytes inthe chevron launch configuration shared memory value to cover all automatic arraysdeclared in the global subroutine.attributes(global) subroutine sub(A, n,integer, value :: n, nbreal, shared :: s(nb*blockdim%x,nb)If a shared array is not a dummy argument and not assumed-size or automatic, it mustbe fixed size. In this case, the allocation for the shared array does not come from thedynamically allocated shared memory area specified in the launch configuration, butrather it is declared statically within the function. If the global routine uses only fixedsize shared arrays, or none at all, no shared memory amount needs to be specified at thelaunch.ReferenceCUDA Fortran Programming Guide and Reference Version2017|283.2.6.Texture dataRead-only real and integer device data can be accessed in device subprograms throughthe texture memory by assigning an F90 pointer variable to the underlying device array.To use texture memory in this manner, follow these steps: 1. Add a declaration to a module declaration section that is used in both the host anddevice code: real, texture, pointer :: t(:) 2. In your host code, add the target attribute to the device data that you wish to accessvia texture memory:Change: real, device :: a(n)To: real, target, device :: a(n)The target attribute is standard F90/F2003 syntax to denote an array or other datastructure that may be "pointed to" by another entity. 3. Tie the texture declaration to the device array by using the F90 pointer assignmentoperator in your host code. A simple expression like the following one performs allthe underlying CUDA texture binding operations. t => aThe CUDA Fortran device code that can refer to t through use or host associationcan now access the elements of t without any change in syntax.In the following example, accesses of t, targeting a, go through the texture cache. ! Vector add, s through device memory, t is through texture memory i = threadIdx%x + (blockIdx%x-1)*blockDim%x s(i) = s(i) + t(i)3.2.7.Value dummy argumentsIn device subprograms, following the rules of Fortran, dummy arguments are passedby default by reference. This means the actual argument must be stored in deviceglobal memory, and the address of the argument is passed to the subprogram. Scalararguments can be passed by value, as is done in C, by adding the value attribute to thevariable declaration.attributes(global) subroutine madd( a, b, n ) real, dimension(n,n) :: a, b integer, value :: nIn this case, the value of n can be passed from the host without needing to reside indevice memory. The variable arrays corresponding to the dummy arguments a and bmust be set up before the call to reside on the device.ReferenceCUDA Fortran Programming Guide and Reference Version2017|293.3.Allocating Device Memory, Pinned Memory,and Managed MemoryThis section describes extensions to the Allocate statement, specifically for dynamicallyallocating device arrays, host pinned arrays, managed arrays, and other supportedmethods for allocating memory specific to CUDA Fortran.3.3.1.Allocating Device MemoryDevice arrays can have the allocatable attribute. These arrays are dynamically allocatedin host subprograms using the Allocate statement, and dynamically deallocated usingthe Deallocate statement. If a device array declared in a host subprogram does not havethe Save attribute, it will be automatically deallocated when the subprogram returns.real, allocatable, device :: b(:)allocate(b(5024),stat=istat)...if(allocated(b)) deallocate(b)Scalar variables can be allocated on the device using the Fortran 2003 allocatable scalarfeature. To use these, declare and initialize the scalar on the host as:integer, allocatable, device :: ndevallocate(ndev)ndev = 100The language also supports the ability to create the equivalent of automatic and localdevice arrays without using the allocate statement. These arrays will also have a lifetimeof the subprogram as is usual with the Fortran language:subroutine vfunc(a,c,n) real, device :: adev(n) real, device :: atmp(4) ...end subroutine vfunc ! adev and atmp are deallocated3.3.2.Allocating Device Memory Using Runtime RoutinesFor programmers comfortable with the CUDA C programming environment, Fortraninterfaces to the CUDA memory management runtime routines are provided. Thesefunctions return memory which will bypass certain Fortran allocatable properties suchas automatic deallocation, and thus the arrays are treated more like C malloced areas.Mixing standard Fortran allocate/deallocate with the runtime Malloc/Free for a givenarray is not supported.The cudaMalloc function can be used to allocate single-dimensional arrays of thesupported intrinsic data-types, and cudaFree can be used to free it:real, allocatable, device :: v(:)istat = cudaMalloc(v, 100)...istat = cudaFree(v)For a complete list of the memory management runtime routines, refer to MemoryManagement.ReferenceCUDA Fortran Programming Guide and Reference Version2017|303.3.3.Allocate Pinned MemoryAllocatable arrays with the pinned attribute are dynamically allocated using theAllocate statement. The compiler will generate code to allocate the array in host page-locked memory, if available. If no such memory space is available, or if it is exhausted,the compiler allocates the array in normal paged host memory. Otherwise, pinnedallocatable arrays work and act like any other allocatable array on the host.real, allocatable, pinned :: p(:)allocate(p(5000),stat=istat)...if(allocated(p)) deallocate(p)To determine whether or not the allocation from page-locked memory was successful,an additional PINNED keyword is added to the allocate statement. It returns a logicalsuccess value.logical plogallocate(p(5000), stat=istat, pinned=plog)if (.not. plog) then. . .3.3.4.Allocating Managed MemoryManaged arrays may or may not have the allocatable attribute. These arrays are alldynamically allocated just as device arrays are.real, allocatable, managed :: b(:)allocate(b(5024),stat=istat)...if(allocated(b)) deallocate(b)CUDA Fortran supports the ability to create the equivalent of automatic and localmanaged arrays without using the allocate statement. These arrays will also have alifetime of the subprogram as is usual with the Fortran language:subroutine vfunc(a,c,n) real, managed :: aman(n) real, managed :: atmp(4) ...end subroutine vfunc ! aman and atmp are deallocated3.3.5.Allocating Managed Memory Using RuntimeRoutinesThe cudaMallocManaged function can be used to allocate single-dimensional managedarrays of the supported intrinsic data-types, and cudaFree can be used to free it:use cudaforreal, allocatable, managed :: v(:)istat = cudaMallocManaged(v, 100, cudaMemAttachHost)...istat = cudaFree(v)For a complete list of the memory management runtime routines, refer to MemoryManagement.ReferenceCUDA Fortran Programming Guide and Reference Version2017|313.4.Data transfer between host and devicememoryThis section provides methods to transfer data between the host and device memory.3.4.1.Data Transfer Using Assignment StatementsYou can copy variables and arrays from the host memory to the device memory byusing simple assignment statements in host subprograms. By default, using assignmentstatements to read or write device, managed, or constant data implicitly uses CUDAstream zero. This means such data copies are synchronous, and the data copy waitsuntil all previous kernels and data copies complete. Alternatively, you can use thecudaforSetDefaultStream call to associate one or more device and managedvariables to a particular stream. After this call has occurred, assignment statements onthose variables will run asynchronously on the specified stream.Specific information on assignment statements: An assignment statement where the left hand side is a device variable or devicearray or array section, and the right hand side is a host variable or host array orarray section, copies data from the host memory to the device global memory. An assignment statement where the left hand side is a host variable or host array orarray section, and the right hand side is a device variable or device array or arraysection, copies data from the device global memory to the host memory. An assignment statement with a device variable or device array or array section onboth sides of the assignment statement copies data between two device variables orarrays.Similarly, you can use simple assignment statements to copy or assign variables orarrays with the constant attribute.Specific information on assignment statements and managed data: An assignment statement where the left hand side is a managed variable ormanaged array, and the right hand side is a conforming scalar constant, hostvariable, host array or array section, copies data from the host memory to the deviceglobal memory using cudaMemcpy, memset, or a similar operation. An assignment statement where the left hand side is a managed array section andthe right hand side is any host variable copies data using generated host code. An assignment statement where the left hand side is a managed variable, managedarray or array section, and the right hand side is a device variable or device array orarray section, copies data from the device global memory to the host memory usingcudaMemcpy or a similar operation. An assignment statement where the right hand side is a managed variable ormanaged array, and the left hand side is a host variable, host array or array section,copies data from the device global memory to the host memory using cudaMemcpyor a similar operation.ReferenceCUDA Fortran Programming Guide and Reference Version2017|32 An assignment statement where the right hand side is a managed array section andthe left hand side is any host or managed variable copies data using generated hostcode. An assignment statement where the right hand side is a managed variable, managedarray or array section, and the left hand side is a device variable or device array orarray section, copies data using cudaMemcpy and accesses the data from the device.More information on Memcpy and Memset behavior with managed memory can befound in Appendix J. of the CUDA_C_Programming_Guide.3.4.2.Implicit Data Transfer in ExpressionsSome limited data transfer can be enclosed within expressions. In general, the ruleof thumb is all arithmetic or operations must occur on the host, which normally onlyallows one device array to appear on the right-hand-side of an expression. Temporaryarrays are generated to accommodate the host copies of device data as needed. Forinstance, if a, b, and c are conforming host arrays, and adev, bdev, and cdev areconforming device arrays, the following expressions are legal:a = adevadev = ab = a + adevc = x * adev + bThe following expressions are not legal as they either promote a false impression ofwhere the actual computation occurs, or would be more efficient written in another way,or both:c = adev + bdevadev = adev + ab = sqrt(adev)Elemental transfers are supported by the language but perform poorly. Array slices arealso supported, and their performance is dependent on the size of the slice, the amountof contiguous data in the slices, and the implementation.3.4.3.Data Transfer Using Runtime RoutinesFor programmers comfortable with the CUDA C programming environment, Fortraninterfaces to the CUDA memory management runtime routines are provided. Thesefunctions can transfer data either from the host to device, device to host, or from onedevice array to another.The cudaMemcpy function can be used to copy data between the host and the GPU:real, device :: wrk(1024)real cur(512)istat = cudaMemcpy(wrk, cur, 512)For those familiar with the CUDA C routines, the kind parameter to the Memcpyroutines is optional in Fortran because the attributes of the arrays are explicitly declared.Counts expressed in arguments to the Fortran runtime routines are expressed in terms ofdata type elements, not bytes.ReferenceCUDA Fortran Programming Guide and Reference Version2017|33For a complete list of memory management runtime routines, refer to MemoryManagement.3.5.Invoking a kernel subroutineA call to a kernel subroutine must give the execution configuration for the call. Theexecution configuration gives the size and shape of the grid and thread blocks thatexecute the function as well as the amount of shared memory to use for assumed-sizeshared memory arrays and the associated stream.The execution configuration is specified after the subroutine name in the call statement;it has the form:> grid is an integer, or of type(dim3). If it is type(dim3), the value of grid%zmust be one. The product grid%x*grid%y gives the number of thread blocks tolaunch. If grid is an integer, it is converted to dim3(grid,1,1). bl block is an integer, or of type(dim3). If it is type(dim3), the number of threadsper thread block is block%x*block%y*block%z, which must be less thanthe maximum supported by the device. If block is an integer, it is converted todim3(block,1,1). bytes is optional; if present, it must be a scalar integer, and specifies the numberof bytes of shared memory to be allocated for each thread block to use for assumed-size shared memory arrays. For more information, refer to Shared Data. If notspecified, the value zero is used. stream is optional; if present, it must be an integer, and have a value of zero, or avalue returned by a call to cudaStreamCreate. See Section 4.5 on page 41. It specifiesthe stream to which this call is enqueued. Beginning with PGI 15.4, the streamconstant value cudaStreamPerThread may be specified. This will use a uniquestream for each CPU thread. Also, starting in PGI 15.4, the default stream value,normally zero, can be changed by a call to cudaforSetDefaultStream().For instance, a kernel subroutineattributes(global) subroutine sub( a )can be called like:call sub > ( A )The function call fails if the grid or block arguments are greater than the maximumsizes allowed, or if bytes is greater than the shared memory available. Shared memorymay also be consumed by fixed-sized shared memory declarations in the kernel andfor other dedicated uses, such as function arguments and execution configurationarguments.3.6.Device codeReferenceCUDA Fortran Programming Guide and Reference Version2017|343.6.1.Datatypes AllowedVariables and arrays with the device, constant, or shared attributes, or declared in devicesubprograms, are limited to the types described in this section. They may have any ofthe intrinsic datatypes in the following table.Table2 Device Code Intrinsic DatatypesType Type Kindinteger 1,2,4(default),8logical 1,2,4(default),8real 4(default),8double precision equivalent to real(kind=8)complex 4(default),8character(len=1) 1 (default)Additionally, they may be of derived type, where the members of the derived type haveone of the allowed intrinsic datatypes, or another allowed derived type.The system module cudafor includes definitions of the derived type dim3, defined astype(dim3) integer(kind=4) :: x,y,zend type3.6.2.Built-in variablesSeveral CUDA Fortran read-only predefined variables are available in device code. Theyare declared as follows:type(dim3) :: threadidx, blockdim, blockidx, griddiminteger(4), parameter :: warpsize = 32 The variable threadidx contains the thread index within its thread block; forone- or two-dimensional thread blocks, the threadidx%y and/or threadidx%zcomponents have the value one. The variable blockdim contains the dimensions of the thread block; blockdim hasthe same value for all threads in the same grid; for one- or two-dimensional threadblocks, the blockdim%y and/or blockdim%z components have the value one. The variable blockidx contains the block index within the grid; as withthreadidx, for one-dimensional grids, blockidx%y has the value one. The valueof blockidx%z is always one. The value of blockidx is the same for all threads inthe same thread block. The variable griddim contains the dimensions of the grid; the value of griddim%z is always one. The value of griddim is the same for all threads in the same grid;the value of griddim%z is always one; the value of griddim%y is one for one-dimensional grids. The variables threadidx, blockdim, blockidx, and griddim are available onlyin device subprograms.ReferenceCUDA Fortran Programming Guide and Reference Version2017|35 The constant warpsize contains the number of threads in a warp. It is currentlydefined to be 220.127.116.11.Fortran IntrinsicsThis section lists the Fortran intrinsic functions allowed in device subprograms.Table3 Fortran Numeric and Logical IntrinsicsName Argument Datatypes Name Argument Datatypesabs integer, real, complex int integer, real, complexaimag complex logical logicalaint real max integer, realanint real min integer, realceiling real mod integer, realcmplx real or (real,real) modulo integer, realconjg complex nint realdim integer, real real integer, real, complexfloor real sign integer, realTable4 Fortran Mathematical IntrinsicsName Argument Datatypes Name Argument Datatypesacos real cosh realacosh real erf realasin real erfc realasinh real exp real, complexatan real gamma realatanh real hypot (real,real)atan2 (real,real) log real, complexbessel_j0 real log10 realbessel_j1 real log_gamma realbessel_jn (int,real) sin real, complexbessel_y0 real sinh realbessel_y1 real sqrt real, complexbessel_yn (int,real) tan realcos real, complex tanh realReferenceCUDA Fortran Programming Guide and Reference Version2017|36Table5 Fortran Numeric Inquiry IntrinsicsName Argument Datatypes Name Argument Datatypesbit_size integer precision real, complexdigits integer, real radix integer, realepsilon real range integer, real, complexhuge integer, real selected_int_kind integermaxexponent real selected_real_kind(integer,integer)minexponent real tiny realTable6 Fortran Bit Manipulation IntrinsicsName Argument Datatypes Name Argument Datatypesbtest integer ishft integeriand integer ishftc integeribclr integer leadz integeribits integer mvbits integeribset integer not integerieor integer popcnt integerior integer poppar integerTable7 Fortran Reduction IntrinsicsName Argument Datatypes Name Argument Datatypesall logical minloc integer, realany logical minval integer, realcount logical product integer, real, complexmaxloc integer, real sum integer, real, complexmaxval integer, realTable8 Fortran Random Number IntrinsicsName Argument Datatypesrandom_number realrandom_seed integer3.6.4.New Intrinsic FunctionsThis section describes the new intrinsic functions and subroutines supported in devicesubprograms.ReferenceCUDA Fortran Programming Guide and Reference Version2017|37Synchronization FunctionsThe synchronization functions control the synchronization of various threads duringexecution of thread blocks.syncthreadssyncthreads_countsyncthreads_andsyncthread_orthreadfencethreadfence_blockthreadfence_systemFor detailed information on these functions, refer to Thread Management.SYNCTHREADSThe syncthreads intrinsic subroutine acts as a barrier synchronization for all threadsin a single thread block; it has no arguments:void syncthreads()Sometimes threads within a block access the same addresses in shared or globalmemory, thus creating potential read-after-write, write-after-read, or write-after-write hazards for some of these memory accesses. To avoid these potential issues, usesyncthreads()to specify synchronization points in the kernel. This intrinsic actsas a barrier at which all threads in the block must wait before any thread is allowedto proceed. Threads within a block cooperate and share data by synchronizing theirexecution to coordinate memory accesses.Each thread in a thread block pauses at the syncthreads call until all threads havereached that call. If any thread in a thread block issues a call to syncthreads, allthreads must also reach and execute the same call statement, or the kernel fails tocomplete correctly.SYNCTHREADS_ANDinteger syncthreads_and(int_value)syncthreads_and. like syncthreads, acts as a barrier at which all threadsin the block must wait before any thread is allowed to proceed. In addition,syncthreads_and evaluates the integer argument int_value for all threads of the blockand returns non-zero if and only if int_value evaluates to non-zero for all of them.SYNCTHREADS_COUNTinteger syncthreads_count(int_value)syncthreads_count, like syncthreads, acts as a barrier at which all threadsin the block must wait before any thread is allowed to proceed. In addition,syncthreads_count evaluates the integer argument int_value for all threads of theblock and returns the number of threads for which int_value evaluates to non-zero.ReferenceCUDA Fortran Programming Guide and Reference Version2017|38SYNCTHREADS_ORinteger syncthreads_or(int_value)syncthreads_or. like syncthreads, acts as a barrier at which all threads in the blockmust wait before any thread is allowed to proceed. In addition, syncthreads_orevaluates the integer argument int_value for all threads of the block and returns non-zeroif and only if int_value evaluates to non-zero for any of them.Memory FencesIn general, when a thread issues a series of writes to memory in a particular order, otherthreads may see the effects of these memory writes in a different order. You can usethreadfence(), threadfence_block(), and threadfence_system() to create amemory fence to enforce ordering.For example, suppose you use a kernel to compute the sum of an array of N numbersin one call. Each block first sums a subset of the array and stores the result in globalmemory. When all blocks are done, the last block done reads each of these partial sumsfrom global memory and sums them to obtain the final result. To determine which blockis finished last, each block atomically increments a counter to signal that it is done withcomputing and storing its partial sum. If no fence is placed between storing the partialsum and incrementing the counter, the counter might increment before the partial sum isstored.THREADFENCEvoid threadfence()threadfence acts as a memory fence, creating a wait. Typically, when a thread issuesa series of writes to memory in a particular order, other threads may see the effects ofthese memory writes in a different order. threadfence() is one method to enforce aspecific order. All global and shared memory accesses made by the calling thread priorto threadfence() are visible to: All threads in the thread block for shared memory accesses All threads in the device for global memory accessesTHREADFENCE_BLOCKvoid threadfence_block()threadfence_block acts as a memory fence, creating a wait until all global andshared memory accesses made by the calling thread prior to threadfence_block()are visible to all threads in the thread block for all accesses.THREADFENCE_SYSTEMvoid threadfence_system()ReferenceCUDA Fortran Programming Guide and Reference Version2017|39threadfence_system acts as a memory fence, creating a wait until all global andshared memory accesses made by the calling thread prior to threadfence_system()are visible to: All threads in the thread block for shared memory accesses All threads in the device for global memory accesses Host threads for page-locked host memory accessesthreadfence_system() is only supported by devices of compute capability 2.0 orhigher.3.6.5.Warp-Vote OperationsWarp-vote operations are only supported by devices with compute capability 1.2 andhigher. Each of these functions has a single argument.ALLTHREADSThe allthreads function is a warp-vote operation with a single scalar logicalargument:if( allthreads(a(i)ReferenceCUDA Fortran Programming Guide and Reference Version2017|403.6.6.Atomic FunctionsThe atomic functions read and write the value of their first operand, which must bea variable or array element in shared memory (with the shared attribute) or in deviceglobal memory (with the device attribute). Atomic functions are only supported bydevices with compute capability 1.1 and higher. Compute capability 1.2 or higher isrequired if the first argument has the shared attribute. Certain real(4) and real(8) atomicfunctions may require compute capability 2.0 and higher.The atomic functions return correct values even if multiple threads in the sameor different thread blocks try to read and update the same location without anysynchronization.Arithmetic and Bitwise Atomic FunctionsThese atomic functions read and return the value of the first argument. They alsocombine that value with the value of the second argument, depending on the function,and store the combined value back to the first argument location. For atomicadd,atomicsub, atomicmax, atomicmin, and atomicexch, the data types may be integer(4),integer(8), real(4), or real(8). For atomicand, atomicor, and atomicxor, only integer(4)arguments are supported.The return value for each of these functions is the first argument, mem.These functions are:Table9 Arithmetic and Bitwise Atomic FunctionsFunction Additional Atomic Updateatomicadd( mem, value ) mem = mem + valueatomicsub( mem, value ) mem = mem valueatomicmax( mem, value ) mem = max(mem,value)atomicmin( mem, value ) mem = min(mem,value)atomicand( mem, value ) mem = iand(mem,value)atomicor( mem, value ) mem = ior(mem,value)atomicxor( mem, value ) mem = ieor(mem,value)atomicexch( mem, value ) mem = valueReferenceCUDA Fortran Programming Guide and Reference Version2017|41Counting Atomic FunctionsThese atomic functions read and return the value of the first argument. They alsocompare the first argument with the second argument, and stores a new value back tothe first argument location, depending on the result of the comparison. These functionsare intended to implement circular counters, counting up to or down from a maximumvalue specified in the second argument. Both arguments must be of type integer(kind=4).The return value for each of these functions is the first argument, mem.These functions are:Table10 Counting Atomic FunctionsFunction Additional Atomic Updateatomicinc( mem, imax ) if (memReferenceCUDA Fortran Programming Guide and Reference Version2017|423.6.7.Fortran I/OThe underlying CUDA printf implementation limits the number of print statements in akernel launch to 4096. Users should take this limit into account when making use of thisfeature.220.127.116.11.PRINT ExampleBy adding the compiler option -Mcuda=charstring, some limited support for characterstrings, character substrings, character variables, and string assignment is also nowavailable in CUDA Fortran device code. Here is a short example:attributes(global) subroutine printtest()character*12 ci = threadIdx%xif (i/2*2.eq.i) then c = "Even Thread:"else c = " Odd Thread:"endifprint *,c,c(6:11),iend subroutine3.6.8.Shuffle FunctionsPGI 14.1 enables CUDA Fortran device code to access compute capability 3.x shufflefunctions. These functions enable access to variables between threads within a warp,referred to as lanes. In CUDA Fortran, lanes use Fortran's 1-based numbering scheme.__shfl()__shfl() returns the value of var held by the thread whose ID is given by srcLane.If the srcLane is outside the range of 1:width, then the thread's own value of varis returned. The width argument is optional in all shuffle functions and has a defaultvalue of 32, the current warp size. integer(4) function __shfl(var, srcLane, width) integer(4) var, srcLane integer(4), optional :: width real(4) function __shfl(var, srcLane, width) real(4) :: var integer(4) :: srcLane integer(4), optional :: width real(8) function __shfl(var, srcLane, width) real(8) :: var integer(4) :: srcLane integer(4), optional :: width__shfl_up()__shfl_up() calculates a source lane ID by subtracting delta from the caller's threadID. The value of var held by the resulting thread ID is returned; in effect, var is shiftedup the warp by delta lanes.ReferenceCUDA Fortran Programming Guide and Reference Version2017|43The source lane index will not wrap around the value of width, so the lower deltalanes are unchanged. integer(4) function __shfl_up(var, delta, width) integer(4) var, delta integer(4), optional :: width real(4) function __shfl_up(var, delta, width) real(4) :: var integer(4) :: delta integer(4), optional :: width real(8) function __shfl_up(var, delta, width) real(8) :: var integer(4) :: delta integer(4), optional :: width__shfl_down()__shfl_down() calculates a source lane ID by adding delta to the caller's threadID. The value of var held by the resulting thread ID is returned: this has the effect ofshifting var down the warp by delta lanes. The ID number of the source lane will notwrap around the value of width, so the upper delta lanes remain unchanged. integer(4) function __shfl_down(var, delta, width) integer(4) var, delta integer(4), optional :: width real(4) function __shfl_down(var, delta, width) real(4) :: var integer(4) :: delta integer(4), optional :: width real(8) function __shfl_down(var, delta, width) real(8) :: var integer(4) :: delta integer(4), optional :: width__shfl_xor()__shfl_xor() uses ID-1 to calculate the source lane ID by performing a bitwise XORof the caller's lane ID with the laneMask. The value of var held by the resulting laneID is returned. If the resulting lane ID falls outside the range permitted by width, thethread's own value of var is returned. This mode implements a butterfly addressingpattern such as is used in tree reduction and broadcast. integer(4) function __shfl_xor(var, laneMask, width) integer(4) var, laneMask integer(4), optional :: width real(4) function __shfl_xor(var, laneMask, width) real(4) :: var integer(4) :: laneMask integer(4), optional :: width real(8) function __shfl_xor(var, laneMask, width) real(8) :: var integer(4) :: laneMask integer(4), optional :: widthReferenceCUDA Fortran Programming Guide and Reference Version2017|44Here is an example using __shfl_xor() to compute the sum of each thread's variablecontribution within a warp: j = . . . k = __shfl_xor(j,1); j = j + k k = __shfl_xor(j,2); j = j + k k = __shfl_xor(j,4); j = j + k k = __shfl_xor(j,8); j = j + k k = __shfl_xor(j,16); j = j + k3.6.9.RestrictionsThis section lists restrictions on statements and features that can appear in devicesubprograms. Recursive subroutines and functions are not allowed. PAUSE statements are not allowed. Most Input/Output statements are not allowed at all: READ, FORMAT, NAMELIST,OPEN, CLOSE, BACKSPACE, REWIND, ENDFILE, INQUIRE. List-directed PRINT and WRITE statements to the default unit may be used whencompiling for compute capability 2.0 and higher; all other uses of PRINT andWRITE are disallowed. Alternate return specifications are not allowed. ENTRY statements are not allowed. Floating point exception handling is not supported. Fortran intrinsic functions not listed in Section 3.6.3 are not supported. Cray pointers are not supported.3.7.Host codeHost subprograms may use intrinsic functions, such as the new sizeof intrinsicfunction.3.7.1.SIZEOF IntrinsicA call to sizeof(A), where A is a variable or expression, returns the number of bytesrequired to hold the value of A.integer(kind=4) :: i, jj = sizeof(i) ! this assigns the value 4 to j3.7.2.Overloaded Reduction IntrinsicsBeginning in PGI 15.1, the sum, maxval, and minval host intrinsics are overloaded toaccept device or managed arrays when the cudafor module is used. The tuned kernelswritten to implement these operations use the shuffle functions, so compute capability3.0 or higher is required to use these reductions. If the mask optional argument is used,the mask argument must be either a device logical array, or an expression containingmanaged operands and constants, i.e. the mask must be computable on the host butreadable on the device.ReferenceCUDA Fortran Programming Guide and Reference Version2017|45Here is a complete example which performs the sum and maxval reductions on theGPU: program multidimred use cudafor real(8), managed :: a(5,5,5,5,5) real(8), managed :: b(5,5,5,5) real(8) :: c call random_number(a) do idim = 1, 5 b = sum(a, dim=idim) c = max(maxval(b), c) end do print *,"Max along any dimension",c end programArray slices are also supported. This may run less efficiently on the GPU, but is verypowerful nonetheless, and useful for debugging: real(4), managed :: a(n,m) reslt(ix) = sum(a(2:n-1,:)) reslt(ix) = sum(a(:,3:m-2)) reslt(ix) = sum(a(n2:n,m2:m)) reslt(ix) = sum(a(1:n3,1:m3)) reslt(ix) = sum(a(n2:n3,m2:m3))By default, intrinsic reductions that are supported on the device will be executed on thedevice for (large enough) managed arrays. There may be occasions where one wouldlike to perform reductions on managed data on the host. This can be accomplished usingthe rename feature of the "use" statement, for example: program reductionRename use cudafor, gpusum => sum implicit none integer, managed :: m(3000) integer :: istat m = 1 istat = cudaDeviceSynchronize() write(*,*) sum(m) ! executes on host write(*,*) gpusum(m) ! executes on device end programBy default, these reductions will run on a nonzero stream. If a unique per-thread defaultstream was set via a call to cudaforSetDefaultStream, the reduction initializationwill pick that up. Users can further control which stream reductions run on, or forcestream 0, by using the cudaforReductionSetStream() call. The current reductionstream can be queried using the cudaforReductionGetStream() call.SUM() on complex managed or device data is not currently supported. This support willbe added in a future PGI release3.8.Fortran ModulesCUDA Fortran Modules are available to help programmers access features of the CUDAruntime environment, which might otherwise not be accessible from Fortran withoutsignificant effort from the programmer. These modules might be either device modulesor host modules.ReferenceCUDA Fortran Programming Guide and Reference Version2017|463.8.1.Device ModulesPGI provides a device module which allows access and interfaces to many of the CUDAdevice built-in routines.To access this module, do one of the following: Add this line to your Fortran program:use cudadevice Add this line to your C program:#include You can use these routines in CUDA Fortran global and device subprograms, in CUFkernels, and in PGI Accelerator compute regions in Fortran as well as in C. Further, thePGI compilers come with implementations of these routines for host code, though theseimplementations are not specifically optimized for the host.CUDA Built-in Routines lists the CUDA built-in routines that are available:Table12 CUDA Built-in Routines__brev __brevll clock clock64__clz __clzll __cosf __dadd_rd__dadd_rn __dadd_ru __dadd_rz __ddiv_rd__ddiv_rn __ddiv_ru __ddiv_rz __dmul_rd__dmul_rn __dmul_ru __dmul_rz __double2float_rd__double2float_rn __double2float_ru __double2float_rz __double2hiint__double2int_rd __double2int_rn __double2int_ru __double2int_rz__double2loint __double2ll_rd __double2ll_rn __double2ll_ru__double2ll_rz __double2uint_rd __double2uint_rn __double2uint_ru__double2uint_rz __double2ull_rd __double2ull_rn __double2ull_ru__double2ull_rz __double_as_longlong __drcp_rd __drcp_rn__drcp_ru __drcp_rz __dsqrt_rd __dsqrt_rn__dsqrt_ru __dsqrt_rz __exp10f __expf__fadd_rd __fadd_rn __fadd_ru __fadd_rz__fdiv_rd __fdiv_rn __fdiv_ru __fdiv_rzfdivide fdividef __fdividef __ffs__ffsll __float2half_rn __float2int_rd __float2int_rn__float2int_ru __float2int_rz __float2ll_rd __float2ll_rn__float2ll_ru __float2ll_rz __float_as_int __fma_rd__fma_rn __fma_ru __fma_rz __fmaf_rd__fmaf_rn __fmaf_ru __fmaf_rz __fmul_rdReferenceCUDA Fortran Programming Guide and Reference Version2017|47__fmul_rn __fmul_ru __fmul_rz __frcp_rd__frcp_rn __frcp_ru __frcp_rz __fsqrt_rd__fsqrt_rn __fsqrt_ru __fsqrt_rz __half2float__hiloint2double __int2double_rn __int2float_rd __int2float_rn__int2float_ru __int2float_rz __int_as_float __ll2double_rd__ll2double_rn __ll2double_ru __ll2double_rz __ll2float_rd__ll2float_rn __ll2float_ru __ll2float_rz __log10f__log2f __logf __longlong_as_double __mul24__mulhi __popc __popcll __powf__sad __saturatef __sinf __tanf__uint2double_rn __uint2float_rd __uint2float_rn __uint2float_ru__uint2float_rz __ull2double_rd __ull2double_rn __ull2double_ru__ull2double_rz __ull2float_rd __ull2float_rn __ull2float_ru__ull2float_rz __umul24 __umulhi __usadPGI also provides a device module which provides interfaces to standard libm functionswhich are not in the Fortran intrinsic library.To access this module, add this line to your Fortran subprogram:use libmThese interfaces are defined in the libm device module:Table13 CUDA Device libm RoutinesName Argument Datatypes Name Argument Datatypescbrt,cbrtf real(8),real(4) returnsrealllround,llroundf real(8),real(4) returnsintegerceil,ceilf real(8),real(4) returnsreallrint,lrintf real(8),real(4) returnsintegercopysign,copysignf 2*real(8),real(4) returnsreallround,lroundf real(8),real(4) returnsintegerexpm1,expm1f real(8),real(4) returnsreallogb,logbf real(8),real(4) returnsrealexp10,exp10f real(8),real(4) returnsreallog1p,log1pf real(8),real(4) returnsrealexp2,exp2f real(8),real(4) returnsreallog2,log2f real(8),real(4) returnsrealfabs,fabsf real(8),real(4) returnsrealmodf,modff 2*real(8),real(4) returnsrealfloor,floorf real(8),real(4) returnsrealnearbyint,nearbyintfreal(8),real(4) returnsrealReferenceCUDA Fortran Programming Guide and Reference Version2017|48fma,fmaf 3*real(8),real(4) returnsrealnextafter,nextafterf2*real(8),real(4) returnsrealfmax,fmaxf 2*real(8),real(4) returnsrealremainder,remainderf2*real(8),real(4) returnsrealfmin,fminf 2*real(8),real(4) returnsrealremquo,remquof 2*real(8),real(4) integerreturns realfrexp,frexpf real(8),real(4) integerreturns realrint,rintf real(8),real(4) returnsrealilogb,ilogbf real(8),real(4) returnsrealscalbn,scalbnf real(8),real(4) integerreturns realldexp,ldexpf real(8),real(4) integerreturns realscalbln,scalblnf real(8),real(4) integerreturns realllrint,llrintf real(8),real(4) returnsintegertrunc,truncf real(8),real(4) returnsrealOn NVIDIA GPUs which support CUDA Compute Capability 3.5 and above, PGIprovides a device module which provides interfaces to the device CUBLAS library.To access this module, add this line to your Fortran subprogram:use cublas_deviceHere is a simple example of using the CUBLAS device library:module mblasdcontains attributes(global) subroutine testblas( a, b, n ) use cublas_device real, device :: a(*), b(*) type(cublasHandle) :: h integer, value :: n j = cublasCreate_v2(h) i = threadIdx%x if (i.eq.1) then j = cublasSswap_v2(h,n,a,1,b,1) end if j = cublasDestroy_v2(h) return end subroutineend module mblasd! Compile with "pgfortran -Mcuda=cc35 cublasdev.cuf -lcublas_device"program cublasdevuse mblasdinteger, parameter :: n = 1000real, managed :: a(n), b(n)a = 1.0; b = 2.0call testblas (a,b,n)istat = cudaDeviceSynchronize()print *,all(a.eq.2*b)end3.8.2.Host ModulesPGI provides a module which defines interfaces to the CUBLAS Library from PGICUDA Fortran. These interfaces are made accessible by placing the following statementin the CUDA Fortran host-code program unit.use cublasReferenceCUDA Fortran Programming Guide and Reference Version2017|49The interfaces are currently in three forms: Overloaded traditional BLAS interfaces which take device arrays as argumentsrather than host arrays, i.e. call saxpy(n, a, x, incx, y, incy)where the arguments x and y have the device attribute. Portable legacy CUBLAS interfaces which interface directly with CUBLAS versions
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