title: gcc/assembler does not recognize aarch64 gicv3 register(s)
body: <p>On Ubuntu 17.04, with aarch64-linux-gnu-as installed, compile the following assembly code:</p>
  
  <pre><code># code.s
  _start:
     mrs x0,icc_igrpen0_el1
  </code></pre>
  
  <p>with </p>
  
  <p><code>aarch64-linux-gnu-as code.s</code></p>
  
  <p>gives the following error:</p>
  
  <pre><code>as.s: Assembler messages:
  as.s:3: Error: unknown or missing system register name at operand 2 -- `mrs x0,icc_igrpen0_el1'
  </code></pre>
  
  <p>However, icc_igrpen0_el1 is described in documentation of ARMv8 profile,cortext-a53 TRM and GICv3 specification.</p>
  
  <p>And the same error applies to registers starts with ICC_.Why gnu assembler does not recognize those ICC_* registers?</p>