title: What is the default for gcc -march option?
body: <p>The gcc info file says in the section on x86-64 specific flags, among
  other things:</p>
  <pre><code>There is no `-march=generic' option because `-march'
  indicates the instruction set the compiler can use, and there
  is no generic instruction set applicable to all processors.
  In contrast, `-mtune' indicates the processor (or, in this
  case, collection of processors) for which the code is
  optimized.
  </code></pre>
  <p>My question then is, what instruction (sub-)set does gcc compile for
  when no -march option is given?  There is a lot of related information
  about -march and -mtune in the webosphere, but none that I could find
  which answers this simple question.  It can't be march=native, or else
  it would be impossible to compile generic distribution kernels and
  binary packages.</p>