label reference missed

[Ulrich Drepper <drepper at redhat dot com>]

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I have this little piece of code:


int
foo (int a)
{
  asm volatile ("jmp %0"::"m" (*&&fff));
  if (1)
    {
      puts ("if");
    }
  else
    {
    fff:
      puts ("else");
    }
  return 0;
}


It compiles fine but gcc doesn't realize that the label fff is 
referenced.  The result is that the entire 'else' branch is gone.  Note 
that this construct works fine if the condition of the 'if' is changed 
from '1' to 'a'.  But this defeats the purpose (in part).

The possibilities opened by code like that above are quite big.  On x86, 
for instance, we often have sequences like this (after macro expansion) 
in lowlevel code:

  if (({ unsigned char r;
         asm ("cmpxchgl %3, %0; sete %1"
              : "=m" (mem), "=q" (r)
              : "0" (mem), "r" (oldval));
         r; }) != 0)
    ...

which expands to something like

   cmpxchgl ...,...
   sete ...
   testl ..., ...
   jne ...

With a construct like that above this could be reduced to

  cmpxchgl ...,...
  je ...


I imagine that the change necessary to achieve this is minimal.  It 
would only require to recognize label references and mark the basic 
blocks they introduce as alive.

I'll file a bug in gnats if it is agreed on that this is useful and 
feasable.

- -- 
- ---------------.                          ,-.   1325 Chesapeake Terrace
Ulrich Drepper  \    ,-------------------'   \  Sunnyvale, CA 94089 USA
Red Hat          `--' drepper at redhat.com   `------------------------
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