[Bug lto/62249] New: Spurious FORTIFY_SOURCE warning with -flto, poll

["zackw at panix dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62249

            Bug ID: 62249
           Summary: Spurious FORTIFY_SOURCE warning with -flto, poll
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zackw at panix dot com

Created attachment 33388
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33388&action=edit
test case (compile on recent glibc system)

Compile the attached program on a recent-glibc system with -D_FORTIFY_SOURCE=2
-O2 -flto and you get a spurious warning:

$ gcc -flto -O2 -D_FORTIFY_SOURCE=2 test.c
In function â__poll_aliasâ,
    inlined from âmainâ at test.c:17:9:
/usr/include/x86_64-linux-gnu/bits/poll2.h:41:2: warning: call to
â__poll_chk_warnâ declared with attribute warning: poll called with fds buffer
too small file nfds entries
  return __poll_chk (__fds, __nfds, __timeout, __bos (__fds));

$ gcc -fwhole-program -O2 -D_FORTIFY_SOURCE=2 test.c
$

Inspection of bits/poll2.h leads me to believe that the glibc folks expect this
construct ...

__fortify_function int
poll (struct pollfd *__fds, nfds_t __nfds, int __timeout)
{
  if (__bos (__fds) != (__SIZE_TYPE__) -1)
    {
      if (! __builtin_constant_p (__nfds))
        return __poll_chk (__fds, __nfds, __timeout, __bos (__fds));
      else if (__bos (__fds) / sizeof (*__fds) < __nfds)
        return __poll_chk_warn (__fds, __nfds, __timeout, __bos (__fds));
    }

  return __poll_alias (__fds, __nfds, __timeout);
}

... to have the dead arm of the inner 'if' eliminated (one of them must be
dead, because the condition is a call to __builtin_constant_p) before warnings
are issued.