[Bug c/64918] invalid (?) warning when initializing structure

["manu at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64918

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |manu at gcc dot gnu.org

--- Comment #6 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Øystein Schønning-Johansen from comment #2)
> Really insightful, Joseph. I do understand the warning a bit better now. I
> have not looked into the GCC parsing code, but based on your description and
> my (limited) understanding of the problem, I guess a fix is not trivial? Can
> I suppress this warning in any way?

I haven't looked at the code, but my guess is that at the time of the warning,
it should be possible to check the overriden initializer, if it is a compound
literal, recur on its initializers and check for side-effects again.
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From: "jamrial at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/65871] New: bzhi builtin/intrinsic wrongly assumes bzhi instruction doesn't set the ZF flag
Date: Thu, 23 Apr 2015 23:34:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?ide871

            Bug ID: 65871
           Summary: bzhi builtin/intrinsic wrongly assumes bzhi
                    instruction doesn't set the ZF flag
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jamrial at gmail dot com

unsigned foo(void);
int main(void)
{
    if (__builtin_ia32_bzhi_si(foo(), foo()))
        return 1;
    return 0;
}

Compiled with -mbmi2 -O3

0000000000000000 <main>:
   0:   53                      push   rbx
   1:   e8 00 00 00 00          call   6 <main+0x6>
   6:   89 c3                   mov    ebx,eax
   8:   e8 00 00 00 00          call   d <main+0xd>
   d:   c4 e2 60 f5 c0          bzhi   eax,eax,ebx
  12:   85 c0                   test   eax,eax
  14:   0f 95 c0                setne  al
  17:   0f b6 c0                movzx  eax,al
  1a:   5b                      pop    rbx
  1b:   c3                      ret

It generates a redundant test instruction. According to
http://www.felixcloutier.com/x86/BZHI.html bzhi already sets the ZF flag on its
own.
Same happens when using inline assembly instead of the builtin to generate the
bzhi instruction. In all cases reproducible with GCC 4.9.2 and GCC 5.1.0.
Didn't test the 4.8 branch or trunk.

This aside, it would be nice if gcc could generate a bzhi instruction on its
own if it detects "X & ((1 << Y) - 1)" where Y is not a constant, same as it
does for several other bmi and tbm instructions, instead of needing to use the
builtin (Which is only available when targeting bmi2).
I can open a new bug report for that if needed.