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From: "ubizjak at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/109483] New: Unoptimal jump threading with
 assembler flag output
Date: Wed, 12 Apr 2023 08:54:33 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: ubizjak at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109483

            Bug ID: 109483
           Summary: Unoptimal jump threading with assembler flag output
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ubizjak at gmail dot com
  Target Milestone: ---

Following testcase (int3 mnemonic is for marker only):


--cut here--
_Bool foo (int cnt)
{
  if (cnt =3D=3D -1)
    {
      _Bool success;
      asm volatile("int3" : "=3D@ccz" (success));

      if (!success)
        return 0;
    }

  asm volatile ("" ::: "memory");
  return 1;
}
--cut here--

compiles w/ -O2 on x86_64 to:

0000000000000000 <foo>:
   0:   83 ff ff                cmp    $0xffffffff,%edi
   3:   74 0b                   je     10 <foo+0x10>
   5:   b8 01 00 00 00          mov    $0x1,%eax
   a:   c3                      retq=20=20=20
   b:   0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
  10:   cc                      int3=20=20=20
  11:   0f 94 c0                sete   %al
  14:   74 ef                   je     5 <foo+0x5>
  16:   c3                      retq=20=20=20

Please note setting of %al before conditional jump. The instruction could be
moved after the jump, where the register could be cleared using "xor %eax,
%eax", similar to what clang creates:

0000000000000000 <foo>:
   0:   83 ff ff                cmp    $0xffffffff,%edi
   3:   75 06                   jne    b <foo+0xb>
   5:   cc                      int3=20=20=20
   6:   74 03                   je     b <foo+0xb>
   8:   31 c0                   xor    %eax,%eax
   a:   c3                      retq=20=20=20
   b:   b0 01                   mov    $0x1,%al
   d:   c3                      retq=20=20=20

Also note that for ZF=3D1 gcc sets %al to 1, jumps to *5 where the register=
 is
again set to 1. This is not the case in the clang code.=