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From: "drepper.fsp+rhbz at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109045] New: assume attribute and
 std::optional do not mix
Date: Mon, 06 Mar 2023 19:32:14 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: drepper.fsp+rhbz at gmail dot com
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109045

            Bug ID: 109045
           Summary: assume attribute and std::optional do not mix
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: drepper.fsp+rhbz at gmail dot com
  Target Milestone: ---

The assume attribute is meant to help expressing more complex assumptions w=
hich
involve function calls.  Given that interfaces should use std::optional when
the semantics matches this should mean code like this should be optimized:

#include <optional>

std::optional<long> g(long);

long f(long a)
{
  auto r =3D g(a);
  [[assume(!r || *r > 0)]];
  return r.value_or(0) / 2;
}


The generated code should use the unsigned divide by two method but it does
not.  With today's gcc trunk version:

0000000000000000 <_Z1fl>:
   0:   48 83 ec 18             sub    $0x18,%rsp
   4:   e8 00 00 00 00          call   9 <_Z1fl+0x9>
   9:   48 89 04 24             mov    %rax,(%rsp)
   d:   48 89 54 24 08          mov    %rdx,0x8(%rsp)
  12:   31 c0                   xor    %eax,%eax
  14:   80 7c 24 08 00          cmpb   $0x0,0x8(%rsp)
  19:   74 11                   je     2c <_Z1fl+0x2c>
  1b:   48 8b 14 24             mov    (%rsp),%rdx
  1f:   48 89 d0                mov    %rdx,%rax
  22:   48 c1 e8 3f             shr    $0x3f,%rax
  26:   48 01 d0                add    %rdx,%rax
  29:   48 d1 f8                sar    %rax
  2c:   48 83 c4 18             add    $0x18,%rsp
  30:   c3                      ret=20=20=20=20

The instructions from 1f to 28 including are not needed (and the initial lo=
ad
at 1b adjusted).=