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Subject: [Bug tree-optimization/102634] New: Optimization in dom2 pass makes
 wrong signed integer overflow inference
Date: Wed, 06 Oct 2021 22:21:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102634

            Bug ID: 102634
           Summary: Optimization in dom2 pass makes wrong signed integer
                    overflow inference
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mk.aard.gcc at gmail dot com
  Target Milestone: ---

Created attachment 51563
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D51563&action=3Dedit
Sample code which fails. Integer with 'NaN' handling user type using min
integer value as NaN

Compiled using GCC 10.2.1 on Debian 11 Bullseye, amd64:

g++ -c -O2 -std=3Dc++17 example.cpp

This generates the following assembly for strange_code() (see attached .ii)

00000000000000d0 <strange_code()>:
  d0:   55                      push   %rbp
  d1:   48 83 ec 10             sub    $0x10,%rsp
  d5:   48 c7 44 24 08 00 00    movq   $0x0,0x8(%rsp)
  dc:   00 00=20
  de:   48 8d 6c 24 08          lea    0x8(%rsp),%rbp
  e3:   48 89 ef                mov    %rbp,%rdi
  e6:   e8 00 00 00 00          callq  eb <strange_code()+0x1b>
  eb:   48 89 ef                mov    %rbp,%rdi
**ee:   48 be 00 00 00 00 00    movabs $0x8000000000000000,%rsi
  f5:   00 00 80=20
  f8:   e8 00 00 00 00          callq  fd <strange_code()+0x2d>
  fd:   48 83 c4 10             add    $0x10,%rsp
 101:   b8 01 00 00 00          mov    $0x1,%eax
 106:   5d                      pop    %rbp
 107:   c3                      retq

The use of 0x8000000000000000 as the argument for the second call we believ=
e is
an incorrect optimization.

Although -fwrapv resolves this issue for us, and bug reporting guidelines
indicate that -fwrapv fixes are likely user error, I and several teammates =
have
been unable to find any signed integer overflow issues in the example provi=
ded.
As far as we can tell, each usage of the problematic value avoids any opera=
tion
which could cause overflow.

I apologize that the example isn't smaller, changing virtually anything in =
it
causes the issue to go away.=