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From: "davidsondfgl at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/102666] New: Wrong loop optimization for
 unsigned types
Date: Sat, 09 Oct 2021 17:03:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102666

            Bug ID: 102666
           Summary: Wrong loop optimization for unsigned types
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: davidsondfgl at gmail dot com
  Target Milestone: ---

The code below produces incorrect result when any optimizations (-O1/-O2/-O=
3)
are enabled:

$ cat unsigned-bug.c
int main(void) {
    for (unsigned a =3D 0, b =3D 0; a < 6; a +=3D 1, b +=3D 2)
        if (b < a)
            return 1;
    return 0;
}

$ gcc -Wall -Wextra unsigned-bug.c; ./a.out; echo $?
0
$ gcc -Wall -Wextra unsigned-bug.c -O1; ./a.out; echo $?
1
$ gcc -Wall -Wextra unsigned-bug.c -O2; ./a.out; echo $?
1
$ gcc -Wall -Wextra unsigned-bug.c -O3; ./a.out; echo $?
1

This affects all releases as of 9.1, including the master branch (commit
2b30143).

A bisect between v9.1 and v8.3 indicates that this may have been introduced=
 in
r266171, as I can revert this commit (a81e2c6) in all releases up to v11.2 =
and
get the expected behavior.

It is also worthwhile to note that this only occurs for unsigned types.=