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From: "ansuelsmth at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/109820] New: False-positive in tautological-compare
Date: Thu, 11 May 2023 21:41:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109820

            Bug ID: 109820
           Summary: False-positive in tautological-compare
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ansuelsmth at gmail dot com
  Target Milestone: ---

Created attachment 55058
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D55058&action=3Dedit
preprocessed file

Hi, while having fun with fixing some compilation warning to enforce WERROR=
 on
kernel linux I faced this interesting false-positive warning.

The simple repro code is this

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <stdint.h>

int main() {
        uint32_t test;

        if (((test >> 4) & 0x15) =3D=3D 0x9)
                return 1;

        return 0;
}

Compiled with gcc -o test test.c -Wall -Werror

Result in warning
test.c: In function =E2=80=98main=E2=80=99:
test.c:10:34: error: bitwise comparison always evaluates to false
[-Werror=3Dtautological-compare]
   10 |         if (((test >> 4) & 0x15) =3D=3D 0x9)
      |                                  ^~
cc1: all warnings being treated as errors

Decreasing the 0x9 to something like 0x5 mute the warning.
It's either a false positive or a warning that gets not triggered in some
corner case.

I have attached the .i as asked in the guidelines. Don't know if -v is need=
ed.
This is simple gcc installed from ubuntu devel

I also verified that this is present in gcc 13.1=