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From: "raanan.lori at getnexar dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/115160] New: Enabling undefined behaviour sanitizer causes
 or'ed bit shift to report wrong result
Date: Mon, 20 May 2024 10:03:24 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 9.4.0
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X-Bugzilla-Who: raanan.lori at getnexar dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115160

            Bug ID: 115160
           Summary: Enabling undefined behaviour sanitizer causes or'ed
                    bit shift to report wrong result
           Product: gcc
           Version: 9.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: raanan.lori at getnexar dot com
  Target Milestone: ---

I've tried to enable gcc undefined behaviour sanitizer recently and one of =
my
tests encountered an issue where bit shifts in certain code snippet will re=
port
a wrong result, i narrowed it down to this snippet.

I used compiler explorer to find that the same behaviour is also in the lat=
est
release of gcc 14.4, tested with gcc 9.4 and 11.4

This was run under Unbuntu LTS 22.04 x86_64

My compile flags: `g++ -std=3Dc++17 -O0  -fsanitize=3Dundefined -Wall
-fno-sanitize-recover main.cpp"

The main.cpp code to replicate this issue:
```
#include <iostream>
#include <cstdint>
#include <vector>

int main() {
    std::vector<uint8_t> data;=20=20=20=20
    data.emplace_back(1);
    data.emplace_back(0);
    data.emplace_back(0);
    data.emplace_back(0);
    auto b =3D data.begin();

    std::cout << (int)( (*b++)  | (*b++ << 8) | (*b++ << 16 ) | ( *b << 24 )
)<< std::endl;
    return 0;
}
```
expected output should be : 1
actual output: 256
note: no error or warning is produced for undefined behaviour=