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From: "obaines at nvidia dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug lto/114263] New: LTO generating incorrect code with -O2
 -std=c++20 -fno-strict-aliasing flags in GCC13
Date: Thu, 07 Mar 2024 05:19:18 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: lto
X-Bugzilla-Version: 13.2.1
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: obaines at nvidia dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114263

            Bug ID: 114263
           Summary: LTO generating incorrect code with -O2 -std=3Dc++20
                    -fno-strict-aliasing flags in GCC13
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: obaines at nvidia dot com
  Target Milestone: ---

I have a minimized test case where LTO seems to be generating incorrect cod=
e.

### Sources ###
A.cpp
```
#include <vector>
#include <cstdint>

struct A {
    A(uint64_t _x, uint8_t _y) : x(_x), y(_y) {}
    uint64_t x;
    uint8_t y;
};

void doA() {
    std::vector<A> v;
    v.emplace_back(0x1111111111111111ULL, 0x2);
    v.emplace_back(0x3333333333333333ULL, 0x4);
}
```

B.cpp
```
#include <iostream>
#include <vector>
#include <cstdint>

struct B {
    B(uint64_t _x, uint64_t _y) : x(_x), y(_y) {}
    uint64_t x;
    uint64_t y;
};

void doB() {
    std::vector<B> v;
    v.emplace_back(0x5555555555555555ULL, 0x6666666666666666ULL);
    v.emplace_back(0x7777777777777777ULL, 0x8888888888888888ULL);
    std::cout << std::hex << "expected: " << 0x6666666666666666ULL << " act=
ual:
" << v[0].y << std::endl;
}
```

AB.cpp
```
void doA();
void doB();

int main() {
    doA();
    doB();
}
```

### Build Command ###
g++ -flto -g -O2 -std=3Dc++20 -fno-strict-aliasing -Wall -Wextra -c *.cpp
g++ -flto -g -O2 -std=3Dc++20 -fno-strict-aliasing -Wall -Wextra *.o

### Notes ###
The code doesn't generate any warnings during compilation.
The output of the program is usually `expected: 6666666666666666 actual:
6666666666666666`, but with these optimization flags the output is `expecte=
d:
6666666666666666 actual: 66`. The bug won't reproduce if I specify C++ stan=
dard
before C++20, or if I don't disable strict-aliasing optimization.
Changing the size of A::y affects the number of bytes that are correctly co=
pied
in the output. The bug reproduces in gcc 13.2.0 and from the latest
releases/gcc-13 branch (g++ (GCC) 13.2.1 20240306), but does not reproduce =
in
gcc-12.3.0 or with the latest trunk branch (g++ (GCC) 14.0.1 20240306
(experimental)).=20
So, as far as I can tell the bug is exclusive to GCC13.=