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From: "18307130172 at fudan dot edu.cn" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/106131] New: -fstrict-aliasing breaks normal program that
 does not use any pointer or reference
Date: Wed, 29 Jun 2022 02:05:06 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106131

            Bug ID: 106131
           Summary: -fstrict-aliasing breaks normal program that does not
                    use any pointer or reference
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: 18307130172 at fudan dot edu.cn
  Target Milestone: ---
            Target: x86_64-pc-linux-gnu

Created attachment 53220
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D53220&action=3Dedit
preprocessed source file

My program does not use any pointer or reference but is affected by
-fstrict-aliasing. Here is a minimal example to manifest this issue:

#include <vector>
#include <iostream>

using Pair =3D std::pair<int, int>;

std::vector<Pair> f =3D {{0, -11}, {0, -8}, {1, 2}};

int foo(Pair x, Pair y) {
    return std::max(x.second, y.second);
}

int main() {
    for (int J =3D 0; J < 1; J++) {
        f[J] =3D f[0];
        if(J =3D=3D 0)
            f[J] =3D f[2];
        std::cout << foo(f[J], f[1]) << std::endl;
    }
}

Save it to main.cpp and compile it with:

g++ -O2 main.cpp

The expected output should be "2", but the optimized build output "-8". If
compiled with "-O2 -fno-strict-aliasing", the output is correct.=