https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116765
Bug ID: 116765
Summary: gcc generate wrong code with -O3 -march=skylake
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: xiaohuba2021 at 163 dot com
Target Milestone: ---
The following code is miscompiled with -O3 -march=skylake:
```
#include <bitset>
#include <iostream>
const int N = 105;
std::bitset<N> ok[N][N];
int n = 5;
int main() {
ok[2][2].set(2);
for (int i = n; i; i--)
for (int j = i; j <= n; j++) {
ok[i][j] = ok[i][j] | ok[i + 1][j] | ok[i][j - 1];
}
std::cout << ok[2][5][2] << '\n';
return 0;
}
```
It outputs 0, however (obviously) the correct answer should be 1.
Turning off O3 or march=skylake gives the right output.
gcc version:
```
COLLECT_GCC=/opt/compiler-explorer/gcc-snapshot/bin/g++
Target: x86_64-linux-gnu
Configured with: ../gcc-trunk-20240918/configure
--prefix=/opt/compiler-explorer/gcc-build/staging
--enable-libstdcxx-backtrace=yes --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu --disable-bootstrap
--enable-multiarch --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --enable-clocale=gnu
--enable-languages=c,c++,fortran,ada,objc,obj-c++,go,d,rust,m2 --enable-ld=yes
--enable-gold=yes --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--enable-linker-build-id --enable-lto --enable-plugins --enable-threads=posix
--with-pkgversion=Compiler-Explorer-Build-gcc-38b5a568f8738c16baa978a62a9e983a85b4c6f0-binutils-2.42
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 15.0.0 20240918 (experimental)
(Compiler-Explorer-Build-gcc-38b5a568f8738c16baa978a62a9e983a85b4c6f0-binutils-2.42)
```
