https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116459
Bug ID: 116459
Summary: gcc 12.0 optimized out functions which has observable
side affect
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: xiaohuba2021 at 163 dot com
Target Milestone: ---
The following code:
#include <iostream>
#include <cstring>
using namespace std;
struct Matrix {
long long mat[2][2];
Matrix() { memset(mat, 0, sizeof(mat)); }
Matrix operator*(Matrix b) {
Matrix ans;
ans.mat[0][0] = (mat[0][0] * b.mat[0][0] + mat[0][1] * b.mat[1][0]);
ans.mat[1][0] = (mat[1][0] * b.mat[0][0] + mat[1][1] * b.mat[1][0]);
ans.mat[0][1] = (mat[0][0] * b.mat[0][1] + mat[0][1] * b.mat[1][1]);
ans.mat[1][1] = (mat[1][0] * b.mat[0][1] + mat[1][1] * b.mat[1][1]);
return ans;
}
};
Matrix st, res;
volatile bool mask = 0;
void qpw(long long k) {
mask = 1;
while (k) {
if (k & 1) st = st * res;
res = res * res, k >>= 1;
}
}
int main() {
st.mat[0][0] = st.mat[0][1] = 1;
res.mat[0][0] = 6;
res.mat[1][0] = -2, res.mat[1][1] = 2;
qpw(2);
cout << st.mat[0][0] << '\n';
return 0;
}
seems to produce wrong output(1) with -O2 or -O1 enabled, while the correct
answer is 20.
Uncomment line 20 (mask = 1) or explicitly add operator= for struct Matrix
solves this problem.
By looking at the assemble code, I found that the compiler did compile function
qpw, however didn't call it. So it's most likely a compiler bug, as the
compiler thought qpw did not have any side affects and optimized it out.
Is it caused by the default copy assignment of a struct containing only 2D
array?
Compiler info:
Using built-in specs.
COLLECT_GCC=/opt/compiler-explorer/gcc-12.2.0/bin/g++
COLLECT_LTO_WRAPPER=/opt/compiler-explorer/gcc-12.2.0/bin/../libexec/gcc/x86_64-linux-gnu/12.2.0/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../gcc-12.2.0/configure
--prefix=/opt/compiler-explorer/gcc-build/staging --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 --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--binutils-2.38
--enable-libstdcxx-backtrace=yes
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 12.2.0 (Compiler-Explorer-Build-gcc--binutils-2.38)
