https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126650
Bug ID: 126650
Summary: Wrong code at -O2/3/s on x86_64-pc-linux-gnu
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: xintong.zhou1 at uwaterloo dot ca
Target Milestone: ---
Compiler Explorer: https://godbolt.org/z/dj7oaT1Tc
$ gcc-trunk -v
Using built-in specs.
COLLECT_GCC=/data/x27zhou/compiler-nightly/install/gcc/bin/gcc
COLLECT_LTO_WRAPPER=/data/x27zhou/compiler-nightly/install/gcc/libexec/gcc/x86_64-pc-linux-gnu/17.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /data/x27zhou/compiler-nightly/src/gcc/configure
--enable-checking=yes --disable-bootstrap --disable-multilib
--enable-languages=c,c++ --prefix=/data/x27zhou/compiler-nightly/install/gcc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 17.0.0 20260801 (experimental) (GCC)
$
$ gcc-trunk -O0 small.c && ./a.out
-64
$ gcc-trunk -O1 small.c && ./a.out
-64
$ gcc-trunk -O2 small.c && ./a.out
0
$ gcc-trunk -O3 small.c && ./a.out
0
$ gcc-trunk -Os small.c && ./a.out
0
$ cat small.c
#include <stdio.h>
long a;
int main() {
struct b {
int : 7;
int : 2;
int : 6;
int : 3;
int c : 7;
};
union {
int d;
struct b bf;
char e[4];
} f;
f.d = a;
f.e[3] = 7;
printf("%d\n", f.bf.c);
}
Bisected to
https://gcc.gnu.org/cgit/gcc/commit/?id=4bdf739f835520ccbc433dc9eac461895741f317
