https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123543
Bug ID: 123543
Summary: [16 Regression] ipa miscompiles python-3.13.11,
libgit2-1.9.2, graphite2-1.3.14 since
r16-6652-ge58b11cd6c9ca3
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: ipa
Assignee: unassigned at gcc dot gnu.org
Reporter: slyfox at gcc dot gnu.org
Target Milestone: ---
I originally noticed the bug on recent gcc-master as a python-3.13.11,
libgit2-1.9.2, graphite2-1.3.14 build (or test) failure. I bisected `gcc` down
to r16-6652-ge58b11cd6c9ca3 and extracted minimal example from libgit2-1.9.2.
The reproducer seems to require 2 files (I suspect `noipa` has no effect and
it's probably a bug). Reproducer:
//$ cat a.c
__attribute__((noinline)) static int local_f(int a, int b) { return a + b; }
__attribute__((noinline)) int extern_f(int a, int b) { return
local_f(a, b); }
__attribute__((noipa)) int unused_g1(int b) { return extern_f(0, b);
}
//$ cat b.c
#include <stdio.h>
extern int extern_f(int a, int b);
int main() { printf("%i\n", extern_f(1, 2)); }
Running:
$ gcc/xgcc -Bgcc a.c b.c -o a -O2 && ./a
3 # good
$ gcc/xgcc -Bgcc a.c b.c -o a -O3 && ./a
2 # bad
Note: we sum up 1+2 here. My speculation is that somehow presence of
`extern_f(0, b)` call tricks IPA into considering `extern_f` as local with the
first parameter always being `0`.
gcc/xgcc -Bgcc -v
Reading specs from gcc/specs
COLLECT_GCC=gcc/xgcc
COLLECT_LTO_WRAPPER=gcc/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /home/slyfox/dev/git/gcc/configure --disable-multilib
--disable-bootstrap --disable-lto --disable-libsanitizer --enable-languages=c
CFLAGS='-O1 -g0' CXXFLAGS='-O1 -g0' LDFLAGS='-O1 -g0'
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 16.0.0 20260111 (experimental) (GCC)
