https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127967
Bug ID: 127967
Summary: [13/14/15/16/17 Regression] wrong code at -O2 with
bit-field of a struct passed by value (ipa-cp)
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Keywords: wrong-code
Severity: normal
Priority: P3
Component: ipa
Assignee: unassigned at gcc dot gnu.org
Reporter: kyuwoncho18 at gmail dot com
Target Milestone: ---
Target: x86_64-pc-linux-gnu
struct S { unsigned p : 4, q : 8; };
__attribute__((noinline)) static int f(struct S s) { return s.q; }
int main() {
union { unsigned char b[4]; struct S s; } u = {{16}};
return f(u.s);
}
godbolt: https://godbolt.org/z/v88TTEd4s
With -O0, -O1 or clang -O2, it returns 1, but with GCC -O2, -O3 or -Os, it
returns 16. -fno-ipa-cp avoids it; GCC 13.3, 14.4, 15.3 and 16.2 are also
affected, GCC 12.5 returns 1.
Disclaimer: This test case was generated by a fuzzer, and the report was
written with LLM assistance.
