https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114176
Bug ID: 114176
Summary: Failure to optimize out useless stack spill when array
is present in union
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
struct Vec3
{
union {
float v[3];
float x, y, z;
};
};
float Vec3Dot(struct Vec3 vec1, struct Vec3 vec2)
{
return vec1.x;
}
on x86-64, with -O3, GCC outputs:
Vec3Dot:
movq QWORD PTR [rsp-16], xmm0
movss xmm0, DWORD PTR [rsp-16]
ret
LLVM instead outputs this:
Vec3Dot:
ret
Stack spilling here seems to occur for no reason when the `float v[3];`
declaration is present.
This looks like an RTL issue to me as the final optimized tree pass looks the
same with or without the `float v[3];` declaration being present, and the issue
seems to be present on multiple targets, though to different degrees: only on
x86-64 have I seen it actually result in a stack spill (on e.g. AArch64 and
RISC-V, it only makes GCC outputs unnecessary instructions to adjust `sp` but
does not actually spill the value).
