https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115351
Bug ID: 115351
Summary: [14 regression] pointless movs when passing by value
on x86-64
Product: gcc
Version: 14.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: michael.kenzel at gmail dot com
Target Milestone: ---
When passing structs of certain shape by value on x86-64 (seems to be specific
to this target), gcc 14 emits (fails to optimize away?) a number of redundant
moves:
struct complex
{
double real;
double imag;
};
complex blub(complex z)
{
return {
z.real * z.real - z.imag * z.imag,
2 * z.real * z.imag
};
}
on gcc 13 with -O3 results in
blub(complex):
movapd xmm3, xmm1
movapd xmm2, xmm0
mulsd xmm3, xmm1
addsd xmm2, xmm2
mulsd xmm0, xmm0
mulsd xmm1, xmm2
subsd xmm0, xmm3
ret
gcc 14 and later with -O3 emit
blub(complex):
movq rax, xmm1
movq rdx, xmm0
xchg rdx, rax
movq xmm2, rax
movq xmm1, rdx
movapd xmm0, xmm2
movapd xmm3, xmm1
mulsd xmm3, xmm1
mulsd xmm0, xmm2
addsd xmm2, xmm2
mulsd xmm1, xmm2
subsd xmm0, xmm3
ret
godbolt: https://godbolt.org/z/hzEfs3ob4
