https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102438
Bug ID: 102438
Summary: [x86-64] Failure to optimize out random extra
store+load in vector code when memcpy is used
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
#include <stddef.h>
typedef double simde_float64x1_t __attribute__((__vector_size__(8)));
simde_float64x1_t simde_vabs_f64(simde_float64x1_t a) {
simde_float64x1_t r;
r[0] = -a[0];
return (simde_float64x1_t)r;
}
On AMD64 with -O3, this is outputted:
simde_vabs_f64(double __vector(1)):
movsd xmm0, QWORD PTR [rsp+8]
xorpd xmm0, XMMWORD PTR .LC0[rip]
mov rax, rdi
movsd QWORD PTR [rsp-24], xmm0
mov rdx, QWORD PTR [rsp-24]
mov QWORD PTR [rdi], rdx
ret
If we instead just return `r` (without the cast) this is instead outputted:
simde_vabs_f64(double __vector(1)):
movsd xmm0, QWORD PTR [rsp+8]
xorpd xmm0, XMMWORD PTR .LC0[rip]
mov rax, rdi
movsd QWORD PTR [rdi], xmm0
ret
It seems as though the presence of a cast (to the same type, no less) confuses
GCC into spilling the result into memory.
The GIMPLE optimized output is different for the two, so idk how much this
target-specific to x86, but I haven't been able to reproduce it anywhere else,
so ¯\_(ツ)_/¯.
PS: The same bug can also be reproduced with -m32
