https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103781
Bug ID: 103781
Summary: [AArch64, 11 regr.] Failed partial vectorization of
mulv2di3
Product: gcc
Version: 11.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: husseydevin at gmail dot com
Target Milestone: ---
As of GCC 11, the AArch64 backend is very greedy in trying to vectorize
mulv2di3. However, there is no mulv2di3 routine so it extracts from the vector.
The bad codegen should be obvious.
#include <stdint.h>
void fma_u64(uint64_t *restrict acc, const uint64_t *restrict x, const uint64_t
*restrict y)
{
for (int i = 0; i < 16384; i++){
acc[0] += *x++ * *y++;
acc[1] += *x++ * *y++;
}
}
gcc-11 -O3
fma_u64:
.LFB0:
.cfi_startproc
ldr q1, [x0]
add x6, x1, 262144
.p2align 3,,7
.L2:
ldr x4, [x1], 16
ldr x5, [x2], 16
ldr x3, [x1, -8]
mul x4, x4, x5
ldr x5, [x2, -8]
fmov d0, x4
ins v0.d[1], x5
mul x3, x3, x5
ins v0.d[1], x3
add v1.2d, v1.2d, v0.2d
cmp x1, x6
bne .L2
str q1, [x0]
ret
.cfi_endproc
GCC 10.2.1 emits better code.
fma_u64:
.LFB0:
.cfi_startproc
ldp x4, x3, [x0]
add x9, x1, 262144
.p2align 3,,7
.L2:
ldr x8, [x1], 16
ldr x7, [x2], 16
ldr x6, [x1, -8]
ldr x5, [x2, -8]
madd x4, x8, x7, x4
madd x3, x6, x5, x3
cmp x9, x1
bne .L2
stp x4, x3, [x0]
ret
.cfi_endproc
However, the ideal code would be a 2 iteration unroll.
Side note: why not ldp in the loop?
