https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116582
Bug ID: 116582
Summary: gather is a win in some cases on zen CPUs
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: hubicka at gcc dot gnu.org
Target Milestone: ---
While the sparse multiply in parest and tsvc does not seem to work well gather
the following benchmark likes it:
T a[M], b[M];
__attribute__ ((noipa))
void
test ()
{
for (int i = 0; i < 1024* 16; i++)
a[i] += b[indices[i]];
}
int
main()
{
for (int i = 0 ; i < M; i++)
indices[i] = (i * 8) % M;
for (int i = 0 ; i < 10000; i++)
test ();
return 0;
}
jan@localhost:/tmp> g++ -DT=float -march=native gather.c -Ofast
-mtune-ctrl=^use_gather_4parts -mtune-ctrl=^use_gather_8parts
-mtune-ctrl=^use_gather -mtune=native -fdump-tree-all-details ; objdump -d
a.out | grep gather ; perf stat ./a.out
Performance counter stats for './a.out':
3,499.60 msec task-clock:u # 1.000 CPUs
utilized
0 context-switches:u # 0.000 /sec
0 cpu-migrations:u # 0.000 /sec
221 page-faults:u # 63.150 /sec
14,526,193,995 cycles:u # 4.151 GHz
467,072,127 stalled-cycles-frontend:u # 3.22% frontend
cycles idle
577,324,069 instructions:u # 0.04 insn per
cycle
# 0.81 stalled cycles per
insn
41,578,204 branches:u # 11.881 M/sec
50,517 branch-misses:u # 0.12% of all
branches
3.500660600 seconds time elapsed
3.497150000 seconds user
0.003333000 seconds sys
jan@localhost:/tmp> g++ -DT=float -march=native gather.c -Ofast
-mtune-ctrl=use_gather_4parts -mtune-ctrl=use_gather_8parts
-mtune-ctrl=use_gather -mtune=native -fdump-tree-all-details ; objdump -d a.out
| grep gather ; perf stat ./a.out
401250: c4 e2 65 92 04 8d 40 vgatherdps
%ymm3,0x404040(,%ymm1,4),%ymm0
Performance counter stats for './a.out':
1,263.87 msec task-clock:u # 0.922 CPUs
utilized
0 context-switches:u # 0.000 /sec
0 cpu-migrations:u # 0.000 /sec
222 page-faults:u # 175.651 /sec
5,172,067,789 cycles:u # 4.092 GHz
93,135,962 stalled-cycles-frontend:u # 1.80% frontend
cycles idle
167,783,419 instructions:u # 0.03 insn per
cycle
# 0.56 stalled cycles per
insn
21,097,560 branches:u # 16.693 M/sec
24,253 branch-misses:u # 0.11% of all
branches
1.370533592 seconds time elapsed
1.265143000 seconds user
0.000000000 seconds sys
Non-gather loop is:
.L2:
movslq indices(%rax), %rcx
movslq indices+8(%rax), %rdi
addq $16, %rax
movslq indices-12(%rax), %rdx
movslq indices-4(%rax), %rsi
vmovss b(,%rdi,4), %xmm1
vmovss b(,%rcx,4), %xmm0
vinsertps $0x10, b(,%rsi,4), %xmm1, %xmm1
vinsertps $0x10, b(,%rdx,4), %xmm0, %xmm0
vmovlhps %xmm1, %xmm0, %xmm0
vaddps a-16(%rax), %xmm0, %xmm0
vmovaps %xmm0, a-16(%rax)
cmpq $65536, %rax
while gather loop:
.L2:
vmovdqa indices(%rax), %ymm1
vmovaps %ymm2, %ymm3
addq $32, %rax
vgatherdps %ymm3, b(,%ymm1,4), %ymm0
vaddps a-32(%rax), %ymm0, %ymm0
vmovaps %ymm0, a-32(%rax)
cmpq $65536, %rax
jne .L2
