https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90579
Bug ID: 90579
Summary: Huge store forward stall due to vectorizer
Product: gcc
Version: 9.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: hjl.tools at gmail dot com
Target Milestone: ---
Target: x86-64
loop/avx256 branch at
https://gitlab.com/x86-benchmarks/microbenchmark
shows huge store forward stall due to vectorizer in
---
extern double r[6];
extern double a[];
double
loop (int k, double x)
{
int i;
double t=0;
for (i=0;i<6;i++)
r[i] = x * a[i + k];
for (i=0;i<6;i++)
t+=r[5-i];
return t;
}
---
when compiled with -O3 -march=skylake:
[hjl@gnu-cfl-1 microbenchmark]$ perf stat -e ld_blocks.store_forward ./event
loop: 229408
Performance counter stats for './event':
1 ld_blocks.store_forward:u
0.000478529 seconds time elapsed
0.000502000 seconds user
0.000000000 seconds sys
[hjl@gnu-cfl-1 microbenchmark]$ perf stat -e ld_blocks.store_forward
./event-avx128
loop: 191390
Performance counter stats for './event-avx128':
1 ld_blocks.store_forward:u
0.000526154 seconds time elapsed
0.000507000 seconds user
0.000000000 seconds sys
[hjl@gnu-cfl-1 microbenchmark]$ perf stat -e ld_blocks.store_forward
./event-avx256
loop: 1312864
Performance counter stats for './event-avx256':
30,001 ld_blocks.store_forward:u
0.000756643 seconds time elapsed
0.000723000 seconds user
0.000000000 seconds sys
[hjl@gnu-cfl-1 microbenchmark]$
