https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79245
Bug ID: 79245
Summary: [7 Regression] Inefficient loop distribution to memcpy
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jgreenhalgh at gcc dot gnu.org
Target Milestone: ---
Created attachment 40591
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40591&action=edit
Code generation for -Ofast -fomit-frame-pointer -mcpu=cortex-a72+crypto
-fno-tree-loop-distribute-patterns
After r242038 we start to distribute the memcpy in the inner loop in this
testcase:
#define k 128000
double a[k][k];
double b[k][k];
double c[k][k];
int x;
int y;
void
foo (void)
{
for (int j = 0; j < x; j++)
{
for (int i = 0; i < y; i++)
{
c[j][i] = b[j][i] - a[j][i];
a[j][i] = b[j][i];
}
}
}
Such that (on AArch64) we first do a vectorized subtract, then a call to
memcpy. That's a suboptimal cache access pattern. We already have a[j][i],
b[j][i], c[j][i] in cache as we walk the inner loop - loop distribution just
causes us to have to pull them in again for the memcpy.
Attached are compilation runs on aarch64-none-elf with -Ofast
-fomit-frame-pointer -mcpu=cortex-a72+crypto with
-ftree-loop-distribute-patterns on/off which show the issue.
