https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96053
Bug ID: 96053
Summary: Miss optimization:Finding SLP sequences from
reductions sometimes is better than finding from
reduction chains
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: zhoukaipeng3 at huawei dot com
Target Milestone: ---
command:
gcc -S -O2 -ftree-vectorize test.c -funsafe-math-optimizations
-fno-tree-reassoc -march=armv8.2-a+sve -msve-vector-bits=128
gcc version 11.0.0 20200629
In vectorization, finding SLP sequences from reduction chains has priority over
from reductions. But sometimes, finding SLP sequences from reductions is a
better way to do vectorization than from reduction chains.
testcase:
double f(double *a, double *b)
{
double res1 = 0;
double res0 = 0;
for (int i = 0 ; i < 1000; i+=4) {
res0 += a[i] * b[i];
res1 += a[i+1] * b[i*1];
res0 += a[i+2] * b[i+2];
res1 += a[i+3] * b[i+3];
}
return res0 + res1;
}
I have two imperfect solutions, one is to add a control option, and the other
is to use the cost model to evaluate which is better. The first one is very
difficult for users to use, and the second one is difficult to implement.
Does anyone have a better suggestion?
