https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100076
Bug ID: 100076
Summary: eembc/automotive/basefp01 has 30.3% regression compare
-O2 -ftree-vectorize with -O2 on SKX/CLX
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: crazylht at gmail dot com
CC: hjl.tools at gmail dot com
Target Milestone: ---
Refer to https://godbolt.org/z/e3nfz3xvW
cat testcase.c
int
t_run_test(double a)
{
static double P1, Q1;
static varsize polyX1[9];
polyX1[1] = a;
P1 = (varsize)constantP[0];
polyX1[1] = a;
// Loop 1
for( int i1 = 2 ; i1 <= 8 ; i1++ )
{
polyX1[i1] = polyX1[i1 - 1] * polyX1[1] ;
}
P1 = (varsize)constantP[0] ;
// Loop 2
for( int i1 = 1 ; i1 <= 8 ; i1++ )
{
P1 += (varsize)constantP[i1] * polyX1[i1] ;
}
Q1 = (varsize)constantQ[0] ;
// Loop 3
for( int i1 = 1 ; i1 <= 8 ; i1++ )
{
Q1 += (varsize)constantQ[i1] * polyX1[i1] ;
}
return a = a * P1 / Q1 ;
}
Loop 1 write array polyX1 which is used by Loop2 and Loop 3, with
-ftree-vectorize -O2, Loop2 and Loop 3 are vectorized, but Loop 1 is not since
it have inter-iterative dependence, then for array polyX1, there're 64-bit
stores in loop 1 and 128-bit load in Loop2 and Loop 3, and it causes store
forwarding stalls which hurt performance.
