https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94792
Bug ID: 94792
Summary: Missed SLP optimization in pr65930-2.c variation
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: iii at linux dot ibm.com
Target Milestone: ---
gcc commit cf3a909cf455. Consider the following variation of pr65930-2.c:
$ cat pr65930-2b.c
#include "tree-vect.h"
int __attribute__((noipa))
bar (unsigned int *x, int n)
{
unsigned int sum = 4;
x = __builtin_assume_aligned (x, __BIGGEST_ALIGNMENT__);
for (int i = 0; i < n; ++i)
sum += x[i*4+0]+ x[i*4 + 1] + x[i*4 + 2] + x[i*4 + 3];
return sum;
}
int
main ()
{
static int a[16] __attribute__((aligned(__BIGGEST_ALIGNMENT__)))
= { 1, 3, 5, 8, 9, 10, 17, 18, 23, 29, 30, 55, 42, 2, 3, 1 };
check_vect ();
if (bar (a, 4) != 260)
abort ();
return 0;
}
This differs from pr65930-2.c only in that sum type is unsigned int, which
should be on cast less. And yet:
$ gcc pr65930-2b.c -fno-diagnostics-show-caret
-fno-diagnostics-show-line-numbers -fdiagnostics-color=never
-fdiagnostics-urls=never -msse2 -ftree-vectorize
-fno-tree-loop-distribute-patterns -fno-vect-cost-model -fno-common -O2
-fdump-tree-vect-details -lm -o ./pr65930-2.exe ; grep SLP
pr65930-2b.c.161t.vect | wc -l
0
whereas for the original version:
$ gcc pr65930-2.c -fno-diagnostics-show-caret
-fno-diagnostics-show-line-numbers -fdiagnostics-color=never
-fdiagnostics-urls=never -msse2 -ftree-vectorize
-fno-tree-loop-distribute-patterns -fno-vect-cost-model -fno-common -O2
-fdump-tree-vect-details -lm -o ./pr65930-2.exe ; grep SLP
pr65930-2.c.161t.vect | wc -l
33
The resulting assembly is also noticeably larger and uses regular adds for at
least part of the data.
