gcc could not vectorize this simple reversed loop:
int a[100], b[100];
void foo(int n)
{
int i;
for(i=n-2; i>=0; i--)
a[i+1] = a[i] + b[i];
}
chf...@pathscale:~/gcc$ gcc -O3 -ftree-vectorizer-verbose=2 -c foo.c
foo.c:6: note: not vectorized: complicated access pattern.
foo.c:3: note: vectorized 0 loops in function.
open64 can vectorize this loop:
chf...@pathscale:~/gcc$ opencc -O3 -LNO:simd_verbose=on -c foo.c
(foo.c:0) LOOP WAS VECTORIZED.
--
Summary: reversed loop is not vectorized
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: changpeng dot fang at amd dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43422
