https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91435
Bug ID: 91435
Summary: Better induction variable for vectorization
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: glisse at gcc dot gnu.org
Target Milestone: ---
Target: x86_64-*-*
(from https://stackoverflow.com/q/57465290/1918193)
long RegularTest(int n) {
long sum = 0;
for (int i = 0; i < n; ++i)
if (i % 2 != 0)
sum += i + 1;
return sum;
}
Compiling with -O3 -march=skylake, this gets vectorized, but the result has
# vect_vec_iv_.14_60 = PHI <{ 0, 1, 2, 3, 4, 5, 6, 7 }(5),
vect_vec_iv_.14_61(6)>
vect_vec_iv_.14_61 = vect_vec_iv_.14_60 + { 8, 8, 8, 8, 8, 8, 8, 8 };
vect__3.17_66 = vect_vec_iv_.14_60 + { 2, 2, 2, 2, 2, 2, 2, 2 };
(those are the only uses of vect_vec_iv_.14_6[01])
If we are only ever going to use x+2, why not use that instead, initialize with
{2,3,4,...}, and skip the +2 at every iteration?
(there are other things to discuss about optimizing this testcase, for instance
clang is clever enough to unroll by a factor of 2 and remove the condition, but
let's stick to the induction variable for this PR)
