https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111905
Bug ID: 111905
Summary: -O3 vectorization terribly pessimizes the code for an
already unrolled loop
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: kamkaz at windowslive dot com
Target Milestone: ---
https://godbolt.org/z/rK4nEWovc
With -O2, the code is the way you'd expect it to be - with performance benefit
for the code "manually unrolled", that processes data in chunks (using
assumption that w > 16).
With -O3 however, auto-vecorization kicks in for the already unrolled loop,
with the results being abysmal, a lot of unnecessary checks and (what I assume
to be) dead code.
Clang doesn't seem to have this problem.
