https://bugs.llvm.org/show_bug.cgi?id=43578
Bug ID: 43578
Summary: h264 (SPEC) : 8% percent perf regression with
-march=haswell compared to plain -O3
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Loop Optimizer
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
I measured a significant performance drop for SPEC benchmark h264 with
-march=haswell:
417 seconds -O3 -march=haswell
418 seconds -O3 -mprefer-vector-width=128 -march=haswell
385 seconds -O3
I didnt go very deep, I went to check known hotspot function/loop of this
benchmark. I reduced benchmark so anybody can look at this hotspot on godbolt:
https://godbolt.org/z/i8vvYN
(if we improve this reduced test case; this benchmark will improve too)
As I can see, the reason of this slowdown is aggresive vectorization; with
-march=haswell we probably destroy loop perf due to use of
vpextrd, vextracti128, vpmovzxwd (SLP vectorizer somehow decided that
vectorization is profitable; clearly not; cost model issue?).
GCC/ICC performs no vectorization of this loop with -march=haswell.
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