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cmd/compile: aggressive IMUL rewrites on AMD64 sometimes lead to performance regressions #25358

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Quasilyte opened this Issue May 11, 2018 · 3 comments

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Quasilyte commented May 11, 2018

image/draw

name        old time/op  new time/op  delta
CopyOver-4  1.18ms ± 0%  1.52ms ± 0%  +28.49%  (p=0.008 n=5+5)

Bisecting from tip leads to b1df8d6.
This is regression from go1.10.2.
Performance delta is shown for b1df8d6 itself and it's parent (previous commit).

Single IMUL is rewritten into mov+shift+lea.
I can't reliably reproduce those results in free context, but they're stable for CopyOver.

@Quasilyte Quasilyte changed the title from cmd/compile: aggressive IMUL rewrites sometimes lead to performance regressions to cmd/compile: aggressive IMUL rewrites on AMD64 sometimes lead to performance regressions May 11, 2018

@josharian

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josharian commented May 11, 2018

Duplicate of #21735?

@Quasilyte

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Quasilyte commented May 11, 2018

Maybe.

I'm reporting regression and it hopefully can be fixed.
Shouldn't be unnoticed.

At the very least it adds related benchmark that can be used to check the results.

@bradfitz

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bradfitz commented May 11, 2018

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