Messages in this thread | | | From | Linus Torvalds <> | Date | Tue, 5 Jun 2018 08:47:24 -0700 | Subject | Re: x86/asm: __clear_user() micro-optimization (was: "Re: [GIT PULL] x86/asm changes for v4.18") |
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On Tue, Jun 5, 2018 at 8:05 AM Ingo Molnar <mingo@kernel.org> wrote: > > Ok, fair point and agreed - if Alexey sends some measurements to back the change > I'll keep this, otherwise queue up a revert.
I don't think it needs to be reverted, it's not like it's likely to hurt on any modern CPU's. The issues I talked about are fairly historical - barely even 64-bit cpus - and I'm not sure an extra uop to carry a constant around even matters in that code sequence.
It was more a generic issue - any micro-optimization should be based on numbers (and there should be some numbers in the commit message), not on "this should be faster". Because while intuitively immediates _should_ be faster than registers, that's simply not always "obviously true". It _may_ be true. But numbers talk.
Linus
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