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SubjectRe: [RFC PATCH] fs: micro-optimization remove branches by adjusting flag values
On Mon, Sep 14, 2020 at 07:06:29PM +0100, Matthew Wilcox wrote:
> $ objdump -d test.o
> 0000000000000000 <a>:
> 0: 89 f8 mov %edi,%eax
> 2: 83 e0 03 and $0x3,%eax
> 5: c3 retq
>
> Please stop submitting uglifying patches without checking they actually
> improve anything. GCC is smarter than you think it is.

His main point isn't that - it's reshuffling LOOKUP_... bits to make that
kind of optimisation possible. However, doing that sets us up for PITA
down the road (e.g. reshuffling LOOKUP_... bits becomes forbidden, etc.)
and I'd rather not go there unless we have a real-world evidence that it
does buy us anything.

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