Re: [RFC PATCH] fs: micro-optimization remove branches by adjusting flag values

[Al Viro]

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.