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

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Date:   Tue, 15 Sep 2020 01:31:50 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     mateusznosek0@...il.com
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fs: micro-optimization remove branches by adjusting
 flag values

On Mon, Sep 14, 2020 at 07:43:38PM +0200, mateusznosek0@...il.com wrote:
> From: Mateusz Nosek <mateusznosek0@...il.com>
> 
> When flags A and B have equal values than the following code
> 
> if(flags1 & A)
> 	flags2 |= B;
> 
> is equivalent to
> 
> flags2 |= (flags1 & A);
> 
> The latter code should generate less instructions and be faster as one
> branch is omitted in it.
> 
> Introduced patch changes the value of 'LOOKUP_EMPTY' and makes it equal
> to the value of 'AT_EMPTY_PATH'. Thanks to that, few branches can be
> changed in a way showed above which improves both performance and the
> size of the code.

No.  AT_EMPTY_PATH is a part of userland ABI; to tie LOOKUP_EMPTY to it
means that we can't ever modify the sucker.  Worse, it restricts any
possible reshuffling of the LOOKUP_... bits in the future.

So unless you can show an effect on the real-world profiles, there are
fairly strong reasons to avoid that headache.