From: Al Viro <viro@zeniv.linux.org.uk>
To: mateusznosek0@gmail.com
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] fs: micro-optimization remove branches by adjusting flag values
Date: Tue, 15 Sep 2020 01:31:50 +0100 [thread overview]
Message-ID: <20200915003150.GJ3421308@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20200914174338.9808-1-mateusznosek0@gmail.com>
On Mon, Sep 14, 2020 at 07:43:38PM +0200, mateusznosek0@gmail.com wrote:
> From: Mateusz Nosek <mateusznosek0@gmail.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.
prev parent reply other threads:[~2020-09-15 0:32 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-14 17:43 [RFC PATCH] fs: micro-optimization remove branches by adjusting flag values mateusznosek0
2020-09-14 18:06 ` Matthew Wilcox
2020-09-15 0:38 ` Al Viro
2020-09-15 0:31 ` Al Viro [this message]
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