From: Al Viro <viro@zeniv.linux.org.uk>
To: Matthew Wilcox <willy@infradead.org>
Cc: mateusznosek0@gmail.com, 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 01:38:58 +0100 [thread overview]
Message-ID: <20200915003858.GK3421308@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20200914180629.GT6583@casper.infradead.org>
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.
next prev parent reply other threads:[~2020-09-15 0:39 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 [this message]
2020-09-15 0:31 ` Al Viro
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