linux-kernel - Re: [PATCH v2] block: optimise bvec_iter

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Message-ID: <20191130220458.GA297712@rani.riverdale.lan>
Date:   Sat, 30 Nov 2019 17:05:00 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>, nivedita@...m.mit.edu,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] block: optimise bvec_iter_advance()

On Sat, Nov 30, 2019 at 11:23:52PM +0300, Pavel Begunkov wrote:
> bvec_iter_advance() is quite popular, but compilers fail to do proper
> alias analysis and optimise it good enough. The assembly is checked
> for gcc 9.2, x86-64.
> 
> - remove @iter->bi_size from min(...), as it's always less than @bytes.
> Modify at the beginning and forget about it.
> 
> - the compiler isn't able to collapse memory dependencies and remove
> writes in the loop. Help it by explicitely using local vars.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
> 
> v2: simplify code (Arvind Sankar)
> 

Thanks :)

Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>

Btw, I discovered that gcc 9.2 doesn't optimize away the second
comparison in something like

	m = min(a,b);
	return m>a;

So the WARN_ONCE bit doesn't get optimized away even in cases like
bio_for_each_bvec where it's guaranteed at compile-time to not trigger.