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

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Date:   Sun, 1 Dec 2019 03:04:36 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] block: optimise bvec_iter_advance()

On 01/12/2019 01:05, Arvind Sankar wrote:
> 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 explicitly 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

That's unfortunate. IDK, whether it could be easily done in gcc,
but maybe relay it to compiler guys?

> 
> 	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.
> 

That's more like a starting point. The idea is to revise and
tune/rewrite iteration helpers including *for_each_bvec(). I assume,
all those are really poorly optimisable (especially with
-fno-strict-aliasing). So, noted to consider

-- 
Pavel Begunkov



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