linux-kernel - Re: [PATCH 2/2] iov_iter: optimise iter type checking

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Date:   Thu, 19 Nov 2020 17:12:44 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] iov_iter: optimise iter type checking

On 19/11/2020 17:03, Christoph Hellwig wrote:
> On Thu, Nov 19, 2020 at 03:29:43PM +0000, Pavel Begunkov wrote:
>> The problem here is that iov_iter_is_*() helpers check types for
>> equality, but all iterate_* helpers do bitwise ands. This confuses
>> a compiler, so even if some cases were handled separately with
>> iov_iter_is_*(), it can't eliminate and skip unreachable branches in
>> following iterate*().
> 
> I think we need to kill the iov_iter_is_* helpers, renumber to not do
> the pointless bitmask and just check for equality (might turn into a
> bunch of nice switch statements actually).

There are uses like below though, and that would also add some overhead
on iov_iter_type(), so it's not apparent to me which version would be
cleaner/faster in the end. But yeah, we can experiment after landing
this patch.

if (type & (ITER_BVEC|ITER_KVEC))

-- 
Pavel Begunkov