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

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Message-ID: <20201119170340.GA6179@infradead.org>
Date:   Thu, 19 Nov 2020 17:03:40 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Pavel Begunkov <asml.silence@...il.com>
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 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).