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

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Message-ID: <20210127183034.GO740243@zeniv-ca>
Date:   Wed, 27 Jan 2021 18:30:34 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     David Laight <David.Laight@...lab.com>
Cc:     'Pavel Begunkov' <asml.silence@...il.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Jens Axboe <axboe@...nel.dk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iov_iter: optimise iter type checking

On Wed, Jan 27, 2021 at 04:28:38PM +0000, David Laight wrote:

> I'd definitely leave the type as a bitmap.

What the hell for?  Microoptimizations in places where we have
much heavier stuff to be done are bloody pointless.

It's already overcomplicated.  And compiler is _not_ going to
be able to prove that we'll only ever have one bit set, so
you would be making it harder to optimize, not to mention
reason about.