linux-kernel - Re: [PATCH] fs: optimise kiocb_set_rw

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Message-ID: <20200801153711.GV23808@casper.infradead.org>
Date:   Sat, 1 Aug 2020 16:37:11 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: optimise kiocb_set_rw_flags()

On Sat, Aug 01, 2020 at 01:36:33PM +0300, Pavel Begunkov wrote:
> Use a local var to collect flags in kiocb_set_rw_flags(). That spares
> some memory writes and allows to replace most of the jumps with MOVEcc.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>

Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>

If you want to improve the codegen here further, I would suggest that
renumbering the IOCB flags to match the RWF flags would lead to better
codegen (can't do it the other way around; RWF flags are userspace ABI,
IOCB flags are not).  iocb_flags() probably doesn't get any worse because
the IOCB_ flags don't have the same numbers as the O_ bits (which differ
by arch anyway).