From: Matthew Wilcox <willy@infradead.org>
To: Pavel Begunkov <asml.silence@gmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fs: optimise kiocb_set_rw_flags()
Date: Thu, 16 Jan 2020 17:21:23 -0800 [thread overview]
Message-ID: <20200117012123.GA9226@bombadil.infradead.org> (raw)
In-Reply-To: <7d493d4872b75fc59556a63ee62c43b30c661ff9.1579223790.git.asml.silence@gmail.com>
On Fri, Jan 17, 2020 at 04:16:41AM +0300, Pavel Begunkov wrote:
> kiocb_set_rw_flags() generates a poor code with several memory writes
> and a lot of jumps. Help compilers to optimise it.
>
> Tested with gcc 9.2 on x64-86, and as a result, it its output now is a
> plain code without jumps accumulating in a register before a memory
> write.
Nice!
> static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
> {
> + int kiocb_flags = 0;
> +
> if (unlikely(flags & ~RWF_SUPPORTED))
> return -EOPNOTSUPP;
>
> if (flags & RWF_NOWAIT) {
> if (!(ki->ki_filp->f_mode & FMODE_NOWAIT))
> return -EOPNOTSUPP;
> - ki->ki_flags |= IOCB_NOWAIT;
> + kiocb_flags |= IOCB_NOWAIT;
> }
> if (flags & RWF_HIPRI)
> - ki->ki_flags |= IOCB_HIPRI;
> + kiocb_flags |= IOCB_HIPRI;
> if (flags & RWF_DSYNC)
> - ki->ki_flags |= IOCB_DSYNC;
> + kiocb_flags |= IOCB_DSYNC;
> if (flags & RWF_SYNC)
> - ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
> + kiocb_flags |= (IOCB_DSYNC | IOCB_SYNC);
> if (flags & RWF_APPEND)
> - ki->ki_flags |= IOCB_APPEND;
> + kiocb_flags |= IOCB_APPEND;
> +
> + if (kiocb_flags)
> + ki->ki_flags |= kiocb_flags;
> return 0;
> }
Might it generate even better code to do ...
int kiocb_flags = 0;
+ if (!flags)
+ return 0;
if (unlikely(flags & ~RWF_SUPPORTED))
return -EOPNOTSUPP;
...
- if (kiocb_flags)
- ki->ki_flags |= kiocb_flags;
+ ki->ki_flags |= kiocb_flags;
next prev parent reply other threads:[~2020-01-17 1:21 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-17 1:16 [PATCH] fs: optimise kiocb_set_rw_flags() Pavel Begunkov
2020-01-17 1:21 ` Matthew Wilcox [this message]
2020-01-17 1:23 ` Pavel Begunkov
2020-01-17 1:32 ` [PATCH v2] " Pavel Begunkov
2020-01-31 13:01 ` Pavel Begunkov
2020-02-12 12:57 ` Pavel Begunkov
2020-08-01 10:36 [PATCH] " Pavel Begunkov
2020-08-01 10:42 ` Pavel Begunkov
2020-08-01 15:37 ` Matthew Wilcox
2020-08-01 17:01 ` Jens Axboe
2020-08-02 8:33 ` Pavel Begunkov
2020-08-02 8:41 ` Pavel Begunkov
2020-08-01 17:02 ` Jens Axboe
2020-08-02 8:21 ` Pavel Begunkov
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