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

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Date:   Thu, 16 Jan 2020 17:21:23 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     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 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;