linux-kernel - Re: [PATCH] fs/open.c: micro-optimization by avoiding branch on common path

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Message-ID: <20200919002033.GD3421308@ZenIV.linux.org.uk>
Date:   Sat, 19 Sep 2020 01:20:33 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     mateusznosek0@...il.com
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/open.c: micro-optimization by avoiding branch on
 common path

On Sat, Sep 19, 2020 at 02:10:21AM +0200, mateusznosek0@...il.com wrote:
> From: Mateusz Nosek <mateusznosek0@...il.com>
> 
> If file is a directory it is surely not regular. Therefore, if 'S_ISREG'
> check returns false one can be sure that vfs_truncate must returns with
> error. Introduced patch refactors code to avoid one branch in 'likely'
> control flow path. Moreover, it marks the proper check with 'unlikely'
> macro to improve both branch prediction and readability. Changes were
> tested with gcc 8.3.0 on x86 architecture and it is confirmed that
> slightly better assembly is generated.

Does it measurably show in any profiles?

> Signed-off-by: Mateusz Nosek <mateusznosek0@...il.com>
> ---
>  fs/open.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/open.c b/fs/open.c
> index 9af548fb841b..69658ea27530 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -74,10 +74,12 @@ long vfs_truncate(const struct path *path, loff_t length)
>  	inode = path->dentry->d_inode;
>  
>  	/* For directories it's -EISDIR, for other non-regulars - -EINVAL */
> -	if (S_ISDIR(inode->i_mode))
> -		return -EISDIR;
> -	if (!S_ISREG(inode->i_mode))
> -		return -EINVAL;
> +	if (unlikely(!S_ISREG(inode->i_mode))) {
> +		if (S_ISDIR(inode->i_mode))
> +			return -EISDIR;
> +		else
> +			return -EINVAL;
> +	}

If anything, turn the second if into return S_ISDIR(...) ? -EISDIR : -EINVAL;