linux-kernel - Re: [PATCH] shm: Remove silly double assignment

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Date:	Wed, 11 Jun 2008 03:45:06 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Neil Horman <nhorman@...driver.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH] shm: Remove silly double assignment

On Tue, 10 Jun 2008 08:53:39 -0400 Neil Horman <nhorman@...driver.com> wrote:

>     Found a silly double assignment of err is do_shmat.  Silly, but good to clean up
>     the useless code.
>     
>     Signed-off-by: Neil Horman <nhorman@...driver.com>
> 
> diff --git a/ipc/shm.c b/ipc/shm.c
> index 554429a..d05f6b5 100644
> --- a/ipc/shm.c
> +++ b/ipc/shm.c
> @@ -894,8 +894,6 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg, ulong *raddr)
>  	if (!sfd)
>  		goto out_put_dentry;
>  
> -	err = -ENOMEM;
> -
>  	file = alloc_file(path.mnt, path.dentry, f_mode, &shm_file_operations);
>  	if (!file)
>  		goto out_free;

Not silly, really.  Look:

	err = -ENOMEM;
	if (expr1)
		goto out;

	err = -ENOMEM;
	if (expr2)
		goto out;

each of these two units is a separate, self-contained clause.  Removing
the second assignment to `err' breaks that separation and will make one
clause undesirably dependent upon the other.

Example: if someone later comes up and does

	err = -ENOMEM;
	if (expr1)
		goto out;

+	er = -EINVAL;
+	if (expr3)
+		goto out;

	if (expr2)
		goto out;

then whoops, it broke.

The compiler should optimise away the second assignment anyway.
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