linux-kernel - Re: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches

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Date:   Wed, 23 Jan 2019 11:18:02 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>, dev@...nvswitch.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        netdev@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-mm@...ck.org,
        linux-security-module@...r.kernel.org,
        kernel-hardening@...ts.openwall.com,
        intel-wired-lan@...ts.osuosl.org, linux-fsdevel@...r.kernel.org,
        xen-devel@...ts.xenproject.org, Laura Abbott <labbott@...hat.com>,
        linux-kbuild@...r.kernel.org,
        Alexander Popov <alex.popov@...ux.com>
Subject: Re: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of
 switches

On Wed, Jan 23, 2019 at 04:17:30PM +0200, Jani Nikula wrote:
> Can't have:
> 
> 	switch (i) {
> 		int j;
> 	case 0:
>         	/* ... */
> 	}
> 
> because it can't be turned into:
> 
> 	switch (i) {
> 		int j = 0; /* not valid C */
> 	case 0:
>         	/* ... */
> 	}
> 
> but can have e.g.:
> 
> 	switch (i) {
> 	case 0:
> 		{
> 			int j = 0;
> 	        	/* ... */
> 		}
> 	}
> 
> I think Kees' approach of moving such variable declarations to the
> enclosing block scope is better than adding another nesting block.

Another nesting level would be bad, but I think this is OK:

	switch (i) {
	case 0: {
		int j = 0;
        	/* ... */
	}
	case 1: {
		void *p = q;
		/* ... */
	}
	}

I can imagine Kees' patch might have a bad effect on stack consumption,
unless GCC can be relied on to be smart enough to notice the
non-overlapping liveness of the vriables and use the same stack slots
for both.