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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.
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