From: H. Peter Anvin <hpa@zytor.com>
Date: Sat, 28 Dec 2013 00:52:47 +0000 (-0800)
Subject: x86: Slightly tweak the access_ok() C variant for better code
X-Git-Tag: v3.14-rc1~167^2
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=a740576a4abf933de8f50787f24f24456cebd761;p=pandora-kernel.git

x86: Slightly tweak the access_ok() C variant for better code

gcc can under very specific circumstances realize that the code
sequence:

	foo += bar;
	if (foo < bar) ...

... is equivalent to a carry out from the addition.  Tweak the
implementation of access_ok() (specifically __chk_range_not_ok()) to
make it more likely that gcc will make that connection.  It isn't
fool-proof (sometimes gcc seems to think it can make better code with
lea, and ends up with a second comparison), still, but it seems to be
able to connect the two more frequently this way.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/CA%2B55aFzPBdbfKovMT8Edr4SmE2_=%2BOKJFac9XW2awegogTkVTA@mail.gmail.com
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
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