linux-kernel - Re: [tip:x86/mm] x86, mm: Use a bitfield to mask nuisance get

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Message-ID: <5119EB71.6080508@zytor.com>
Date:	Mon, 11 Feb 2013 23:12:49 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jamie Lokier <jamie@...reable.org>,
	ville.syrjala@...ux.intel.com, Borislav Petkov <bp@...en8.de>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-tip-commits@...r.kernel.org, "H.J. Lu" <hjl.tools@...il.com>
Subject: Re: [tip:x86/mm] x86, mm: Use a bitfield to mask nuisance get_user()
 warnings

Just to be sure, I re-ran the comparison using gcc 4.6.3 instead of gcc
4.7.2.  With gcc 4.6.3 I consistently get a few hundred bytes longer
with the bitfield variant than with the pre-get_user() baseline.

I looked at some of the diffed disassembly, and the differences seem to
be in the code generated downstream of __get_user_1 and __get_user_2,
which I guess is to be expected, mostly in the form of padding.

Annoyingly enough in *both* cases I found unnecessary instructions like:

+c12f6fbb:      0f b7 d2                movzwl %dx,%edx

	-hpa

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