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

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Message-ID: <CA+55aFx7KA-inJJog0v_cAsU0S2FeAN61pyvoUO8pbaxEAWFag@mail.gmail.com>
Date:	Mon, 11 Feb 2013 20:42:37 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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

On Mon, Feb 11, 2013 at 8:21 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 02/11/2013 07:33 PM, Linus Torvalds wrote:
>
>> Has anybody run this past any gcc developers? And if so, did they run
>> away screaming?
>
> I haven't no... H.J., any comments on this patch?

I'd be most worried about any known pitfalls about bitfield code
generation. Looking at your code size numbers, it actually seems to
*improve* code generation except for the odd i386.pae case (bigger
code but also a different data size - odd) and i386 noconfig
(different bss, bigger code).

The code/data changes makes me wonder if the variable sometimes gets
flushed to memory as a 8-byte entry, and maybe there are things gcc
people can suggest..

But I don't see anything fundamentally wrong with it. Certainly it
looks much better than the disgusting and warning-prone

    unsigned long long __val_gu8

thing.

               Linus
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