linux-kernel - Re: [tip:core/types] bitops: Add sign

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Message-ID: <54C00A7B.1000808@zytor.com>
Date:	Wed, 21 Jan 2015 12:22:19 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	jsrhbz@...argh.force9.co.uk,
	christoph.muellner@...obroma-systems.com, linux@...ck-us.net,
	linux@...musvillemoes.dk, paulmck@...ux.vnet.ibm.com,
	tglx@...utronix.de, mingo@...nel.org, akpm@...ux-foundation.org,
	maxime.coquelin@...com, linux-kernel@...r.kernel.org,
	martink@...teo.de, tytso@....edu, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:core/types] bitops: Add sign_extend8(), 16 and 64 functions

On 01/19/2015 02:04 AM, Peter Zijlstra wrote:
> On Mon, Jan 19, 2015 at 07:54:22AM +1200, Linus Torvalds wrote:
>> Why?
>>
>> The 8- and 16- bit versions are the same as the 32-bit one. This seems
>> pointless. If you want something where the sign is in bit 3, they all
>> return the same value, just the return type differs, but that's really a
>> *caller* thing, no?
> 
> Even for the 8bit ones? Since we have the *H and *L register we have
> more 8 bit regs than we have 16/32 bit regs and it might just be worth
> it.

Fewer, actually.  gcc doesn't really use the H registers much, and
instead considers 8-bit values to occupy the whole register, but that
means only four are available in 32-bit mode.

	-hpa

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