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

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Message-ID: <54D39CB7.6080504@zytor.com>
Date:	Thu, 05 Feb 2015 08:39:19 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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, 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 02/04/2015 11:17 PM, Ingo Molnar wrote:
>>>
>>> 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,
>
> Is that true for other compilers as well?
>

Probably.  It wrecks havoc on register allocation, and it doesn't buy 
that much, especially on 64 bits where all the registers can be used for 
8-bit operations.

	-hpa

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