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

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Date:	Mon, 19 Jan 2015 16:00:23 +1200
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	jsrhbz@...argh.force9.co.uk,
	christoph.muellner@...obroma-systems.com,
	Guenter Roeck <linux@...ck-us.net>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Anvin <hpa@...or.com>,
	Maxime Coquelin <maxime.coquelin@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>, martink@...teo.de,
	"Theodore Ts'o" <tytso@....edu>
Cc:	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:core/types] bitops: Add sign_extend8(), 16 and 64 functions

On Mon, Jan 19, 2015 at 7:54 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> The 8- and 16- bit versions are the same as the 32-bit one.

Side note: the 64-bit one is actually different on 32-bit
architectures, not because it would generate a different value, but
because the 64-bit overhead likely kills you. It might be that the
compiler is smart enough that we'd *only* need the 64-bit version, but
for non-constant bit numbers, it's likely hard for gcc to generate
sane single-register versions, and so the 64-bit version is likely
fine.

But I don't see the 8-bit and 16-bit versions generating better code
on any sane architecture.

           Linus
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