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

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Message-ID: <20150205071734.GA3203@gmail.com>
Date:	Thu, 5 Feb 2015 08:17:34 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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


* H. Peter Anvin <hpa@...or.com> wrote:

> 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, 

Is that true for other compilers as well?

> and instead considers 8-bit values to occupy the whole 
> register, but that means only four are available in 32-bit 
> mode.

So where are we with this? Should I consider:

  7e9358073d3f ("bitops: Add sign_extend8(), 16 and 64 functions")

NAK-ed due to having marginal benefits, or due to having no 
benefits whatsoever?

How about the two patch series from Martin Keppling - that does 
seem to be both beneficial and correct, agreed?

Thanks,

	Ingo
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