linux-kernel - Re: [tip:x86/asm] x86, bitops: Change bitops to be native operand size

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Message-ID: <1384224883.4771.28.camel@joe-AO722>
Date:	Mon, 11 Nov 2013 18:54:43 -0800
From:	Joe Perches <joe@...ches.com>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	linux-arch <linux-arch@...r.kernel.org>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	james.t.kukunas@...el.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [tip:x86/asm] x86, bitops:  Change bitops to be native operand
 size

On Mon, 2013-11-11 at 15:34 -0800, H. Peter Anvin wrote:
> On 11/10/2013 06:22 PM, Joe Perches wrote:
> > 
> > Perhaps the current x86 bitops asm code is being conflated
> > with the ideal implementation?
> > 
> Yes, by you.

Really?  I don't think so.

How does the use of signed long for an index where
no negative values are possible or the use of a
negative int for BIT_MASK make sense?

> x86 has instructions that operate on signed bitindicies.

indices.

> It doesn't
> have instructions that operate on unsigned bitindicies.  Unless someone
> is willing to do the work to prove that shift and mask is actually
> faster than using the hardware instructions (which I doubt, but it is
> always a possibility), that's what we have.

That doesn't mean x86 is the ideal implementation.


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