linux-kernel - Re: [RFC] Design for flag bit outputs from asms

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Date:	Mon, 04 May 2015 13:42:01 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Richard Henderson <rth@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Vladimir Makarov <vmakarov@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	"gcc@....gnu.org" <gcc@....gnu.org>
Subject: Re: [RFC] Design for flag bit outputs from asms

On 05/04/2015 01:35 PM, Linus Torvalds wrote:
> On Mon, May 4, 2015 at 1:14 PM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> I would argue that for x86 what you actually want is to model the
>> *conditions* that are available on the flags, not the flags themselves.
> 
> Yes. Otherwise it would be a nightmare to try to describe simple
> conditions like "le", which a rather complicated combination of three
> of the actual flag bits:
> 
>     ((SF ^^ OF) || ZF) = 1
> 
> which would just be ridiculously painful for (a) the user to describe
> and (b) fior the compiler to recognize once described.
> 
> Now, I do admit that most of the cases where you'd use inline asm with
> condition codes would probably fall into just simple "test ZF or CF".
> But I could certainly imagine other cases.
> 

Yes, although once again I'm more than happy to let gcc do the boolean
optimizations if it already has logic to do so (which it might have/want
for its own reasons.)

	-hpa


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