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

["H. Peter Anvin" <hpa@zytor.com>]

On 05/04/2015 01:35 PM, Linus Torvalds wrote:
> On Mon, May 4, 2015 at 1:14 PM, H. Peter Anvin <hpa@zytor.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