[RFC] Design for flag bit outputs from asms

Linus Torvalds torvalds@linux-foundation.org
Mon May 4 20:36:00 GMT 2015

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.


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