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

[Linus Torvalds]

On Mon, May 4, 2015 at 1:14 PM, H. Peter Anvin <hpa@xxxxxxxxx> 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.

                       Linus
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/