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

[Linus Torvalds]

On Tue, May 5, 2015 at 6:50 AM, Segher Boessenkool
<segher@xxxxxxxxxxxxxxxxxxx> wrote:
>
> Since it is pre-processed, there is no real reason to overlap this with
> the constraints namespace; we could have e.g. "=@[xy]" (and "@[xy]" for
> inputs) mean the target needs to do some "xy" transform here.

In fact, standing out visually would be just a good thing, since it's
pretty special even from a usage standpoint.

And are you actually planning to have flags as inputs? Because *that*
sounds like a bad idea. It's pretty hard to turn a boolean into a flag
value, while pretty much any archiecture has an operation like "setcc"
to go the other way. And I don't think your machine descriptions have
anything to "generate flags". You'd have to add fragile and complex
machinery for something it is unlikely anybody ever wants.

Flag *outputs* people definitely want. Flag inputs? Yeah, I can
absolutely see the carry flag being useful for multi-precision
arithmetic, but it's *so* hard to guarantee that it still is live,
that in practice the compiler would likely have to re-generate it from
a value anyway, so ...

So I'd go for output-only, and make the syntax be something very
visually unambiguous. That "=@[xy]" format looks fine, where "xy"
would be very architecture-dependent.

Or make it even *more* specific by using "CC" for condition codes, and
make the syntax "=@CC[xy]", in case you ever want to use the "@"
marker for any other kind of magic constraint.

                         Linus
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