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Re: [RFC] Design for flag bit outputs from asms
- From: Linus Torvalds <torvalds at linux-foundation dot org>
- To: Segher Boessenkool <segher at kernel dot crashing dot org>
- Cc: Richard Henderson <rth at redhat dot com>, Peter Zijlstra <peterz at infradead dot org>, Vladimir Makarov <vmakarov at redhat dot com>, Jakub Jelinek <jakub at redhat dot com>, Ingo Molnar <mingo at kernel dot org>, "H. Peter Anvin" <hpa at zytor dot com>, Thomas Gleixner <tglx at linutronix dot de>, Linux Kernel Mailing List <linux-kernel at vger dot kernel dot org>, Borislav Petkov <bp at alien8 dot de>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Tue, 5 May 2015 08:37:01 -0700
- Subject: Re: [RFC] Design for flag bit outputs from asms
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On Tue, May 5, 2015 at 6:50 AM, Segher Boessenkool
> 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.