[cfe-commits] [PATCH] Asm multiple alternative constraints revisited

[John Thompson]

Eli,

Could you have a look at ChooseConstraint in LLVM and confirm whether or not
it is choosing multiple alternative constraints?  It seems to me that the
backend is in a better position to choose them.  Having studied
ChooseConstraintTuple, I was in the process of trying something similar in
the front end, when my boss suggested that the back end was a better place,
leading me to have a look inside LLVM.  It looks like there is some basic
support for it, though not all constraint types are supported.  I even saw a
comment somewhere about "|" being used in the constrainst, though I didn't
see any support for it, but it suggested to me that it might have been
considered as a means for separating the alternative constraints.  One thing
I also noticed is that the back-end doesn't consider multiple constraints in
parallel, as ChooseConstraintTuple does, but it seems like it should be
okay, as long as the backend can find suitable instrustions.

-John

On Thu, Aug 5, 2010 at 6:08 PM, Eli Friedman <eli.friedman at gmail.com> wrote:

> On Thu, Aug 5, 2010 at 5:40 PM, John Thompson
> <john.thompson.jtsoftware at gmail.com> wrote:
> > May I check this in?
>
> No, it's wrong; the correct approach is
> http://llvm.org/bugs/show_bug.cgi?id=7338#c6 .  I think you're not
> understanding the part of multi-alternative constraints that requires
> only one alternative to be selected.
>
> -Eli
>



-- 
John Thompson
John.Thompson.JTSoftware at gmail.com
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