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

John Thompson john.thompson.jtsoftware at gmail.com
Thu Aug 5 18:24:57 PDT 2010


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.


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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