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

[John Thompson]

Eli,

So, then this is a problem with the back-end, right?  Do you agree that the
back end is a better place to try to choose constraints, given that it knows
better which things are already in registers and so forth?

-John

On Thu, Aug 5, 2010 at 6:28 PM, Eli Friedman <eli.friedman at gmail.com> wrote:

> On Thu, Aug 5, 2010 at 6:24 PM, John Thompson
> <john.thompson.jtsoftware at gmail.com> wrote:
> > Eli,
> >
> > 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.
>
> That's precisely the issue; multiple-alternative constraints require
> examining all the constraints at once.  From the gcc docs: "For
> example, on the 68000, a logical-or instruction can combine register
> or an immediate value into memory, or it can combine any kind of
> operand into a register; but it cannot combine one memory location
> into another. "
>
> -Eli
>



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