[LLVMdev] Predicate registers/condition codes question

[Hal Finkel]

On Mon, 4 Jun 2012 11:41:43 -0500
Sebastian Pop <spop at codeaurora.org> wrote:

> On Mon, Jun 4, 2012 at 11:22 AM, Sebastian Pop <spop at codeaurora.org>
> wrote:
> >> Why don't you call it with "Promote" instead of
> >> "Custom" and let the Legalizer do the job? Does it not work?
> >
> > I tried this, and the legalizer will happily say that i8 is a legal
> > type and just return the exact same node: this is because we
> > declared that Hexagon has a register for i8, that makes i8 legal
> > for all promotions.
> 
> As Hal mentioned, the problem is linked to the fact that type
> legalization happens before register class assignments.
> 
> One way to solve this problem would be to teach type legalization
> about the predicate register class: if a processor can perform only
> boolean arithmetic it would declare a type to be in the PredRegs
> class, whereas a processor that can do both integer and boolean
> arithmetic on a type would declare the register to be in both the
> IntRegs and PredRegs class, or just in the IntRegs class.
> 
> Opinions?  How hard is it to teach type legalization about register
> classes?
> 

Out of curiosity, have you some up with a workable scheme for this?
FWIW, PowerPC also has these predicate boolean-operation instructions.
Currently, the backend does not even know about them, but it might be
worth changing that.

If nothing else, maybe some kind of pre-register-allocation
peephole pass would be sufficient: A pass that looks for boolean
operations where all inputs have just been copied from the predicate
registers, and replaces the operation with the predicate operation (and
a copy-out). Or if you arrange things so that you have the opposite
problem, add pseudo instructions (for everything else) and have the
peephole pass (or regular pseudo expansion) pull the operands into
regular registers and then replace the pseudo with the real operation.

 -Hal

> Thanks,
> Sebastian
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-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory