[LLVMdev] Storage-Only Register Class?

[Tom Stellard]

On Thu, Aug 08, 2013 at 01:51:53PM -0400, Justin Holewinski wrote:
> Good to know, thanks!
> 
> Currently I'm just not declaring an i8 register class since we only have
> load/store/convert available for that type.  This works fine for most uses,
> but becomes a hassle when dealing with function parameters.  For example,
> if the function argument is i8, the code in LowerFormalArguments sees it as
> an i16 since that is the next target legal type.  For my target, we need to
> load function parameters from a special memory pool; so I need to emit
> any-extend loads of an 8-bit memory VT to a 16-bit register VT.  But I
> don't see any way to get at the information that the function argument is
> really 8 bits unless I go back to the llvm::Function definition.  This is
> do-able, but feels wrong to be doing in TargetLowering.  I was trying to
> get around this by declaring i8 as a legal type (strictly speaking, it *is*
> legal in the target), and relying on the legalizer to perform any
> extensions for me; but now it looks like it may be easier to just teach
> SelectionDAGBuilder to pass along the original parameter type in
> LowerFormalArguments and friends.
>

R600 has the same issue and we just use the llvm::Function definition to
determine the type.  The only other alternative is to make i8 a legal
type, loop through all ISD opcodes and set them all to promote
for i8, and then fix whatever bugs you find in the SelectionDAG.

It would be really nice if it were possible to mark a type as load/store
only, but I'm not sure how to begin implementing that.

-Tom
> 
> On Thu, Aug 8, 2013 at 1:37 PM, Jim Grosbach <grosbach at apple.com> wrote:
> 
> > Hi Justin,
> >
> > This is a big weakness of the current SelectionDAG infrastructure. There's
> > not a really clean way to do this. The legalizer assumes that if a type is
> > "legal" at all, the target can do at least basic arithmetic on that type.
> >
> > Theoretically, your approach of setting the operations to
> > "TargetLowering::Promote" for i8 should work. I think it would be
> > reasonable to fix SelectionDAG to allow that. It's probably a non-trivial
> > task, though.
> >
> > -Jim
> >
> > On Aug 8, 2013, at 7:35 AM, Justin Holewinski <justin.holewinski at gmail.com>
> > wrote:
> >
> > > Is there a way to define a register class that is storage-only?  I want
> > to have an i8 register class that I can use for loads/stores/converts, but
> > that does not support arithmetic.
> > >
> > > It seems addOperationAction(ISD::ADD, MVT::i8, Promote) and
> > SetPromotedToType(ISD::ADD, MVT::i8, MVT::i16) are not sufficient, as the
> > legalizer just looks at whether or not the underlying type is legal (which
> > it is).  So come instruction selection time I still have i8 adds.  Do I
> > need to custom handle all 8-bit arithmetic, or is there some way to have
> > the legalizer do the promotion like it would for an illegal type?
> > >
> > >
> > > --
> > >
> > > Thanks,
> > >
> > > Justin Holewinski
> > > _______________________________________________
> > > LLVM Developers mailing list
> > > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
> >
> 
> 
> -- 
> 
> Thanks,
> 
> Justin Holewinski

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