[LLVMdev] Matching addsub

[Hal Finkel]

Duncan,

Thanks for the suggestion; I'll look at it.

 -Hal

On Tue, 2011-10-18 at 10:15 +0200, Duncan Sands wrote:
> Hi Hal, you should probably add a target specific DAG combine that
> synthesizes the appropriate target instruction.  This is how I
> handled x86 horizontal add (see the FHADD X86 opcode).  If it turns
> out that the same thing is useful for other targets then it can be
> generalized later.
> 
> Ciao, Duncan.
> 
> On 10/18/11 00:40, Hal Finkel wrote:
> > How should I go about matching floating-point addsub-like vector
> > instructions? My first inclination is to write something which matches
> > build_vector 1.0, -1.0, and then use that in combination with a match on
> > fadd, but that does not seem to work. I think this is because
> > BUILD_VECTOR cannot ever be "Legal", and so it is always turned into a
> > constant load before instruction selection.
> >
> > Specifically, in SDValue SelectionDAGLegalize::LegalizeOp(SDValue Op):
> >
> > case ISD::BUILD_VECTOR:
> > // A weird case: legalization for BUILD_VECTOR never legalizes the
> > // operands!
> > // FIXME: This really sucks... changing it isn't semantically incorrect,
> > // but it massively pessimizes the code for floating-point BUILD_VECTORs
> > // because ConstantFP operands get legalized into constant pool loads
> > // before the BUILD_VECTOR code can see them.  It doesn't usually bite,
> > // though, because BUILD_VECTORS usually get lowered into other nodes
> > // which get legalized properly.
> > SimpleFinishLegalizing = false;
> > break;
> >
> > and then:
> >
> > case ISD::BUILD_VECTOR:
> >    switch (TLI.getOperationAction(ISD::BUILD_VECTOR,
> > Node->getValueType(0))) {
> >    default: assert(0&&  "This action is not supported yet!");
> >    case TargetLowering::Custom:
> >      Tmp3 = TLI.LowerOperation(Result, DAG);
> >      if (Tmp3.getNode()) {
> >        Result = Tmp3;
> >        break;
> >      }
> >      // FALLTHROUGH
> >    case TargetLowering::Expand:
> >      Result = ExpandBUILD_VECTOR(Result.getNode());
> >      break;
> >    }
> >    break;
> > (so there is not even branch for TargetLowering::Legal). Maybe I'm just
> > missing something.
> >
> > Thanks in advance,
> > Hal
> >
> 
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-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory
1-630-252-0023
hfinkel at anl.gov