[llvm-commits] [llvm] r37851 - /llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

[Chris Lattner]

>>> +  // fold (or x, undef) -> -1
>>> +  if (N1.getOpcode() == ISD::UNDEF)
>>> +    return DAG.getConstant(-1, VT);
>>
>> Is this the right way to get an all-bits-one value?
>
> I guess DAG.getConstant(~0ULL, VT) is the way its written  
> elsewhere. And I
> have to fix it for vectors. I'll work on it.

Either way works.  "-1" is a signed integer that sign extends to  
uint64_t.  ~0ULL is more explicit though.

> Looking at it a little more, I think the instcombine folds are  
> actually
> wrong here. For example, it does X + undef -> undef, but that's  
> wrong if
> X is a NaN.

Ah, good point.

-Chris