[VP] Fix VPintrinsic::getStaticVectorLength for vp.merge|select

[simoll]

Summary

VPIntrinsic::getStaticVectorLength infers the operational vector length of a VPIntrinsic instance from a type that is used with the intrinsic. The function used the mask operand before. Yet, vp.merge|select do not have a mask operand (in the predicating sense that the other VP intrinsics are using them - it is a selection mask for them). Fallback to the return type to fix this.

[kaz7]

llvm/lib/IR/IntrinsicInst.cpp

302–308

Please add assert in then closure to see whether it is vp.select or vp.merge.
Also please add comments to describe why vp.select and vp.merge don't have a mask operand here for future references.

[simoll]

Document that vp.merge, vp.select do not have a VP mask and why.

[kaz7]

Thank you. LGTM.

llvm/include/llvm/IR/VPIntrinsics.def

HELPER_REGISTER_REDUCTION_SEQ_VP(vp_reduce_fmul, VP_REDUCE_FMUL,
                                  experimental_vector_reduce_fmul)
 
 #undef HELPER_REGISTER_REDUCTION_SEQ_VP
 
 ///// } Reduction
 
 ///// Shuffles {
 
+// The mask 'cond' operand of llvm.vp.select and llvm.vp.merge are not reported
+// as masks with the BEGIN_REGISTER_VP_* macros.  This is because, unlike other
+// VP intrinsics, these two have a defined result on lanes where the mask is
+// false.
+//
 // llvm.vp.select(cond,on_true,on_false,vlen)
 BEGIN_REGISTER_VP(vp_select, None, 3, VP_SELECT, -1)
 VP_PROPERTY_FUNCTIONAL_OPC(Select)
 END_REGISTER_VP(vp_select, VP_SELECT)
 
 // llvm.vp.merge(cond,on_true,on_false,pivot)
 BEGIN_REGISTER_VP(vp_merge, None, 3, VP_MERGE, -1)
 END_REGISTER_VP(vp_merge, VP_MERGE)

llvm/lib/IR/IntrinsicInst.cpp

 ElementCount VPIntrinsic::getStaticVectorLength() const {
   auto GetVectorLengthOfType = [](const Type *T) -> ElementCount {
     const auto *VT = cast<VectorType>(T);
     auto ElemCount = VT->getElementCount();
     return ElemCount;
   };
 
   Value *VPMask = getMaskParam();
-  assert(VPMask && "No mask param?");
+  if (!VPMask) {
+    assert((getIntrinsicID() == Intrinsic::vp_merge ||
+            getIntrinsicID() == Intrinsic::vp_select) &&
+           "Unexpected VP intrinsic without mask operand");
+    return GetVectorLengthOfType(getType());
+  }
   return GetVectorLengthOfType(VPMask->getType());
 }
 
 Value *VPIntrinsic::getMaskParam() const {
   if (auto MaskPos = getMaskParamPos(getIntrinsicID()))
     return getArgOperand(MaskPos.getValue());
   return nullptr;
 }