[SVE] Remove invalid usage of VectorType::getNumElements in Function

[ctetreau]

Summary

Removes usage of VectorType::getNumElements identified by test located
at CodeGen/aarch64-sve-intrinsics/acle_sve_dot.c. This code explicitly
converts a potentially fixed length vector to scalable vector by
constructing the ElementCount = {getNumElements(), true}

[ctetreau]

llvm/lib/IR/Function.cpp

1079

This behavior was introduced in D65930. It seems to me that this would have just returned the decoded type if it were expected that the decoded type would be scalable.

[efriedma]

llvm/lib/IR/Function.cpp

1080

It's intentional. See EncodeFixedType in llvm/utils/TableGen/IntrinsicEmitter.cpp . Actually, Ty should always be a FixedVectorType here, I think; maybe we can just cast<FixedVectorType> here?

[ctetreau]

address code review issues

[ctetreau]

llvm/lib/IR/Function.cpp

1080

It's not super clear to me because the types and values don't line up exactly, but it seems like this all has to do with serialization, and in the stream there's something like:

VALUE    -> PREFIX SUFFIX
PREFIX   -> SCALABLE |
SCALABLE -> [value that specifies if the following is actually a scalable vector]
SUFFIX   -> [value for a fixed width vector]

... seems like a bit of an abuse to me, but doing anything about it would involve changing the data layout.

[efriedma]

LGTM

llvm/lib/IR/Function.cpp

case IITDescriptor::VecOfBitcastsToInt: {
     VectorType *VTy = dyn_cast<VectorType>(Ty);
     assert(VTy && "Expected an argument of Vector Type");
     return VectorType::getInteger(VTy);
   }
   case IITDescriptor::VecOfAnyPtrsToElt:
     // Return the overloaded type (which determines the pointers address space)
     return Tys[D.getOverloadArgNumber()];
   case IITDescriptor::ScalableVecArgument: {
-    auto *Ty = cast<VectorType>(DecodeFixedType(Infos, Tys, Context));
-    return VectorType::get(Ty->getElementType(), {Ty->getNumElements(), true});
+    auto *Ty = cast<FixedVectorType>(DecodeFixedType(Infos, Tys, Context));
+    return ScalableVectorType::get(Ty->getElementType(), Ty->getNumElements());
   }
   }
   llvm_unreachable("unhandled");
 }
 
 FunctionType *Intrinsic::getType(LLVMContext &Context,
                                  ID id, ArrayRef<Type*> Tys) {
   SmallVector<IITDescriptor, 8> Table;
   getIntrinsicInfoTableEntries(id, Table);