IRBuilder has a method CreateNot which may invoke BinaryOperator::CreateNot. However there isn’t actually a
not operation in LLVM IR so BinaryOperator::CreateNot acts as a helper by creating a
xor against a suitable constant of all ones, the exact nature of such constant depends on the operand and may end being a (fixed-size) vector.
However the creation of the “all ones” constant is not feasible when the operand we want to
not is a scalable vector (vector constants are described by extension).
I guess we can change IRBuilder::CreateNot to check this case and emit the LLVM IR we customarily use for splats via IRBuilder::CreateVectorSplat so we get a value of a scalable vector of “all ones”.
However, a cursory
grep reveals a few direct users of BinaryOperator::CreateNot (note that this is a pessimistic stance as not all of them may end being invoked on vectors) so we might want to do better here.
I don’t have a good solution here but I will assume that introducing a “splatted constant” in LLVM IR is not desirable.
One option may be asserting in BinaryOperator::CreateNot (and other helpers impacted by the same issue). This seems a bit hostile for users of BinaryOperator.
Maybe move around the code in IRBuilder::CreateVectorSplat so it is useable outside IRBuilder and then use it in BinaryOperator::CreateNot?
Perhaps there are better alternatives already used in practice for those working with scalable vectors?
Thanks a lot,