Support casting of OpConstantNull to SPIRVConstant

[vmaksimo]

Current design does not imply inheritance between SPIRVConstantNull and SPIRVConstant classes, however it is possible that we do the static_cast of SPIRVValue to SPIRVConstant for null constant. Return zero-value in that case.

[vmaksimo]

Without the fix the test case fails on the validation of matrix type here: https://github.com/KhronosGroup/SPIRV-LLVM-Translator/blob/main/lib/SPIRV/libSPIRV/SPIRVType.cpp#L375
And without validation it does not fail, but translates zero as 43793728 here: https://github.com/KhronosGroup/SPIRV-LLVM-Translator/blob/main/lib/SPIRV/libSPIRV/SPIRVType.cpp#L375

[Naghasan]

nit

Suggested change

	if (OpCode == OpConstantNull)
	return 0;
	if (OpCode == OpConstantNull) {
	assert(std::is_arithmetic_v<T> && "OpConstantNull has no defined value for this type");
	return 0;
	}

[MrSidims]

I actually have another Q. In which contexts getValue is called? Can it be called to get value of an object of Event type or a Coop Matrix or any composite? If yes note, that ConstantNull is defined as:

Result Type must be one of the following types:
- Scalar or vector Boolean type
- Scalar or vector integer type
- Scalar or vector floating-point type
- Pointer type
- Event type
- Device side event type
- Reservation id type
- Queue type
- Composite type

hence here we should:
a. not add the proposed assetion;
b. return not null, but a value constructed by the T's default c'tor.

[MrSidims]

Actually, I got the answer: the method is protected and used only by:

  uint64_t getZExtIntValue() const { return getValue<uint64_t>(); }
  float getFloatValue() const { return getValue<float>(); }
  double getDoubleValue() const { return getValue<double>(); }

hence we can even make the proposed by Viktor assert to be a static_assert

[Naghasan]

Actually, I got the answer: the method is protected and used only by:

yeah, that's why I suggested the nit, I suggested an assert because the function is all constants not just null constant.