[ImportVerilog] Support parameter constants.

[angelzzzzz]

Create a new op NamedConstantOp for three elaboration-time constants: parameter, localparam, and specparam, which distinguishes 'parameter' from constants and variables.

[hailongSun2000]

If this op is approved. Maybe we should add ConstantLike and Pure to its trait list.

[fabianschuiki]

Cool! It's great to have an op to materialize the compile-time constants and preserve them. I don't think you can use ConstantLike here since the op takes a single operand, unlike ConstantOp which has no operands and just a single constant value attribute. Pure could work, but that would mean that if the named_constant result is unused (which I think it will be in a lot of cases, for example if the constant value was only used to parametrize types) then CSE/dead code elimination will delete the named_constant op again. That might not be what we want.

[hailongSun2000]

I get it! Looks like I misunderstod.

[hailongSun2000]

The OptionalTypesMatchWith is different from TypesMatchWith. It's just used to either lhs or rhs are optional. SameOperandsAndResultType is a better choice.

[fabianschuiki]

👍 for SameOperandsAndResultType 😃

[angelzzzzz]

Good idea! I will add SameOperandsAndResultType for it.

[fabianschuiki]

Cool! It's great to have an op to materialize the compile-time constants and preserve them. I don't think you can use ConstantLike here since the op takes a single operand, unlike ConstantOp which has no operands and just a single constant value attribute. Pure could work, but that would mean that if the named_constant result is unused (which I think it will be in a lot of cases, for example if the constant value was only used to parametrize types) then CSE/dead code elimination will delete the named_constant op again. That might not be what we want.

[fabianschuiki]

If there is no initializer, I don't think you can create a NamedConstantOp, because there is no value to pass to that constant. I think you can just skip/ignore parameters that have no assigned value. So something like:

Suggested change

	Value initial;
	if (const auto *init = paramNode.getInitializer()) {
	initial = context.convertExpression(*init);
	if (!initial)
	return failure();
	}
	const auto *init = paramNode.getInitializer();
	if (!init)
	return success(); // skip parameters without value
	Value initial = context.convertExpression(*init);
	if (!initial)
	return failure();

[fabianschuiki]

Same as above: you probably want to skip specparams without any value.

Suggested change

	Value initial;
	if (const auto *init = spNode.getInitializer()) {
	initial = context.convertExpression(*init);
	if (!initial)
	return failure();
	}
	const auto *init = spNode.getInitializer();
	if (!init)
	return success(); // skip parameters without value
	Value initial = context.convertExpression(*init);
	if (!initial)
	return failure();

[fabianschuiki]

It would be great to also test the code path where there is no value assigned to the parameter. To ensure those cases also work (and potentially don't produce any IR op).

[angelzzzzz]

Thoughtful idea! But this may not work. If no value is assigned to the parameter, Slang will throw an error that "error: parameter declaration is missing an initializer".

[angelzzzzz]

@fabianschuiki Thank you for your generous help! I have resolved the issues and look forward to hearing any other suggestions you may have.

[fabianschuiki]

Looks great to me! Thanks a lot for pushing this forward 🥳