rtlil: add Const::compress helper function

[phsauter]

If one has a Sigspec that is fully constant it is currently non-trivial to get its value.
The main blocking point here is that the SigSpec may be wider than 32bit (an integer).
However, this doesn't mean the value stored in it is necessarily this large/small.

Eg something like 33'd4 or 33'd-2 cannot be easily converted to an int.

This adds a compress(is_signed) function.
It tries to minimize the number of bits used for the representation of the number.
It does so by removing the leading bits while taking signedness into consideration.

With this function, the above can be successfully converted to an integer by first calling compress.

Edit:
Here is a possible application.
https://github.com/povik/yosys-slang/blob/28cea56840a6bc8a4b6398d36609500d5960ad79/src/builder.cc#L134
With compress it is easy to check if the actual value has a larger representation than 24-bits instead of checking the SigSpec, which again may not use the minimum width.
I am fairly convinced there are places in the Yosys codebase itself this would be helpful, finding them is a bit difficult.

[povik]

Thanks! Some details need fixing

[widlarizer]

What's the motivation for not respecting/setting flags & CONST_FLAG_SIGNED ?

[povik]

I am not sure we want to defer to that. That flag isn't set to a useful value in all contexts, especially when you convert a SigSpec with as_const(), which is the use case which prompted writing the helper.

[phsauter]

Should I also add a .as_int_compress(bool is_signed) to make it even easier to get a constant integer?

[povik]

For that one to be useful you need to know when you can trust the result and when not, so maybe it should return a pair of int, bool. If it can fail, then we may call it try_int.

[widlarizer]

That's what std::optional is for. What could fail?

[phsauter]

If it is larger than 32-bit then it fails as it is not representable but yes, std::optional is a good option there.

[phsauter]

Does this solution look reasonable?

[widlarizer]

As far as I can tell, it does