[P4Testgen] Represent bits of width zero as an empty string.

[fruffy]

Do not return "something" when the bitwidth is zero. Instead handle this case separately. This fixes some problems when trying to test for 0 bit packets emitted in the packet test framework.

[fruffy]

Required for #4697. @vlstill Could you give this a review. This fixes some packet mismatches when generating PTF tests.

[vlstill]

This seems quite weird to me, formatting a number I would not expect to get an empty string. Especially in the case the base prefix is expected. How does an example output in PTF look (a snippet whith the empty packet suffices)? Would it make more sense to handle this special case in the PTF formatter only?

[fruffy]

This seems quite weird to me, formatting a number I would not expect to get an empty string. Especially in the case the base prefix is expected. How does an example output in PTF look (a snippet whith the empty packet suffices)? Would it make more sense to handle this special case in the PTF formatter only?

How do you represent a number with width zero and no value? "0x0" also does not seem correct to me and you can distinguish between an expression with width and one that has none. The empty string signals that you requested a value that returns nothing and you can process this as needed. I can also make this a std::optional<> to distinguish between errors and an empty return.

In PTF this would look like:

exp_pkt = Mask(b'')

In STF a packet that is 0 actually leads to garbled output (02000000). Because we set the expected packet top this value in STF the problem has been masked there. 0x0 also would be incorrect in STF because it is interpreted as a malformed 4 bit output.

[vlstill]

Hmm, that is really a bad design on the testing frameworks :-/, they really should take width with the packet (that is why in my backend, which uses a custom format, I always use the P4 sized notation for literals e.g. it would be 0w0b0). I guess this can't be really applied to existing frameworks though :-/.

[vlstill]

OK, let's go for this to make PTF work, even though it is not very nice.