Configurator: support both negative numbers and expressions in C imports

[emillon]

Rather than using the preprocessor to determine the sign of a value, we store both x and -x. One of them has an all-digit representation.

For example, 1234 is stored as "0000001234000000/.-," and -5678 is stored as "000000+*)(0000005678".

We might be able to do better if we could encode the sign and absolute value in the preprocessor, but it seems tricky to do so in a portable way.

(cc @Chris00 @avsm)

[Chris00]

Well, #1726 does not use the sign-detection by the preprocessor, just compile time evaluation of constant expressions.

[emillon]

Indeed! But this requires the constant to be evaluable by cpp, which is a stronger requirement than what it used to be.

[Chris00]

Are you sure the evaluation is by cpp? I would rather think the compiler does them.

[emillon]

You can see #1720 for a description of the problem, but yes the idea is that #if ... expressions are evaluated only by the C preprocessor. It has access to macros and constant evaluation, but does not have access to C features such as sizeof. You can find the relevant bits in the GCC documentation or in the C18 document in 6.10.1, "conditional inclusion".

[Chris00]

Please see #1726 (the PR I was referring to in my earlier post), there are no #if anymore.

[emillon]

Ah! My bad, I didn't see this one. I'll merge it and close this one. Sorry about the confusion!