Add ImportC compiler to dmd

[Max Haughton]

On Friday, 14 May 2021 at 05:27:27 UTC, Walter Bright wrote:
> On 5/13/2021 2:04 PM, Iain Buclaw wrote:
>> Well, either one of, or two things will happen:
>> 
>> 1. People will raise bugs against dmd because `gdc foo.c` will 
>> be able to compile more code than dmd (and do preprocessing).
>> 
>> 2. People will raise bugs against gdc because `gdc -finline 
>> foo.c bar.d` doesn't inline C functions into bar.
>> 
>> I meant conflicts will happen if I change the current behavior 
>> so make it so the D compiler handles C sources too i.e: `d21 
>> bar.d foo.c -o bar.o`.
>
> Understood.
>
> It's pretty clear that ImportC is not going to be an exact 
> clone of gcc. I've been looking over the __attribute__ 
> documentation, and there doesn't even seem to be a proper 
> grammar for it.
>
> https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html#Attribute-Syntax
>
> There's some wishy-washy wording:
>
> "Where an attribute specifier is applied to a parameter 
> declared as a function or an array, it should apply to the 
> function or array rather than the pointer to which the 
> parameter is implicitly converted, but this is not yet 
> correctly implemented."
>
> "In some other cases, attribute specifiers are permitted by 
> this grammar but not yet supported by the compiler."
>
> "At present, ..."
>
> "An attribute specifier list may, in future, be permitted ..."
>
> "at present this is not implemented and they are ignored"
>
> "some laxity is allowed in the placing of attributes"
>
> which is discouraging. There are the other extensions, too.
>
> Ironically, the one gcc extension ImportC can support fully is 
> inline assembler :-) as you've already seen to that!
>
> The plan is to do as few gcc C extensions as practical.
>
> As for the user having options, that's why we have multiple D 
> compilers. ImportC isn't part of the D language, and so 
> differing flavors of C support is within reason.

Recognizing the syntax and doing nothing would he enough for 
quite a few libraries. For GNU stdio.h to work I had to clean out 
a bunch of attributes, asm, and any structs, but other than that 
it did seem to work.

The only other that I can say is that keep in mind that the 
blocker is using C not compiling C. I don't want dmd to be my C 
compiler - so you only need to understand GNU extensions that are 
used in header files (and don't appear in inline functions in 
those headers, but still).