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jyn514 / saltwater

A C compiler written in Rust, with a focus on good error messages.
BSD 3-Clause "New" or "Revised" License
294 stars 27 forks source link

Close #453. Builtin macros such as `__LINE__` (not function macros like `__has_header()`) #492

Closed hdamron17 closed 4 years ago

hdamron17 commented 4 years ago

This will provide the functionality to define builtin macros. Those in #453 and __has_header() will be implemented as proof of concept.

hdamron17 commented 4 years ago

The __has_header macro is only usable in a #if expression so I think I'll need to fix #449 first.

jyn514 commented 4 years ago

@hdamron17 interested in picking this back up now that #449 is fixed?

hdamron17 commented 4 years ago

@hdamron17 interested in picking this back up now that #449 is fixed?

I'm still thinking through how everything will tie together, especially the function macros (others are easy and I could switch this to only do those if you want).

jyn514 commented 4 years ago

Let's do only object macros for now yeah. That will hit the vast majority of use cases.

hdamron17 commented 4 years ago

The standard says "None of these macro names, nor the identifier defined, shall be the subject of a #define or a #undef preprocessing directive" (6.10.8p2). In clang, it is a warning and then successfully redefines. For us, any of the constantly redefined macros (like __LINE__), this would override any user redefinition. Is this something we care about since the standard says no?

jyn514 commented 4 years ago

'shall not' means this is a constraint on the user, not on the compiler, so technically the behavior is undefined. It would be friendly to give a warning but I'm fine having it do unexpected things since this is a really bad idea anyway.

hdamron17 commented 4 years ago

I think it would be hard to maintain giving a warning since we don't differentiate the builtins anywhere. I'll leave it such that __LINE__ and __FILE__ cannot be replaced but the others can (although none of them should).

hdamron17 commented 4 years ago

Interestingly, clang doesn't define __STDC_IEC_559__.

hdamron17 commented 4 years ago

Also, according to GCC, "GCC does not define __STDC_IEC_559__ or implement the associated standard pragmas"

hdamron17 commented 4 years ago

Even though GCC says it doesn't, __STDC_IEC_559__ is defined as 1 on my Ubuntu machine. On Mac with clang, nothing.

hdamron17 commented 4 years ago

Just waiting on __STDC_IEC_559__ now.

r? @jyn514