[cfe-dev] Clang macro expansion differs from GCC

[Chris Lattner]

On Apr 8, 2010, at 8:34 AM, David Peixotto wrote:

> I have found a discrepancy in behavior between the clang preprocessor and gcc. 
> 
> $ cat t.c 
> #define X 0 .. 1
> 
> 0 .. 1 // Ok
> X      // Bad: space between ..
> 
> $ gcc -E t.c 
> <snip>
> 
> 0 .. 1
> 0 .. 1
> 
> $ clang -E t.c 
> <snip>
> 
> 0 .. 1
> 0 . . 1
> 
> The problem is that when ".." is in the macro and the macro gets expanded, it adds a space between the dots. As you can see, the space is not added when the ".." is entered directly in the text. GCC does not insert a space in either case. 
> 
> I'm not sure this qualifies as a bug, but the reason I need this to work is that I am trying to compile GHC (a Haskell compiler) with clang. It uses GCC to preprocess c-- files (http://www.cminusminus.org) before parsing them and ".." is a valid operator in c--. Adding the extra space messes up the subsequent parsing phase.

Hi David,

I'm not particularly interested in fixing this: people abusing the C preprocessor to preprocess things that aren't C don't get much sympathy from me.  However, if you are able to produce a patch that works, doesn't break anything else, and doesn't affect performance, I'd be happy to apply it.

> 
> I'd be happy to take a look at fixing this myself if someone could point me in the right direction.


lib/Frontend/PrintPreprocessedOutput.cpp is the code that outputs a .i file, the "AvoidConcat" stuff is what you want.  It is what decides to add spaces to prevent three . tokens from looking like a ... for example.

-Chris

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