hanchenye-llvm-project/clang/NOTES.txt

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// Random Notes
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Extensions:

 * "#define_target X Y"
   This preprocessor directive works exactly the same was as #define, but it
   notes that 'X' is a target-specific preprocessor directive.  When used, a
   diagnostic is emitted indicating that the translation unit is non-portable.
   
   If a target-define is #undef'd before use, no diagnostic is emitted.  If 'X'
   were previously a normal #define macro, the macro is tainted.  If 'X' is
   subsequently #defined as a non-target-specific define, the taint bit is
   cleared.
   
 * "#define_other_target X"
    The preprocessor directive takes a single identifier argument.  It notes
    that this identifier is a target-specific #define for some target other than
    the current one.  Use of this identifier will result in a diagnostic.
    
    If 'X' is later #undef'd or #define'd, the taint bit is cleared.  If 'X' is
    already defined, X is marked as a target-specific define. 

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To time GCC preprocessing speed without output, use:
   "time gcc -MM file"
This is similar to -Eonly.

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Interesting fact:
  clang -Eonly INPUTS/carbon-header-C-E.c

is much faster than:
  wc -w INPUTS/carbon-header-C-E.c
!!
   
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The 'portability' model in clang is sufficient to catch translation units (or
their parts) that are not portable, but it doesn't help if the system headers
are non-portable and not fixed.  An alternative model that would be easy to use
is a 'tainting' scheme.  Consider:

inline int foo() {
#ifdef __i386__
  return 1;
#else
  return 2;
#endif
}

It would be trivial to mark 'foo' as being non-portable (tainted) instead of
marking the entire translation unit.  Then, if foo is never called/used by the
current translation unit, the t-u wouldn't be marked non-portable.  However,
there is no good way to handle stuff like:

extern int X, Y;

#ifndef __POWERPC__
#define X Y
#endif

int bar() { return X; }

When compiling for powerpc, the #define is skipped, so it doesn't know that bar
uses a #define that is set on some other target.  In practice, limited cases
could be handled by scanning the skipped region of a #if, but the fully general
case cannot be implemented efficiently.

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