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:

int32_t
OSHostByteOrder(void) {
#if defined(__LITTLE_ENDIAN__)
    return OSLittleEndian;
#elif defined(__BIG_ENDIAN__)
    return OSBigEndian;
#else
    return OSUnknownByteOrder;
#endif
}

It would be trivial to mark 'OSHostByteOrder' as being non-portable (tainted)
instead of marking the entire translation unit.  Then, if OSHostByteOrder 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.  In particular, code like this (from
OSByteOrder.h):

  #if (defined(__ppc__) || defined(__ppc64__))
  #include <libkern/ppc/OSByteOrder.h>
  #elif (defined(__i386__) || defined(__x86_64__))
  #include <libkern/i386/OSByteOrder.h>
  #else
  #include <libkern/machine/OSByteOrder.h>
  #endif

... should be fixed by having an initial #ifdef __llvm__ that defines its
contents in terms of the llvm bswap intrinsics.  Other things should be handled
on a case-by-case basis.


We probably have to do something smarter like this in the future. The C++ header
<limits> contains a lot of code like this:

   static const int digits10 = __LDBL_DIG__;
   static const int min_exponent = __LDBL_MIN_EXP__;
   static const int min_exponent10 = __LDBL_MIN_10_EXP__;
   static const float_denorm_style has_denorm
     = bool(__LDBL_DENORM_MIN__) ? denorm_present : denorm_absent;

 ... since this isn't being used in an #ifdef, it should be easy enough to taint
the decl for these ivars.


/usr/include/sys/cdefs.h contains stuff like this:

#if defined(__ppc__)
#  if defined(__LDBL_MANT_DIG__) && defined(__DBL_MANT_DIG__) && \
	__LDBL_MANT_DIG__ > __DBL_MANT_DIG__
#    if __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__-0 < 1040
#      define	__DARWIN_LDBL_COMPAT(x)	__asm("_" __STRING(x) "$LDBLStub")
#    else
#      define	__DARWIN_LDBL_COMPAT(x)	__asm("_" __STRING(x) "$LDBL128")
#    endif
#    define	__DARWIN_LDBL_COMPAT2(x) __asm("_" __STRING(x) "$LDBL128")
#    define	__DARWIN_LONG_DOUBLE_IS_DOUBLE	0
#  else
#   define	__DARWIN_LDBL_COMPAT(x) /* nothing */
#   define	__DARWIN_LDBL_COMPAT2(x) /* nothing */
#   define	__DARWIN_LONG_DOUBLE_IS_DOUBLE	1
#  endif
#elif defined(__i386__) || defined(__ppc64__) || defined(__x86_64__)
#  define	__DARWIN_LDBL_COMPAT(x)	/* nothing */
#  define	__DARWIN_LDBL_COMPAT2(x) /* nothing */
#  define	__DARWIN_LONG_DOUBLE_IS_DOUBLE	0
#else
#  error Unknown architecture
#endif

An ideal way to solve this issue is to mark __DARWIN_LDBL_COMPAT / 
__DARWIN_LDBL_COMPAT2 / __DARWIN_LONG_DOUBLE_IS_DOUBLE as being non-portable
because they depend on non-portable macros.

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