想要知道在 AIX 上，GCC 有沒有 predefine 一些 macro，好讓我針對 PowerPC/I386、Win32/FreeBSD/Linux/AIX 等不同平台，修改程式片段，以達到 cross-platform 的需求。可是，GCC 的官方文件這麼說：

The C preprocessor normally predefines several macros that indicate what type of system and machine is in use. They are obviously different on each target supported by GCC. This manual, being for all systems and machines, cannot tell you what their names are, but you can use cpp -dM to see them all.

意思是說，GCC 不會明白地在文件裡載明，會有哪些 predefined macros，請自己下指令試，然後猜猜看哪一個是你要找的。XD

所以我就用了下面的指令，列出了 AIX/GCC 的 predefined macros：

SHELL> echo | cpp -dM | sort
#define _AIX 1
#define _AIX32 1
#define _AIX41 1
#define _AIX43 1
#define _AIX51 1
#define _AIX52 1
#define _ARCH_PPC 1
#define _BIG_ENDIAN 1
#define _IBMR2 1
#define _LONG_LONG 1
#define _POWER 1
#define __BIG_ENDIAN__ 1
#define __CHAR_BIT__ 8
#define __CHAR_UNSIGNED__ 1
#define __DBL_DENORM_MIN__ 4.9406564584124654e-324
#define __DBL_DIG__ 15
#define __DBL_EPSILON__ 2.2204460492503131e-16
#define __DBL_HAS_DENORM__ 1
#define __DBL_HAS_INFINITY__ 1
#define __DBL_HAS_QUIET_NAN__ 1
#define __DBL_MANT_DIG__ 53
#define __DBL_MAX_10_EXP__ 308
#define __DBL_MAX_EXP__ 1024
#define __DBL_MAX__ 1.7976931348623157e+308
#define __DBL_MIN_10_EXP__ (-307)
#define __DBL_MIN_EXP__ (-1021)
#define __DBL_MIN__ 2.2250738585072014e-308
#define __DEC128_DEN__ 0.000000000000000000000000000000001E-6143DL
#define __DEC128_EPSILON__ 1E-33DL
#define __DEC128_MANT_DIG__ 34
#define __DEC128_MAX_EXP__ 6144
#define __DEC128_MAX__ 9.999999999999999999999999999999999E6144DL
#define __DEC128_MIN_EXP__ (-6143)
#define __DEC128_MIN__ 1E-6143DL
#define __DEC32_DEN__ 0.000001E-95DF
#define __DEC32_EPSILON__ 1E-6DF
#define __DEC32_MANT_DIG__ 7
#define __DEC32_MAX_EXP__ 96
#define __DEC32_MAX__ 9.999999E96DF
#define __DEC32_MIN_EXP__ (-95)
#define __DEC32_MIN__ 1E-95DF
#define __DEC64_DEN__ 0.000000000000001E-383DD
#define __DEC64_EPSILON__ 1E-15DD
#define __DEC64_MANT_DIG__ 16
#define __DEC64_MAX_EXP__ 384
#define __DEC64_MAX__ 9.999999999999999E384DD
#define __DEC64_MIN_EXP__ (-383)
#define __DEC64_MIN__ 1E-383DD
#define __DECIMAL_DIG__ 17
#define __DEC_EVAL_METHOD__ 2
#define __FINITE_MATH_ONLY__ 0
#define __FLT_DENORM_MIN__ 1.40129846e-45F
#define __FLT_DIG__ 6
#define __FLT_EPSILON__ 1.19209290e-7F
#define __FLT_EVAL_METHOD__ 0
#define __FLT_HAS_DENORM__ 1
#define __FLT_HAS_INFINITY__ 1
#define __FLT_HAS_QUIET_NAN__ 1
#define __FLT_MANT_DIG__ 24
#define __FLT_MAX_10_EXP__ 38
#define __FLT_MAX_EXP__ 128
#define __FLT_MAX__ 3.40282347e+38F
#define __FLT_MIN_10_EXP__ (-37)
#define __FLT_MIN_EXP__ (-125)
#define __FLT_MIN__ 1.17549435e-38F
#define __FLT_RADIX__ 2
#define __GNUC_GNU_INLINE__ 1
#define __GNUC_MINOR__ 2
#define __GNUC_PATCHLEVEL__ 4
#define __GNUC__ 4
#define __GXX_ABI_VERSION 1002
#define __INTMAX_MAX__ 9223372036854775807LL
#define __INTMAX_TYPE__ long long int
#define __INT_MAX__ 2147483647
#define __LDBL_DENORM_MIN__ 4.9406564584124654e-324L
#define __LDBL_DIG__ 15
#define __LDBL_EPSILON__ 2.2204460492503131e-16L
#define __LDBL_HAS_DENORM__ 1
#define __LDBL_HAS_INFINITY__ 1
#define __LDBL_HAS_QUIET_NAN__ 1
#define __LDBL_MANT_DIG__ 53
#define __LDBL_MAX_10_EXP__ 308
#define __LDBL_MAX_EXP__ 1024
#define __LDBL_MAX__ 1.7976931348623157e+308L
#define __LDBL_MIN_10_EXP__ (-307)
#define __LDBL_MIN_EXP__ (-1021)
#define __LDBL_MIN__ 2.2250738585072014e-308L
#define __LONG_LONG_MAX__ 9223372036854775807LL
#define __LONG_MAX__ 2147483647L
#define __NO_INLINE__ 1
#define __PTRDIFF_TYPE__ long int
#define __REGISTER_PREFIX__
#define __SCHAR_MAX__ 127
#define __SHRT_MAX__ 32767
#define __SIZE_TYPE__ long unsigned int
#define __STDC_HOSTED__ 1
#define __STDC__ 1
#define __UINTMAX_TYPE__ long long unsigned int
#define __USER_LABEL_PREFIX__
#define __VERSION__ "4.2.4"
#define __WCHAR_MAX__ 65535U
#define __WCHAR_TYPE__ short unsigned int
#define __WINT_TYPE__ int

嗯，_AIX、_ARCH_PPC 跟 _POWER 應該就是我要的，但還有沒有其他，就天曉得了。