Some compilers with hacks:
====================================
- GNUC
- DEC C
- SCO OpenUNIX 8 cc
- __cplusplus

Useful/undocumented macros in gmp.h:
====================================
__GMPN_ABS(x) - absolute value
__GMPN_MAX(h, i) - max of h and i

__GMPN_UINT_MAX - (int) ~0
__GMPN_ULONG_MAX - (unsigned long) ~0
__GMPN_USHRT_MAX - unsigned short) ~0

__GMPN_LIKELY(cond) - branch likely hint
__GMPN_UNLIKELY(cond) - branch unlikely hint
  - e.g. if UNLIKELY(c >= 0L)....

Note: the __GMPN_ macros are redefined with nicer names in gmp-impl.h

mp_bits_per_limb - number of bits per limb
gmp_version - GMP version number i.j or i.j.k as a null terminated string
gmp_errno - integer corresponding to a GMP error condition:
  0 none
  1 unsupported argument
  2 division by zero
  4 sqrt of negative
  8 invalid argument

Undocumented mpn functions prototyped in gmp.h:
===============================================
mp_limb_t mpn_preinv_mod_1(mp_srcptr sp, mp_size_t sn, mp_limb_t x, mp_limb_t xinv) - 
  compute {sp, sn} mod x given a precomputed inverse xinv of x