A little warning in GMP.h

Torbjorn Granlund tege at swox.com
Sat Sep 25 00:59:24 CEST 2004

Brian Gladman <brg at gladman.plus.com> writes:

   >     return (__gmp_n != 0)?__gmp_l:0;
  which produces:
  mov         edx,dword ptr [__gmp_n]
  mov         ecx,dword ptr [__gmp_l]
  neg         edx
  sbb         edx,edx
  and         edx,ecx
  with Microsoft VC++ (optimiser on, fast code), whereas:
   >>>      return __gmp_l & (-(signed)(mp_limb_t) (__gmp_n != 0));
[This is an edited version of the original GMP code.  The edits
have made it incorrect.]
  mov         ecx,dword ptr [__gmp_n]
  xor         edx,edx
  test        ecx,ecx
  mov         ecx,dword ptr [__gmp_l]
  setne       dl
  neg         edx
  and         edx,ecx
Another reason to avoid the Microsoft compiler; its optimizer
cannot canonicalize equivalent trivial branch-free expressions.
It is as if it statically generates fixed code from known idioms,
but doesn't really perform algebraic equivalence tranformations.


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