A little warning in GMP.h
bhurt at spnz.org
Wed Sep 22 20:53:48 CEST 2004
On Wed, 22 Sep 2004, Francesco Montorsi wrote:
> return __gmp_l & (-(mp_limb_t) (__gmp_n != 0));
On a two's complement binary machine, this returns __gmp_l is __gmp_n !=
0, otherwise it returns 0.
How it works:
__gmp_n != 0 evaluates to 1 if __gmp_n != 0, or to 0 otherwise. Casting
the value to type mp_limb_t increases the precision to be a full limb.
Negating it converts the number to -1, or 0xFFF...FFF (assuming a
twos-complement machine), if __gmp_n != 0, or to 0 or 0x000...000 if
__gmp_n == 0. We then and this value with __gmp_l- if __gmp_n != 0, then
we're evaluation __gmp_l & 0xFFF...FFF, which comes out to __gmp_p. If
__gmp_n == 0, then we're evaluation __gmp_l & 0x000...000, which comes out
The reason for this trickiness is that it's turned an unpredictable branch
into a data dependency.
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