OpenXM_contrib/gmp/mpn/generic/diveby3.c - diff

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Diff for /OpenXM_contrib/gmp/mpn/generic/Attic/diveby3.c between version 1.1 and 1.1.1.2

version 1.1, 2000/09/09 14:12:24

version 1.1.1.2, 2003/08/25 16:06:20

Line 1

/* mpn_divexact_by3 -- mpn division by 3, expecting no remainder. */

This file is part of the GNU MP Library.

Line 26 MA 02111-1307, USA.

#include "gmp-impl.h"

/* Multiplicative inverse of 3, modulo 2^BITS_PER_MP_LIMB.

0xAAAAAAAB for 32 bits, 0xAAAAAAAAAAAAAAAB for 64 bits. */

#define INVERSE_3 ((MP_LIMB_T_MAX / 3) * 2 + 1)

/* The "c += ..."s are adding the high limb of 3*l to c. That high limb

will be 0, 1 or 2. Doing two separate "+="s seems to turn out better

code on gcc (as of 2.95.2 at least).

When a subtraction of a 0,1,2 carry value causes a borrow, that leaves a

limb value of either 0xFF...FF or 0xFF...FE and the multiply by INVERSE_3

limb value of either 0xFF...FF or 0xFF...FE and the multiply by

gives 0x55...55 or 0xAA...AA respectively, producing a further borrow of

MODLIMB_INVERSE_3 gives 0x55...55 or 0xAA...AA respectively, producing a

only 0 or 1 respectively. Hence the carry out of each stage and for the

further borrow of only 0 or 1 respectively. Hence the carry out of each

return value is always only 0, 1 or 2. */

stage and for the return value is always only 0, 1 or 2.

This implementation has each multiply successively dependent due to the

"l=s-c", but the multiply src[i]*MODLIMB_INVERSE_3 could be scheduled

back as far as desired, and the effect of the "-c" applied by subtracting

0, 1 or 2 copies or MODLIMB_INVERSE_3 from the product. With some good

scheduling in assembler it might come down to a dependent chain of maybe

5 simple operations per limb. */

mp_limb_t

#if __STDC__

mpn_divexact_by3c (mp_ptr dst, mp_srcptr src, mp_size_t size, mp_limb_t c)

#else

mpn_divexact_by3c (dst, src, size, c)

mp_ptr dst;

mp_srcptr src;

mp_size_t size;

mp_limb_t c;

#endif

{

mp_size_t i;

ASSERT (size >= 1);

ASSERT (c == 0 || c == 1 || c == 2);

ASSERT (MPN_SAME_OR_SEPARATE_P (dst, src, size));

i = 0;

Line 65 mpn_divexact_by3c (dst, src, size, c)

Line 61 mpn_divexact_by3c (dst, src, size, c)

l = s - c;

c = (l > s);

l *= INVERSE_3;

l = (l * MODLIMB_INVERSE_3) & GMP_NUMB_MASK;

dst[i] = l;

c += (l > MP_LIMB_T_MAX/3);

c += (l > GMP_NUMB_MAX/3);

c += (l > (MP_LIMB_T_MAX/3)*2);

c += (l > (GMP_NUMB_MAX/3)*2);

}

while (++i < size);

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