/* mpn_divexact_by3 -- mpn division by 3, expecting no remainder. */
Copyright 2000, 2001, 2002 Free Software Foundation, Inc.
This file is part of the GNU MP Library.
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/* 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
MODLIMB_INVERSE_3 gives 0x55...55 or 0xAA...AA respectively, producing a
further borrow of only 0 or 1 respectively. Hence the carry out of each
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. */
mpn_divexact_by3c (mp_ptr dst, mp_srcptr src, mp_size_t size, mp_limb_t c)
ASSERT (size >= 1);
ASSERT (c == 0 || c == 1 || c == 2);
ASSERT (MPN_SAME_OR_SEPARATE_P (dst, src, size));
i = 0;
mp_limb_t l, s;
s = src[i];
l = s - c;
c = (l > s);
l = (l * MODLIMB_INVERSE_3) & GMP_NUMB_MASK;
dst[i] = l;
c += (l > GMP_NUMB_MAX/3);
c += (l > (GMP_NUMB_MAX/3)*2);
while (++i < size);