1/* mpn_mul -- Multiply two natural numbers.
2
3Copyright (C) 1991-2021 Free Software Foundation, Inc.
4
5This file is part of the GNU MP Library.
6
7The GNU MP Library is free software; you can redistribute it and/or modify
8it under the terms of the GNU Lesser General Public License as published by
9the Free Software Foundation; either version 2.1 of the License, or (at your
10option) any later version.
11
12The GNU MP Library is distributed in the hope that it will be useful, but
13WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
14or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
15License for more details.
16
17You should have received a copy of the GNU Lesser General Public License
18along with the GNU MP Library; see the file COPYING.LIB. If not, see
19<https://www.gnu.org/licenses/>. */
20
21#include <gmp.h>
22#include "gmp-impl.h"
23
24/* Multiply the natural numbers u (pointed to by UP, with USIZE limbs)
25 and v (pointed to by VP, with VSIZE limbs), and store the result at
26 PRODP. USIZE + VSIZE limbs are always stored, but if the input
27 operands are normalized. Return the most significant limb of the
28 result.
29
30 NOTE: The space pointed to by PRODP is overwritten before finished
31 with U and V, so overlap is an error.
32
33 Argument constraints:
34 1. USIZE >= VSIZE.
35 2. PRODP != UP and PRODP != VP, i.e. the destination
36 must be distinct from the multiplier and the multiplicand. */
37
38/* If KARATSUBA_THRESHOLD is not already defined, define it to a
39 value which is good on most machines. */
40#ifndef KARATSUBA_THRESHOLD
41#define KARATSUBA_THRESHOLD 32
42#endif
43
44mp_limb_t
45mpn_mul (mp_ptr prodp,
46 mp_srcptr up, mp_size_t usize,
47 mp_srcptr vp, mp_size_t vsize)
48{
49 mp_ptr prod_endp = prodp + usize + vsize - 1;
50 mp_limb_t cy;
51 mp_ptr tspace;
52 TMP_DECL (marker);
53
54 if (vsize < KARATSUBA_THRESHOLD)
55 {
56 /* Handle simple cases with traditional multiplication.
57
58 This is the most critical code of the entire function. All
59 multiplies rely on this, both small and huge. Small ones arrive
60 here immediately. Huge ones arrive here as this is the base case
61 for Karatsuba's recursive algorithm below. */
62 mp_size_t i;
63 mp_limb_t cy_limb;
64 mp_limb_t v_limb;
65
66 if (vsize == 0)
67 return 0;
68
69 /* Multiply by the first limb in V separately, as the result can be
70 stored (not added) to PROD. We also avoid a loop for zeroing. */
71 v_limb = vp[0];
72 if (v_limb <= 1)
73 {
74 if (v_limb == 1)
75 MPN_COPY (prodp, up, usize);
76 else
77 MPN_ZERO (prodp, usize);
78 cy_limb = 0;
79 }
80 else
81 cy_limb = mpn_mul_1 (prodp, up, usize, v_limb);
82
83 prodp[usize] = cy_limb;
84 prodp++;
85
86 /* For each iteration in the outer loop, multiply one limb from
87 U with one limb from V, and add it to PROD. */
88 for (i = 1; i < vsize; i++)
89 {
90 v_limb = vp[i];
91 if (v_limb <= 1)
92 {
93 cy_limb = 0;
94 if (v_limb == 1)
95 cy_limb = mpn_add_n (prodp, prodp, up, usize);
96 }
97 else
98 cy_limb = mpn_addmul_1 (prodp, up, usize, v_limb);
99
100 prodp[usize] = cy_limb;
101 prodp++;
102 }
103 return cy_limb;
104 }
105
106 TMP_MARK (marker);
107
108 tspace = (mp_ptr) TMP_ALLOC (2 * vsize * BYTES_PER_MP_LIMB);
109 MPN_MUL_N_RECURSE (prodp, up, vp, vsize, tspace);
110
111 prodp += vsize;
112 up += vsize;
113 usize -= vsize;
114 if (usize >= vsize)
115 {
116 mp_ptr tp = (mp_ptr) TMP_ALLOC (2 * vsize * BYTES_PER_MP_LIMB);
117 do
118 {
119 MPN_MUL_N_RECURSE (tp, up, vp, vsize, tspace);
120 cy = mpn_add_n (prodp, prodp, tp, vsize);
121 mpn_add_1 (prodp + vsize, tp + vsize, vsize, cy);
122 prodp += vsize;
123 up += vsize;
124 usize -= vsize;
125 }
126 while (usize >= vsize);
127 }
128
129 /* True: usize < vsize. */
130
131 /* Make life simple: Recurse. */
132
133 if (usize != 0)
134 {
135 mpn_mul (tspace, vp, vsize, up, usize);
136 cy = mpn_add_n (prodp, prodp, tspace, vsize);
137 mpn_add_1 (prodp + vsize, tspace + vsize, usize, cy);
138 }
139
140 TMP_FREE (marker);
141 return *prod_endp;
142}
143