1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
|
/* Source code for RCC-Legacy.
Copyright (C) 2026 Richard Qin
This file is part of RCC.
RCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 3, or (at your option) any later
version.
RCC is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
for more details.
You should have received a copy of the GNU General Public License
along with RCC; see the file LICENSE. If not see
<http://www.gnu.org/licenses/>. */
/// @file mulopt.cc
/// @brief Constant integer multiplication strength reduction.
///
/// See docs/mulopt.md for architectural notes.
#include "rcc/all.h"
#include <cstdint>
namespace rcc {
namespace {
enum Kind {
KZero,
KCopy,
KShl,
KShlAddShl,
KShlSubShl,
};
struct Rewrite {
Kind kind;
int sh1; // Primary shift for KShl / KShlAddShl / KShlSubShl
int sh2; // Secondary shift for KShlAddShl / KShlSubShl (0 means use x itself)
};
// Classify patterns.
bool classify(uint64_t ac, int max_shift, Rewrite& out) {
if (ac == 0) {
out.kind = KZero;
return true;
}
if (ac == 1) {
out.kind = KCopy;
return true;
}
if ((ac & (ac - 1)) == 0) {
int k = __builtin_ctzll(ac);
if (k >= max_shift)
return false;
out.kind = KShl;
out.sh1 = k;
return true;
}
int a = 63 - __builtin_clzll(ac); // floor log2(ac), 0 < ac, ac is not pow2
if (a >= max_shift)
return false;
uint64_t hi = static_cast<uint64_t>(1) << a;
// pow2 + pow2.
uint64_t low = ac - hi;
if ((low & (low - 1)) == 0) {
int b = __builtin_ctzll(low);
out.kind = KShlAddShl;
out.sh1 = a;
out.sh2 = b;
return true;
}
// pow2 - pow2.
if (a + 1 < max_shift) {
uint64_t hi2 = hi << 1;
uint64_t diff = hi2 - ac;
// Check diff.
if ((diff & (diff - 1)) == 0) {
int b = __builtin_ctzll(diff);
out.kind = KShlSubShl;
out.sh1 = a + 1;
out.sh2 = b;
return true;
}
}
return false;
}
// emit() prepends.
bool lower_mul(Ctx* ctx, Fn* fn, Ins* i, Blk* b, bool* rebuilt) {
if (i->op != Omul)
return false;
if (i->cls != Kw && i->cls != Kl)
return false;
if (rtype(i->arg[1]) != RCon)
return false;
Con* c = &fn->con[i->arg[1].val];
if (c->type != Con::CBits)
return false;
int cls = i->cls;
int64_t cv;
int max_shift;
if (cls == Kw) {
cv = static_cast<int32_t>(
c->bits.i); // Narrow to i32 to avoid interference from Kl residual bits
max_shift = 32;
} else {
cv = c->bits.i;
max_shift = 64;
}
bool neg = (cv < 0);
uint64_t ac = static_cast<uint64_t>(cv);
if (neg)
ac = -ac; // wrap-around produces 2^31 / 2^63 for INT_MIN / INT64_MIN respectively
Rewrite r;
if (!classify(ac, max_shift, r))
return false;
Ref dst = i->to;
Ref x = i->arg[0];
// Move unprocessed tail.
if (!*rebuilt) {
ctx->curi = ctx->insb + NIns;
ulong tail = static_cast<ulong>((b->ins + b->nins) - (i + 1));
ctx->curi -= tail;
icpy(ctx->curi, i + 1, tail);
*rebuilt = true;
}
switch (r.kind) {
case KZero:
// 0 * x = 0.
emit(ctx, Ocopy, cls, dst, getcon(ctx, 0, fn), Ref::none());
break;
case KCopy:
if (neg)
emit(ctx, Oneg, cls, dst, x, Ref::none());
else
emit(ctx, Ocopy, cls, dst, x, Ref::none());
break;
case KShl: {
if (neg) {
Ref t = newtmp(ctx, "mulopt", cls, fn);
emit(ctx, Oneg, cls, dst, t, Ref::none());
emit(ctx, Oshl, cls, t, x, getcon(ctx, r.sh1, fn));
} else {
emit(ctx, Oshl, cls, dst, x, getcon(ctx, r.sh1, fn));
}
break;
}
case KShlAddShl: {
// sum.
Ref sum = neg ? newtmp(ctx, "mulopt", cls, fn) : dst;
Ref t1 = newtmp(ctx, "mulopt", cls, fn);
Ref t2 = (r.sh2 == 0) ? x : newtmp(ctx, "mulopt", cls, fn);
if (neg)
emit(ctx, Oneg, cls, dst, sum, Ref::none());
emit(ctx, Oadd, cls, sum, t1, t2);
if (r.sh2 != 0)
emit(ctx, Oshl, cls, t2, x, getcon(ctx, r.sh2, fn));
emit(ctx, Oshl, cls, t1, x, getcon(ctx, r.sh1, fn));
break;
}
case KShlSubShl: {
Ref diff = neg ? newtmp(ctx, "mulopt", cls, fn) : dst;
Ref t1 = newtmp(ctx, "mulopt", cls, fn);
Ref t2 = (r.sh2 == 0) ? x : newtmp(ctx, "mulopt", cls, fn);
if (neg)
emit(ctx, Oneg, cls, dst, diff, Ref::none());
emit(ctx, Osub, cls, diff, t1, t2);
if (r.sh2 != 0)
emit(ctx, Oshl, cls, t2, x, getcon(ctx, r.sh2, fn));
emit(ctx, Oshl, cls, t1, x, getcon(ctx, r.sh1, fn));
break;
}
}
return true;
}
} // namespace
void mulopt(Ctx* ctx, Fn* fn) {
for (Blk* b = fn->start; b; b = b->link) {
bool rebuilt = false;
for (Ins* i = b->ins + b->nins; i != b->ins;) {
--i;
if (lower_mul(ctx, fn, i, b, &rebuilt))
continue;
if (rebuilt)
emiti(ctx, *i);
}
if (rebuilt)
idup(ctx, b, ctx->curi, static_cast<ulong>(ctx->insb + NIns - ctx->curi));
}
}
} // namespace rcc
|
