1/* Macro for explicit loop unrolling.
2 Copyright (C) 2019-2020 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <https://www.gnu.org/licenses/>. */
18
19#ifndef _LOOP_UNROLL_H
20#define _LOOP_UNROLL_H
21
22/* Loop unroll macro to be used for explicit force loop unrolling with a
23 configurable number of iterations. The idea is to make the loop unrolling
24 independent of whether the compiler is able to unroll through specific
25 optimizations options (-funroll-loops or -funroll-all-loops).
26
27 For instance, to implement strcpy with SRC being the source input and
28 DEST the destination buffer, it is expected the macro to be used in this
29 way:
30
31 #define ITERATION(index) \
32 ({ char c = *str++; *dest++ = c; c != '\0' })
33
34 while (1)
35 UNROLL_REPEAT (4, ITERATION)
36
37 The loop will be manually unrolled 4 times. Another option is to do
38 the index update after the tests:
39
40 #define ITERATION(index) \
41 ({ char c = *(str + index); *(dest + index) = c; c != '\0' })
42 #define UPDATE(n) \
43 str += n; dst += n
44
45 while (1)
46 UNROLL_REPEAT_UPDATE (4, ITERATION, UPDATE)
47
48 The loop will be manually unrolled 4 times and the SRC and DEST pointers
49 will be updated only after the last iteration.
50
51 Currently, both macros unroll the loop 8 times at maximum. */
52
53#define UNROLL_REPEAT_1(X) if (!X(0)) break;
54#define UNROLL_REPEAT_2(X) UNROLL_REPEAT_1 (X) if (!X (1)) break;
55#define UNROLL_REPEAT_3(X) UNROLL_REPEAT_2 (X) if (!X (2)) break;
56#define UNROLL_REPEAT_4(X) UNROLL_REPEAT_3 (X) if (!X (3)) break;
57#define UNROLL_REPEAT_5(X) UNROLL_REPEAT_4 (X) if (!X (4)) break;
58#define UNROLL_REPEAT_6(X) UNROLL_REPEAT_5 (X) if (!X (5)) break;
59#define UNROLL_REPEAT_7(X) UNROLL_REPEAT_6 (X) if (!X (6)) break;
60#define UNROLL_REPEAT_8(X) UNROLL_REPEAT_7 (X) if (!X (7)) break;
61
62#define UNROLL_EXPAND(...) __VA_ARGS__
63
64#define UNROLL_REPEAT__(N, X) UNROLL_EXPAND(UNROLL_REPEAT_ ## N) (X)
65#define UNROLL_REPEAT_(N, X) UNROLL_REPEAT__ (N, X)
66
67#define UNROLL_REPEAT(N, X) \
68 (void) ({ \
69 UNROLL_REPEAT_ (UNROLL_EXPAND(N), X); \
70 })
71
72#define UNROLL_REPEAT_UPDATE(N, X, U) \
73 (void) ({ \
74 UNROLL_REPEAT_ (UNROLL_EXPAND(N), X); \
75 UPDATE (N); \
76 })
77
78#endif
79