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1 | /* Macro for explicit loop unrolling. |
2 | Copyright (C) 2019 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 | <http://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 |