1 /* Macro for explicit loop unrolling. 2 Copyright (C) 2019-2022 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