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/* Macro for explicit loop unrolling.
Copyright (C) 2019-2024 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library 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
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#ifndef _LOOP_UNROLL_H
#define _LOOP_UNROLL_H
/* Loop unroll macro to be used for explicit force loop unrolling with a
configurable number of iterations. The idea is to make the loop unrolling
independent of whether the compiler is able to unroll through specific
optimizations options (-funroll-loops or -funroll-all-loops).
For instance, to implement strcpy with SRC being the source input and
DEST the destination buffer, it is expected the macro to be used in this
way:
#define ITERATION(index) \
({ char c = *str++; *dest++ = c; c != '\0' })
while (1)
UNROLL_REPEAT (4, ITERATION)
The loop will be manually unrolled 4 times. Another option is to do
the index update after the tests:
#define ITERATION(index) \
({ char c = *(str + index); *(dest + index) = c; c != '\0' })
#define UPDATE(n) \
str += n; dst += n
while (1)
UNROLL_REPEAT_UPDATE (4, ITERATION, UPDATE)
The loop will be manually unrolled 4 times and the SRC and DEST pointers
will be updated only after the last iteration.
Currently, both macros unroll the loop 8 times at maximum. */
#define UNROLL_REPEAT_1(X) if (!X(0)) break;
#define UNROLL_REPEAT_2(X) UNROLL_REPEAT_1 (X) if (!X (1)) break;
#define UNROLL_REPEAT_3(X) UNROLL_REPEAT_2 (X) if (!X (2)) break;
#define UNROLL_REPEAT_4(X) UNROLL_REPEAT_3 (X) if (!X (3)) break;
#define UNROLL_REPEAT_5(X) UNROLL_REPEAT_4 (X) if (!X (4)) break;
#define UNROLL_REPEAT_6(X) UNROLL_REPEAT_5 (X) if (!X (5)) break;
#define UNROLL_REPEAT_7(X) UNROLL_REPEAT_6 (X) if (!X (6)) break;
#define UNROLL_REPEAT_8(X) UNROLL_REPEAT_7 (X) if (!X (7)) break;
#define UNROLL_EXPAND(...) __VA_ARGS__
#define UNROLL_REPEAT__(N, X) UNROLL_EXPAND(UNROLL_REPEAT_ ## N) (X)
#define UNROLL_REPEAT_(N, X) UNROLL_REPEAT__ (N, X)
#define UNROLL_REPEAT(N, X) \
(void) ({ \
UNROLL_REPEAT_ (UNROLL_EXPAND(N), X); \
})
#define UNROLL_REPEAT_UPDATE(N, X, U) \
(void) ({ \
UNROLL_REPEAT_ (UNROLL_EXPAND(N), X); \
UPDATE (N); \
})
#endif
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