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79 lines
2.9 KiB
C
79 lines
2.9 KiB
C
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/* Macro for explicit loop unrolling.
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Copyright (C) 2019 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#ifndef _LOOP_UNROLL_H
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#define _LOOP_UNROLL_H
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/* Loop unroll macro to be used for explicit force loop unrolling with a
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configurable number of iterations. The idea is to make the loop unrolling
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independent of whether the compiler is able to unroll through specific
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optimizations options (-funroll-loops or -funroll-all-loops).
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For instance, to implement strcpy with SRC being the source input and
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DEST the destination buffer, it is expected the macro to be used in this
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way:
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#define ITERATION(index) \
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({ char c = *str++; *dest++ = c; c != '\0' })
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while (1)
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UNROLL_REPEAT (4, ITERATION)
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The loop will be manually unrolled 4 times. Another option is to do
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the index update after the tests:
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#define ITERATION(index) \
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({ char c = *(str + index); *(dest + index) = c; c != '\0' })
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#define UPDATE(n) \
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str += n; dst += n
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while (1)
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UNROLL_REPEAT_UPDATE (4, ITERATION, UPDATE)
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The loop will be manually unrolled 4 times and the SRC and DEST pointers
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will be updated only after the last iteration.
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Currently, both macros unroll the loop 8 times at maximum. */
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#define UNROLL_REPEAT_1(X) if (!X(0)) break;
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#define UNROLL_REPEAT_2(X) UNROLL_REPEAT_1 (X) if (!X (1)) break;
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#define UNROLL_REPEAT_3(X) UNROLL_REPEAT_2 (X) if (!X (2)) break;
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#define UNROLL_REPEAT_4(X) UNROLL_REPEAT_3 (X) if (!X (3)) break;
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#define UNROLL_REPEAT_5(X) UNROLL_REPEAT_4 (X) if (!X (4)) break;
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#define UNROLL_REPEAT_6(X) UNROLL_REPEAT_5 (X) if (!X (5)) break;
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#define UNROLL_REPEAT_7(X) UNROLL_REPEAT_6 (X) if (!X (6)) break;
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#define UNROLL_REPEAT_8(X) UNROLL_REPEAT_7 (X) if (!X (7)) break;
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#define UNROLL_EXPAND(...) __VA_ARGS__
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#define UNROLL_REPEAT__(N, X) UNROLL_EXPAND(UNROLL_REPEAT_ ## N) (X)
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#define UNROLL_REPEAT_(N, X) UNROLL_REPEAT__ (N, X)
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#define UNROLL_REPEAT(N, X) \
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(void) ({ \
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UNROLL_REPEAT_ (UNROLL_EXPAND(N), X); \
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})
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#define UNROLL_REPEAT_UPDATE(N, X, U) \
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(void) ({ \
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UNROLL_REPEAT_ (UNROLL_EXPAND(N), X); \
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UPDATE (N); \
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})
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#endif
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