include/loop_unroll.h

   1 /* Macro for explicit loop unrolling.
   2    Copyright (C) 2019-2023 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