arch/mips/include/asm/unroll.h

   1 /* SPDX-License-Identifier: GPL-2.0-only */
   2 #ifndef __ASM_UNROLL_H__
   3 #define __ASM_UNROLL_H__
   4
   5 /*
   6  * Explicitly unroll a loop, for use in cases where doing so is performance
   7  * critical.
   8  *
   9  * Ideally we'd rely upon the compiler to provide this but there's no commonly
  10  * available means to do so. For example GCC's "#pragma GCC unroll"
  11  * functionality would be ideal but is only available from GCC 8 onwards. Using
  12  * -funroll-loops is an option but GCC tends to make poor choices when
  13  * compiling our string functions. -funroll-all-loops leads to massive code
  14  * bloat, even if only applied to the string functions.
  15  */
  16 #define unroll(times, fn, ...) do {                             \
  17         extern void bad_unroll(void)                            \
  18                 __compiletime_error("Unsupported unroll");      \
  19                                                                 \
  20         /*                                                      \
  21          * We can't unroll if the number of iterations isn't    \
  22          * compile-time constant. Unfortunately GCC versions    \
  23          * up until 4.6 tend to miss obvious constants & cause  \
  24          * this check to fail, even though they go on to        \
  25          * generate reasonable code for the switch statement,   \
  26          * so we skip the sanity check for those compilers.     \
  27          */                                                     \
  28         BUILD_BUG_ON((CONFIG_GCC_VERSION >= 40700 ||            \
  29                       CONFIG_CLANG_VERSION >= 80000) &&         \
  30                      !__builtin_constant_p(times));             \
  31                                                                 \
  32         switch (times) {                                        \
  33         case 32: fn(__VA_ARGS__); /* fall through */            \
  34         case 31: fn(__VA_ARGS__); /* fall through */            \
  35         case 30: fn(__VA_ARGS__); /* fall through */            \
  36         case 29: fn(__VA_ARGS__); /* fall through */            \
  37         case 28: fn(__VA_ARGS__); /* fall through */            \
  38         case 27: fn(__VA_ARGS__); /* fall through */            \
  39         case 26: fn(__VA_ARGS__); /* fall through */            \
  40         case 25: fn(__VA_ARGS__); /* fall through */            \
  41         case 24: fn(__VA_ARGS__); /* fall through */            \
  42         case 23: fn(__VA_ARGS__); /* fall through */            \
  43         case 22: fn(__VA_ARGS__); /* fall through */            \
  44         case 21: fn(__VA_ARGS__); /* fall through */            \
  45         case 20: fn(__VA_ARGS__); /* fall through */            \
  46         case 19: fn(__VA_ARGS__); /* fall through */            \
  47         case 18: fn(__VA_ARGS__); /* fall through */            \
  48         case 17: fn(__VA_ARGS__); /* fall through */            \
  49         case 16: fn(__VA_ARGS__); /* fall through */            \
  50         case 15: fn(__VA_ARGS__); /* fall through */            \
  51         case 14: fn(__VA_ARGS__); /* fall through */            \
  52         case 13: fn(__VA_ARGS__); /* fall through */            \
  53         case 12: fn(__VA_ARGS__); /* fall through */            \
  54         case 11: fn(__VA_ARGS__); /* fall through */            \
  55         case 10: fn(__VA_ARGS__); /* fall through */            \
  56         case 9: fn(__VA_ARGS__); /* fall through */             \
  57         case 8: fn(__VA_ARGS__); /* fall through */             \
  58         case 7: fn(__VA_ARGS__); /* fall through */             \
  59         case 6: fn(__VA_ARGS__); /* fall through */             \
  60         case 5: fn(__VA_ARGS__); /* fall through */             \
  61         case 4: fn(__VA_ARGS__); /* fall through */             \
  62         case 3: fn(__VA_ARGS__); /* fall through */             \
  63         case 2: fn(__VA_ARGS__); /* fall through */             \
  64         case 1: fn(__VA_ARGS__); /* fall through */             \
  65         case 0: break;                                          \
  66                                                                 \
  67         default:                                                \
  68                 /*                                              \
  69                  * Either the iteration count is unreasonable   \
  70                  * or we need to add more cases above.          \
  71                  */                                             \
  72                 bad_unroll();                                   \
  73                 break;                                          \
  74         }                                                       \
  75 } while (0)
  76
  77 #endif /* __ASM_UNROLL_H__ */