compiler-rt/lib/builtins/clzsi2.c

   1 //===-- clzsi2.c - Implement __clzsi2 -------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements __clzsi2 for the compiler_rt library.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "int_lib.h"
  14
  15 // Returns: the number of leading 0-bits
  16
  17 // Precondition: a != 0
  18
  19 COMPILER_RT_ABI int __clzsi2(si_int a) {
  20   su_int x = (su_int)a;
  21   si_int t = ((x & 0xFFFF0000) == 0) << 4; // if (x is small) t = 16 else 0
  22   x >>= 16 - t;                            // x = [0 - 0xFFFF]
  23   su_int r = t;                            // r = [0, 16]
  24   // return r + clz(x)
  25   t = ((x & 0xFF00) == 0) << 3;
  26   x >>= 8 - t; // x = [0 - 0xFF]
  27   r += t;      // r = [0, 8, 16, 24]
  28   // return r + clz(x)
  29   t = ((x & 0xF0) == 0) << 2;
  30   x >>= 4 - t; // x = [0 - 0xF]
  31   r += t;      // r = [0, 4, 8, 12, 16, 20, 24, 28]
  32   // return r + clz(x)
  33   t = ((x & 0xC) == 0) << 1;
  34   x >>= 2 - t; // x = [0 - 3]
  35   r += t;      // r = [0 - 30] and is even
  36   // return r + clz(x)
  37   //     switch (x)
  38   //     {
  39   //     case 0:
  40   //         return r + 2;
  41   //     case 1:
  42   //         return r + 1;
  43   //     case 2:
  44   //     case 3:
  45   //         return r;
  46   //     }
  47   return r + ((2 - x) & -((x & 2) == 0));
  48 }