1 /* ===-- ctzsi2.c - Implement __ctzsi2 -------------------------------------=== 2 * 3 * The LLVM Compiler Infrastructure 4 * 5 * This file is dual licensed under the MIT and the University of Illinois Open 6 * Source Licenses. See LICENSE.TXT for details. 7 * 8 * ===----------------------------------------------------------------------=== 9 * 10 * This file implements __ctzsi2 for the compiler_rt library. 11 * 12 * ===----------------------------------------------------------------------=== 13 */ 14 15 #include "int_lib.h" 16 17 /* Returns: the number of trailing 0-bits */ 18 19 /* Precondition: a != 0 */ 20 21 COMPILER_RT_ABI si_int 22 __ctzsi2(si_int a) 23 { 24 su_int x = (su_int)a; 25 si_int t = ((x & 0x0000FFFF) == 0) << 4; /* if (x has no small bits) t = 16 else 0 */ 26 x >>= t; /* x = [0 - 0xFFFF] + higher garbage bits */ 27 su_int r = t; /* r = [0, 16] */ 28 /* return r + ctz(x) */ 29 t = ((x & 0x00FF) == 0) << 3; 30 x >>= t; /* x = [0 - 0xFF] + higher garbage bits */ 31 r += t; /* r = [0, 8, 16, 24] */ 32 /* return r + ctz(x) */ 33 t = ((x & 0x0F) == 0) << 2; 34 x >>= t; /* x = [0 - 0xF] + higher garbage bits */ 35 r += t; /* r = [0, 4, 8, 12, 16, 20, 24, 28] */ 36 /* return r + ctz(x) */ 37 t = ((x & 0x3) == 0) << 1; 38 x >>= t; 39 x &= 3; /* x = [0 - 3] */ 40 r += t; /* r = [0 - 30] and is even */ 41 /* return r + ctz(x) */ 42 43 /* The branch-less return statement below is equivalent 44 * to the following switch statement: 45 * switch (x) 46 * { 47 * case 0: 48 * return r + 2; 49 * case 2: 50 * return r + 1; 51 * case 1: 52 * case 3: 53 * return r; 54 * } 55 */ 56 return r + ((2 - (x >> 1)) & -((x & 1) == 0)); 57 } 58
