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