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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-exception6//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-bits16 17// Precondition: a != 018 19COMPILER_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 023 x >>= t; // x = [0 - 0xFFFF] + higher garbage bits24 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 bits28 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 bits32 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 even38 // return r + ctz(x)39 40 // The branch-less return statement below is equivalent41 // 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