compiler-rt/lib/builtins/ctzsi2.c

//===-- ctzsi2.c - Implement __ctzsi2 -------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements __ctzsi2 for the compiler_rt library.
//
//===----------------------------------------------------------------------===//

#include "int_lib.h"

// Returns: the number of trailing 0-bits

// Precondition: a != 0

COMPILER_RT_ABI int __ctzsi2(si_int a) {
  su_int x = (su_int)a;
  si_int t = ((x & 0x0000FFFF) == 0)
             << 4; // if (x has no small bits) t = 16 else 0
  x >>= t;         // x = [0 - 0xFFFF] + higher garbage bits
  su_int r = t;    // r = [0, 16]
  // return r + ctz(x)
  t = ((x & 0x00FF) == 0) << 3;
  x >>= t; // x = [0 - 0xFF] + higher garbage bits
  r += t;  // r = [0, 8, 16, 24]
  // return r + ctz(x)
  t = ((x & 0x0F) == 0) << 2;
  x >>= t; // x = [0 - 0xF] + higher garbage bits
  r += t;  // r = [0, 4, 8, 12, 16, 20, 24, 28]
  // return r + ctz(x)
  t = ((x & 0x3) == 0) << 1;
  x >>= t;
  x &= 3; // x = [0 - 3]
  r += t; // r = [0 - 30] and is even
  // return r + ctz(x)

  //  The branch-less return statement below is equivalent
  //  to the following switch statement:
  //     switch (x)
  //    {
  //     case 0:
  //         return r + 2;
  //     case 2:
  //         return r + 1;
  //     case 1:
  //     case 3:
  //         return r;
  //     }
  return r + ((2 - (x >> 1)) & -((x & 1) == 0));
}