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| //===-- 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 si_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));
}
|