lib/ctzsi2.c

//===-- ctzsi2.c - Implement __ctzsi2 -------------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements __ctzsi2 for the compiler_rt library.
//
//===----------------------------------------------------------------------===//

#include "int_lib.h"

// Returns: the number of trailing 0-bits

// Precondition: a != 0

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));
}