compiler-rt/lib/clzsi2.c

//===-- clzsi2.c - Implement __clzsi2 -------------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements __clzsi2 for the compiler_rt library.
//
//===----------------------------------------------------------------------===//

#include "int_lib.h"

// Returns: the number of leading 0-bits

// Precondition: a != 0

si_int
__clzsi2(si_int a)
{
    su_int x = (su_int)a;
    si_int t = ((x & 0xFFFF0000) == 0) << 4;  // if (x is small) t = 16 else 0
    x >>= 16 - t;      // x = [0 - 0xFFFF]
    su_int r = t;       // r = [0, 16]
    // return r + clz(x)
    t = ((x & 0xFF00) == 0) << 3;
    x >>= 8 - t;       // x = [0 - 0xFF]
    r += t;            // r = [0, 8, 16, 24]
    // return r + clz(x)
    t = ((x & 0xF0) == 0) << 2;
    x >>= 4 - t;       // x = [0 - 0xF]
    r += t;            // r = [0, 4, 8, 12, 16, 20, 24, 28]
    // return r + clz(x)
    t = ((x & 0xC) == 0) << 1;
    x >>= 2 - t;       // x = [0 - 3]
    r += t;            // r = [0 - 30] and is even
    // return r + clz(x)
//     switch (x)
//     {
//     case 0:
//         return r + 2;
//     case 1:
//         return r + 1;
//     case 2:
//     case 3:
//         return r;
//     }
    return r + ((2 - x) & -((x & 2) == 0));
}