__lzcnt16, __lzcnt, __lzcnt64

Microsoft Specific

Counts the number of leading zeros in a 16-, 32-, or 64-bit integer.

Syntax

unsigned short __lzcnt16(
   unsigned short value
);
unsigned int __lzcnt(
   unsigned int value
);
unsigned __int64 __lzcnt64(
   unsigned __int64 value
);

Parameters

value
[in] The 16-, 32-, or 64-bit unsigned integer to scan for leading zeros.

Return value

The number of leading zero bits in the value parameter. If value is zero, the return value is the size of the input operand (16, 32, or 64). If the most significant bit of value is one, the return value is zero.

Requirements

Header file <intrin.h>

Remarks

Each of the intrinsics generates the lzcnt instruction. The size of the value that the lzcnt instruction returns is the same as the size of its argument. In 32-bit mode, there are no 64-bit general-purpose registers, so the 64-bit lzcnt isn't supported.

To determine hardware support for the lzcnt instruction, call the __cpuid intrinsic with InfoType=0x80000001 and check bit 5 of CPUInfo[2] (ECX). This bit will be 1 if the instruction is supported, and 0 otherwise. If you run code that uses the intrinsic on hardware that doesn't support the lzcnt instruction, the results are unpredictable.

On Intel processors that don't support the lzcnt instruction, the instruction byte encoding is executed as bsr (bit scan reverse). If code portability is a concern, consider use of the _BitScanReverse intrinsic instead. For more information, see _BitScanReverse, _BitScanReverse64.

Example

// Compile this test with: /EHsc
#include <iostream>
#include <intrin.h>
using namespace std;

int main()
{
  unsigned short us[3] = {0, 0xFF, 0xFFFF};
  unsigned short usr;
  unsigned int   ui[4] = {0, 0xFF, 0xFFFF, 0xFFFFFFFF};
  unsigned int   uir;

  for (int i=0; i<3; i++) {
    usr = __lzcnt16(us[i]);
    cout << "__lzcnt16(0x" << hex << us[i] << ") = " << dec << usr << endl;
  }

  for (int i=0; i<4; i++) {
    uir = __lzcnt(ui[i]);
    cout << "__lzcnt(0x" << hex << ui[i] << ") = " << dec << uir << endl;
  }
}

__lzcnt16(0x0) = 16
__lzcnt16(0xff) = 8
__lzcnt16(0xffff) = 0
__lzcnt(0x0) = 32
__lzcnt(0xff) = 24
__lzcnt(0xffff) = 16
__lzcnt(0xffffffff) = 0

END Microsoft Specific

Portions of this content are Copyright 2007 by Advanced Micro Devices, Inc. All rights reserved. Reproduced with permission from Advanced Micro Devices, Inc.

See also

Compiler intrinsics