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__lzcnt16, __lzcnt, __lzcnt64

Microsoft Specific

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

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

[in] value

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

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.

Intrinsic

Architecture

__lzcnt16

Advanced Bit Manipulation

__lzcnt

Advanced Bit Manipulation

__lzcnt64

Advanced Bit Manipulation in 64-bit mode.

Header file <intrin.h>

Each of these 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, hence no 64-bit lzcnt.

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 this intrinsic on hardware that does not support the lzcnt instruction, the results are unpredictable.

// 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

Copyright 2007 by Advanced Micro Devices, Inc. All rights reserved. Reproduced with permission from Advanced Micro Devices, Inc.

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