__popcnt16, __popcnt, __popcnt64

Microsoft Specific

Counts the number of one bits (population count) in a 16-, 32-, or 64-byte unsigned integer.

unsigned short __popcnt16(
      unsigned short value
);
unsigned int __popcnt(
      unsigned int value
);
unsigned __int64 __popcnt64(
      unsigned __int64 value
);

Parameters

• [in] value
  The 16-, 32-, or 64-bit unsigned integer for which we want the population count.

Return Value

The number of one bits in the value parameter.

Requirements

Header file <intrin.h>

Remarks

Each of these intrinsics generates the popcnt instruction. The size of the value that the popcnt 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 popcnt.

To determine hardware support for the popcnt instruction, call the __cpuid intrinsic with InfoType=0x00000001 and check bit 23 of CPUInfo[2] (ECX). This bit is 1 if the instruction is supported, and 0 otherwise. If you run code that uses this intrinsic on hardware that does not support the popcnt instruction, the results are unpredictable.

Example

#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 = __popcnt16(us[i]);
    cout << "__popcnt16(0x" << hex << us[i] << ") = " << dec << usr << endl;
  }

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

  __popcnt16(0x0) = 0
  __popcnt16(0xff) = 8
  __popcnt16(0xffff) = 16
  __popcnt(0x0) = 0
  __popcnt(0xff) = 8
  __oopcnt(0xffff) = 16
  __popcnt(0xffffffff) = 32

END Microsoft Specific

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

See Also

Concepts

Compiler Intrinsics