popcnt16, popcnt, __popcnt64

Article
06/05/2017

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The latest version of this topic can be found at __popcnt16, __popcnt, __popcnt64.

Microsoft Specific**

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

Syntax

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

Intrinsic	Architecture
`__popcnt16`	Advanced Bit Manipulation
`__popcnt`	Advanced Bit Manipulation
`__popcnt64`	Advanced Bit Manipulation in 64-bit mode.

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

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