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VPOPCNT: Number of set bits

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In this article at OpenGenus, you must have the complete idea of VPOPCNT assembly instruction that is used to get the number of set bits.

Table of contents:

  1. VPOPCNT
  2. Assembly code with VPOPCNT
  3. C++ Implementation using VPOPCNT with intrinsic

VPOPCNT

VPOPCNT is a vectorized assembly instruction to count the number of set bits in a given register/ data. A data consist of 0 and 1. A set bit refer to 1.

It is supported in AVX512 and AVX256 as well. If you have a system that supports AVX512, it means VPOPCNT can be used to get the number of set bits in 512 bits using one instruction.

VPOPCNT is executed in 3 clock cycles.

Assembly code with VPOPCNT

The code in this section has a data in eax register. It uses vpopcnt to find the number of set bits and stores the result in eax (same register).

Following is the assembly code with VPOPCNT instruction:

section .text
global _start

_start:
  ; input integer in eax
  ; use vpopcnt instruction to count number of set bits
  vpopcnt eax, eax 
  ; result in eax

  ; exit program
  mov eax, 1
  xor ebx, ebx
  int 0x80

C++ Implementation using VPOPCNT with intrinsic

It use 2 main intrinsics:

  • _mm256_popcnt_epi32 intrinsic to count number of set bits in a data of 256 bits
  • _mm256_add_epi32 to add the number of set bits from _mm256_popcnt_epi32 across all data

Following is the complete C++ code using VPOPCNT as an intrinsic:

#include <immintrin.h>

// Compute the number of set bits in the given array of integers
int popcnt(const uint32_t* data, size_t n) {
  size_t i = 0;
  __m256i sum = _mm256_setzero_si256();
  // Process the data in chunks of 8 integers
  for (; i + 8 <= n; i += 8) {
    __m256i chunk = _mm256_loadu_si256(reinterpret_cast<const __m256i*>(data + i));
    sum = _mm256_add_epi32(sum, _mm256_popcnt_epi32(chunk));
  }
  // Process any remaining integers
  uint32_t remaining[8] = {0};
  for (; i < n; ++i) {
    remaining[i % 8] = data[i];
  }
  __m256i chunk = _mm256_loadu_si256(reinterpret_cast<const __m256i*>(remaining));
  sum = _mm256_add_epi32(sum, _mm256_popcnt_epi32(chunk));
  // Compute the total popcount by summing the individual counts
  uint32_t count[8];
  _mm256_storeu_si256(reinterpret_cast<__m256i*>(count), sum);
  return count[0] + count[1] + count[2] + count[3] + count[4] + count[5] + count[6] + count[7];
}

With this article at OpenGenus, you must have the complete idea of VPOPCNT vectorized instruction.

VPOPCNT: Number of set bits
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