Intrinsics are marked as inline now. However gcc knows what they do, and is able to optimize out some code code which uses them - e.g. in example below both count1a() and count2a() are optimized to "mov eax, 5; ret". Because of this intrinsics could be marked as constexpr. This would allow gcc to better optimize code. It also would allow to perform quick code validation in tools like Compiler Explorer, by using code like below.

#include "immintrin.h"

constexpr int data[8] = {0, 1, 0, 2, 0, 3};

constexpr int count1(const int* d)
{
  int cnt = 0;
  for (int n = 0; n < 8; ++n)
    if (0 == d[n])
      ++cnt;
  return cnt;
}

int count1a()
{
  return count1(data);
}

inline int count2(const int* d)
{
  __m256i v = _mm256_loadu_si256((const __m256i*)d);
  v = _mm256_cmpeq_epi32(v, _mm256_setzero_si256());
  return _mm256_movemask_epi8(v) >> 2;
}

int count2a()
{
  return count1(data);
}