Benchmarking code for https://kristerw.blogspot.se/2017/07/a-loadstore-performance-corner-case.html

main.cpp

#include <benchmark/benchmark.h>

unsigned char *init_test_memset(int);
unsigned char *init_test_random(int);
void run_test(unsigned char *, int);
void release_test(unsigned char *);

static void BM_cleared(benchmark::State &state)
{
  const int len = state.range(0);
  unsigned char *p = init_test_memset(len);

  while (state.KeepRunning())
  {
    run_test(p, len);
  }

  release_test(p);
}

static void BM_random(benchmark::State &state)
{
  const int len = state.range(0);
  unsigned char *p = init_test_random(len);

  while (state.KeepRunning())
  {
    run_test(p, len);
  }

  release_test(p);
}

BENCHMARK(BM_cleared)->Arg(1 << 12);
BENCHMARK(BM_random)->Arg(1 << 12);

BENCHMARK_MAIN();

test.cpp

#include <stdlib.h>
#include <string.h>

int histogram[256];

unsigned char *init_test_memset(int len)
{
  unsigned char *p = (unsigned char *)malloc(len);
  if (p == NULL)
    exit(1);
  memset(p, 0, len);
  return p;
}

unsigned char *init_test_random(int len)
{
  unsigned char *p = (unsigned char *)malloc(len);
  if (p == NULL)
    exit(1);
  for (int i = 0; i < len; i++)
    p[i] = rand();
  return p;
}

void run_test(unsigned char *p, int len)
{
  memset(histogram, 0, sizeof(histogram));
  for (int i = 0; i < len; i++)
    histogram[p[i]]++;
}

void release_test(unsigned char *p)
{
  free(p);
}