bench.c

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

#define DoNotOptimize(value)                   \
  ({                                           \
    typeof(value) v = value;                   \
    asm volatile("" : "+r,m"(v) : : "memory"); \
    value;                                     \
  })

#define NANOS (int64_t)(1e9)
#define MEGABYTE (1024 * 1024)

static int64_t timespec_to_nsec(const struct timespec a) {
  return (int64_t)a.tv_sec * NANOS + a.tv_nsec;
}

void bench_strlen() {
  char *buffer = malloc(MEGABYTE);
  memset(buffer, 5, MEGABYTE);
  buffer[MEGABYTE - 1] = 0;

  // Test various lengths.
  for (size_t i = 1; i < MEGABYTE; i = i * 3 - 1) {
    buffer[i] = 0;
  }

  struct timespec start, end;
  clock_gettime(CLOCK_MONOTONIC, &start);

  for (size_t i = 0; i < MEGABYTE;) {
    i += 1 + DoNotOptimize(strlen(buffer + i));
  }

  clock_gettime(CLOCK_MONOTONIC, &end);

  int64_t delta = timespec_to_nsec(end) - timespec_to_nsec(start);
  printf("strlen: %f MB/s\n", (float)NANOS / (float)delta);

  free(buffer);
}

void bench_memchr() {
  char *buffer = malloc(MEGABYTE);
  memset(buffer, 5, MEGABYTE);

  // Test various lengths.
  for (size_t i = 1; i < MEGABYTE; i = i * 3 - 1) {
    buffer[i] = 7;
  }

  struct timespec start, end;
  clock_gettime(CLOCK_MONOTONIC, &start);

  for (char *ptr = buffer; ptr < buffer + MEGABYTE; ptr++) {
    ptrdiff_t avail = (buffer + MEGABYTE) - ptr;
    ptr = DoNotOptimize(memchr(ptr, 7, avail));
    if (ptr == NULL) break;
  }

  clock_gettime(CLOCK_MONOTONIC, &end);

  int64_t delta = timespec_to_nsec(end) - timespec_to_nsec(start);
  printf("memchr: %f MB/s\n", (float)NANOS / (float)delta);

  free(buffer);
}

int main(void) {
  for (int i = 0; i < 10; i++) {
    bench_strlen();
    bench_memchr();
  }
  return 0;
} 

So I can replicate: what commands do you use to compile and run this?

I just dropped it next to the unit tests in test/src/misc/.

Given the infra, just printing some output fails the test, but that was good enough “for now”. Then go look at the failure.