`strstr` and `strncmp+strlen` seems to be indistinguishable for is-prefix check.

microbench-strstr-strncmp.c

#include <math.h>
#include <stdio.h>
#include <string.h>
#define _WIN64

#if defined(__GNUC__)
#include <x86intrin.h>
#elif defined(_MSC_VER)
#include <intrin.h>
#else
#error("???")
#endif

#define repeat10(x)                                                            \
  do {                                                                         \
    { x; }                                                                     \
    { x; }                                                                     \
    { x; }                                                                     \
    { x; }                                                                     \
    { x; }                                                                     \
    { x; }                                                                     \
    { x; }                                                                     \
    { x; }                                                                     \
    { x; }                                                                     \
    { x; }                                                                     \
  } while (0)

#define BENCH(name, x)                                                         \
  do {                                                                         \
    long times[10];                                                            \
    double stddev = 0, avg = 0;                                                \
    for (int j = 0; j < sizeof(times) / sizeof(times[0]); j++) {               \
      long start = __rdtsc();                                                  \
      for (int i = 0; i < 100000; i++) {                                       \
        repeat10(repeat10(volatile int y = (x)));                              \
      }                                                                        \
      long end = __rdtsc();                                                    \
      times[j] = end - start;                                                  \
      avg += times[j];                                                         \
    }                                                                          \
    avg /= sizeof(times) / sizeof(times[0]);                                   \
    for (int j = 0; j < sizeof(times) / sizeof(times[0]); j++) {               \
      stddev += (times[j] - avg) * (times[j] - avg);                           \
    }                                                                          \
    stddev /= sizeof(times) / sizeof(times[0]) - 1;                            \
    printf("%20s: avg=%15.3f std2=%15.3f\n", name, avg, sqrt(stddev));         \
    if (sqrt(stddev) > avg / 20) {                                             \
      for (int j = 0; j < sizeof(times) / sizeof(times[0]); j++) {             \
        printf("  %15.3ld", times[j]);                                         \
      }                                                                        \
      printf("\n");                                                            \
    }                                                                          \
  } while (0)

#define COMPARE(S, N)                                                          \
  do {                                                                         \
    BENCH("strstr  " #S " " #N, strstr(S, N) == S);                            \
    BENCH("strncmp " #S " " #N, !strncmp(S, N, strlen(N)));                    \
  } while (0)

const char S1[151] = "123456789012345678901234567890123456789012345678901234567"
                     "890123456789012345678901234567890123456789012345678901234"
                     "567890123456789012345678901234567890";
const char S2[151] = "nociv1apipuaflc2c74wq1mys7js2tv0ghblue0np866p30r4rr0hp3eg"
                     "26m5btr56tgpr2dkhualaf3t7esfrqysitkaowrp2bs5z3q9z5bck3rn5"
                     "2bxi9a05gs0g623n6tt6iwf12xlnx68e7im0";

const char N1[11] = "1234567890";
const char N2[11] = "!@#$%^&*()";
const char N3[11] = "26m5btr56t";
const char N4[11] = "123456789!";
const char N5[11] = "26m5btr56!";
const char N6[141] = "123456789012345678901234567890123456789012345678901234567"
                     "890123456789012345678901234567890123456789012345678901234"
                     "56789012345678901234567890";
const char N7[141] = "123456789012345678901234567890123456789012345678901234567"
                     "890123456789012345678901234567890123456789012345678901234"
                     "5678901234567890123456789!";

int main() {
  printf("Warming up...\n");
  COMPARE(S1, N1);
  printf("Test:\n");
  COMPARE(S1, N1);
  COMPARE(S1, N2);
  COMPARE(S1, N3);
  COMPARE(S1, N4);
  COMPARE(S1, N5);
  COMPARE(S1, N6);
  COMPARE(S1, N7);
  COMPARE(S2, N1);
  COMPARE(S2, N2);
  COMPARE(S2, N3);
  COMPARE(S2, N4);
  COMPARE(S2, N5);
  COMPARE(S2, N6);
  COMPARE(S2, N7);
}