vlibc/benchmarks/bench_string.c

/*
 * Benchmark harness for the five L1 string functions — strlen, strcmp,
 * memcpy, memmove, memset (musts/BENCHMARKING.md).
 *
 * Deliberately a SYSTEM-headers-only translation unit: it includes no vlibc
 * header, because vlibc's self-contained <stddef.h>/<string.h> (guard
 * VLIBC_STDDEF_H) conflicts with the system <stdio.h>/<time.h> (guard
 * _STDDEF_H) — both define size_t/NULL/offsetof/max_align_t and mixing them
 * hard-errors. Which libc implements the timed calls is therefore decided
 * purely at link time (../libvlibc.la vs the system libc).
 */
#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

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

#define ITERATIONS 50000000ULL

/*
 * Volatile function pointers defeat both GCC builtin substitution and
 * pure-attribute LICM hoisting of the timed loops (-fno-builtin alone does
 * not prevent the latter). The signatures match the system-header
 * declarations exactly: memcpy has restrict, memmove does not.
 */
static size_t (*volatile p_strlen)(const char *) = strlen;
static int (*volatile p_strcmp)(const char *, const char *) = strcmp;
static void *(*volatile p_memcpy)(void *restrict, const void *restrict, size_t) = memcpy;
static void *(*volatile p_memmove)(void *, const void *, size_t) = memmove;
static void *(*volatile p_memset)(void *, int, size_t) = memset;

/*
 * XOR accumulator. Every timed result is folded in here so the compiler
 * cannot discard the indirect calls; volatile keeps the store itself alive.
 */
static volatile unsigned long long sink;

/* Fixed small inputs: 8-byte and 64-byte buffers, sized per the plan. */
static char str8[8] = "abcdefg"; /* 7 non-NUL bytes + NUL: full scan */
static char str64[64] =
    "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789.";
static char cmp8a[8] = "abcdefg";
static char cmp8b[8] = "abcdefg"; /* equal: strcmp runs the full length */
static char cmp64a[64] =
    "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789.";
static char cmp64b[64] =
    "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789.";
static char src8[8];
static char src64[64];
static char dst8[8];
static char dst64[64];

/*
 * Time one expression executed ITERATIONS times through a volatile function
 * pointer, then print `name@size ns/call`. The result of each call is XORed
 * into a local accumulator and folded into `sink` after the loop, so the
 * calls cannot be eliminated. On clock_gettime failure, returns 1 out of
 * main.
 */
#define MEASURE(name, size, call)                                              \
    do                                                                         \
    {                                                                          \
        unsigned long long r = 0;                                              \
        double seconds;                                                        \
        if (clock_gettime(CLOCK_MONOTONIC, &start) != 0)                       \
        {                                                                      \
            return 1;                                                          \
        }                                                                      \
        for (unsigned long long i = 0; i < ITERATIONS; i++)                    \
        {                                                                      \
            r ^= (unsigned long long)(call);                                   \
        }                                                                      \
        if (clock_gettime(CLOCK_MONOTONIC, &end) != 0)                         \
        {                                                                      \
            return 1;                                                          \
        }                                                                      \
        sink ^= r;                                                             \
        seconds = (double)(end.tv_sec - start.tv_sec) +                        \
                  (double)(end.tv_nsec - start.tv_nsec) / 1000000000.0;        \
        printf("%s@%d %.1f\n", name, size,                                     \
               seconds * 1000000000.0 / (double)ITERATIONS);                   \
    } while (0)

int
main(void)
{
    struct timespec start;
    struct timespec end;

    /*
     * Correctness self-asserts, routed through the SAME volatile function
     * pointers as the timed loops. A bare literal call (strlen("hello"))
     * would be constant-folded by GCC even at -O0, and a call through a
     * non-volatile pointer would not exercise the pointer the timing uses.
     */
    {
        char scratch[16];

        if (p_strlen("hello") != 5)
        {
            return 1;
        }
        if (p_strcmp("a", "b") >= 0)
        {
            return 1;
        }
        if (p_strcmp("abc", "abc") != 0)
        {
            return 1;
        }
        if (p_memset(scratch, 0, sizeof scratch) != scratch)
        {
            return 1;
        }
        if (p_memcpy(scratch, "hello", 6) != scratch)
        {
            return 1;
        }
        if (p_strcmp(scratch, "hello") != 0)
        {
            return 1;
        }
        if (p_memmove(scratch + 1, scratch, 6) != scratch + 1)
        {
            return 1;
        }
        if (p_strcmp(scratch + 1, "hello") != 0)
        {
            return 1;
        }
    }

    MEASURE("strlen", 8, p_strlen(str8));
    MEASURE("strlen", 64, p_strlen(str64));
    MEASURE("strcmp", 8, p_strcmp(cmp8a, cmp8b));
    MEASURE("strcmp", 64, p_strcmp(cmp64a, cmp64b));
    MEASURE("memcpy", 8, p_memcpy(dst8, src8, sizeof dst8));
    MEASURE("memcpy", 64, p_memcpy(dst64, src64, sizeof dst64));
    MEASURE("memmove", 8, p_memmove(dst8, src8, sizeof dst8));
    MEASURE("memmove", 64, p_memmove(dst64, src64, sizeof dst64));
    MEASURE("memset", 8, p_memset(dst8, 0x5a, sizeof dst8));
    MEASURE("memset", 64, p_memset(dst64, 0x5a, sizeof dst64));

    /* Keep the accumulated results observable to the compiler. */
    if (sink == 0xdeadbeefULL)
    {
        printf("%llu\n", sink);
    }

    /*
     * Explicit flush: this harness is a host program, but under
     * --with-libc=vlibc its DT_NEEDED order puts libvlibc.so before
     * libc.so.6, so the exit() that runs at process end is vlibc's — which
     * does not flush stdio yet (that hook lands with the stdio todo).
     * Without the flush the timed results above are lost.
     */
    fflush(stdout);

    return 0;
}