bench: replace wall-clock timers with cpu-timers where possible

This commit improves the reliability of benchmarks by removing some of the influence of other background running processes. This is achieved by using CPU bound clocks that aren't influenced by interrupts, sleeps, blocked I/O, etc.

Author: Raimo33

src/bench.c

@@ -4,6 +4,8 @@
  * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
  ***********************************************************************/
 
+#define _POSIX_C_SOURCE 200112L /* for clock_gettime() */
+
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

src/bench.h

@@ -7,30 +7,44 @@
 #ifndef SECP256K1_BENCH_H
 #define SECP256K1_BENCH_H
 
+#if defined(_WIN32)
+# include <windows.h>
+#else /* POSIX */
+# include <time.h>
+#endif
+
 #include <stdlib.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <string.h>
 
-#if (defined(_MSC_VER) && _MSC_VER >= 1900)
-#  include <time.h>
-#else
-#  include <sys/time.h>
-#endif
+static int64_t gettime_us(void) {
+#if defined(_WIN32)
+
+    LARGE_INTEGER freq, counter;
+    QueryPerformanceFrequency(&freq);
+    QueryPerformanceCounter(&counter);
+    return (int64_t)(counter.QuadPart * 1000000 / freq.QuadPart);
+
+#else /* POSIX */
+
+# if defined(CLOCK_PROCESS_CPUTIME_ID)
+    /* In theory, CLOCK_PROCESS_CPUTIME_ID is only useful if the process is locked to a core,
+     * see `man clock_gettime` on Linux. In practice, modern CPUs have synchronized TSCs which
+     * address this issue, see https://docs.amd.com/r/en-US/ug1586-onload-user/Timer-TSC-Stability . */
+    const clockid_t clock_type = CLOCK_PROCESS_CPUTIME_ID;
+# elif defined(CLOCK_MONOTONIC)
+    /* fallback to wall-clock timer */
+    const clockid_t clock_type = CLOCK_MONOTONIC;
+# else
+    /* fallback to less precise wall-clock timer */
+    const clockid_t clock_type = CLOCK_REALTIME;
+# endif
+
+    struct timespec ts;
+    clock_gettime(clock_type, &ts);
+    return (int64_t)ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
 
-static int64_t gettime_i64(void) {
-#if (defined(_MSC_VER) && _MSC_VER >= 1900)
-    /* C11 way to get wallclock time */
-    struct timespec tv;
-    if (!timespec_get(&tv, TIME_UTC)) {
-        fputs("timespec_get failed!", stderr);
-        exit(EXIT_FAILURE);
-    }
-    return (int64_t)tv.tv_nsec / 1000 + (int64_t)tv.tv_sec * 1000000LL;
-#else
-    struct timeval tv;
-    gettimeofday(&tv, NULL);
-    return (int64_t)tv.tv_usec + (int64_t)tv.tv_sec * 1000000LL;
 #endif
 }
 
@@ -105,9 +119,9 @@ static void run_benchmark(char *name, void (*benchmark)(void*, int), void (*setu
         if (setup != NULL) {
             setup(data);
         }
-        begin = gettime_i64();
+        begin = gettime_us();
         benchmark(data, iter);
-        total = gettime_i64() - begin;
+        total = gettime_us() - begin;
         if (teardown != NULL) {
             teardown(data, iter);
         }

src/bench_ecmult.c

@@ -3,6 +3,9 @@
  * Distributed under the MIT software license, see the accompanying    *
  * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
  ***********************************************************************/
+
+#define _POSIX_C_SOURCE 200112L /* for clock_gettime() */
+
 #include <stdio.h>
 #include <stdlib.h>
 
@@ -362,7 +365,6 @@ int main(int argc, char **argv) {
     }
     secp256k1_ge_set_all_gej_var(data.pubkeys, data.pubkeys_gej, POINTS);
 
-
     print_output_table_header_row();
     /* Initialize offset1 and offset2 */
     hash_into_offset(&data, 0);

src/bench_internal.c

@@ -3,6 +3,9 @@
  * Distributed under the MIT software license, see the accompanying    *
  * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
  ***********************************************************************/
+
+#define _POSIX_C_SOURCE 200112L /* for clock_gettime() */
+
 #include <stdio.h>
 #include <stdlib.h>