Add nstime_ns_since which obtains the duration since the input time.

include/jemalloc/internal/nstime.h

@@ -35,6 +35,7 @@ void nstime_isubtract(nstime_t *time, uint64_t subtrahend);
 void nstime_imultiply(nstime_t *time, uint64_t multiplier);
 void nstime_idivide(nstime_t *time, uint64_t divisor);
 uint64_t nstime_divide(const nstime_t *time, const nstime_t *divisor);
+uint64_t nstime_ns_since(const nstime_t *past);
 
 typedef bool (nstime_monotonic_t)(void);
 extern nstime_monotonic_t *JET_MUTABLE nstime_monotonic;

src/nstime.c

@@ -158,6 +158,19 @@ nstime_divide(const nstime_t *time, const nstime_t *divisor) {
 	return time->ns / divisor->ns;
 }
 
+/* Returns time since *past, w/o updating *past. */
+uint64_t
+nstime_ns_since(const nstime_t *past) {
+	nstime_assert_initialized(past);
+
+	nstime_t now;
+	nstime_copy(&now, past);
+	nstime_update(&now);
+
+	assert(nstime_compare(&now, past) >= 0);
+	return now.ns - past->ns;
+}
+
 #ifdef _WIN32
 #  define NSTIME_MONOTONIC true
 static void

test/unit/nstime.c

@@ -201,6 +201,33 @@ TEST_BEGIN(test_nstime_divide) {
 }
 TEST_END
 
+void
+test_nstime_since_once(nstime_t *t) {
+	nstime_t old_t;
+	nstime_copy(&old_t, t);
+
+	uint64_t ns_since = nstime_ns_since(t);
+	nstime_update(t);
+
+	nstime_t new_t;
+	nstime_copy(&new_t, t);
+	nstime_subtract(&new_t, &old_t);
+
+	expect_u64_ge(nstime_ns(&new_t), ns_since,
+	    "Incorrect time since result");
+}
+
+TEST_BEGIN(test_nstime_ns_since) {
+	nstime_t t;
+
+	nstime_init_update(&t);
+	for (uint64_t i = 0; i < 10000; i++) {
+		/* Keeps updating t and verifies ns_since is valid. */
+		test_nstime_since_once(&t);
+	}
+}
+TEST_END
+
 TEST_BEGIN(test_nstime_monotonic) {
 	nstime_monotonic();
 }
@@ -220,5 +247,6 @@ main(void) {
 	    test_nstime_imultiply,
 	    test_nstime_idivide,
 	    test_nstime_divide,
+	    test_nstime_ns_since,
 	    test_nstime_monotonic);
 }