--- a/hotspot/src/share/vm/prims/jvm.cpp Wed Jan 28 09:27:49 2015 +0100
+++ b/hotspot/src/share/vm/prims/jvm.cpp Wed Jan 28 17:52:48 2015 +0100
@@ -300,6 +300,48 @@
return os::javaTimeNanos();
JVM_END
+// The function below is actually exposed by sun.misc.VM and not
+// java.lang.System, but we choose to keep it here so that it stays next
+// to JVM_CurrentTimeMillis and JVM_NanoTime
+
+const jlong MAX_DIFF_SECS = 0x0100000000; // 2^32
+const jlong MIN_DIFF_SECS = -MAX_DIFF_SECS; // -2^32
+
+JVM_LEAF(jlong, JVM_GetNanoTimeAdjustment(JNIEnv *env, jclass ignored, jlong offset_secs))
+ JVMWrapper("JVM_GetNanoTimeAdjustment");
+ jlong seconds;
+ jlong nanos;
+
+ os::javaTimeSystemUTC(seconds, nanos);
+
+ // We're going to verify that the result can fit in a long.
+ // For that we need the difference in seconds between 'seconds'
+ // and 'offset_secs' to be such that:
+ // |seconds - offset_secs| < (2^63/10^9)
+ // We're going to approximate 10^9 ~< 2^30 (1000^3 ~< 1024^3)
+ // which makes |seconds - offset_secs| < 2^33
+ // and we will prefer +/- 2^32 as the maximum acceptable diff
+ // as 2^32 has a more natural feel than 2^33...
+ //
+ // So if |seconds - offset_secs| >= 2^32 - we return a special
+ // sentinel value (-1) which the caller should take as an
+ // exception value indicating that the offset given to us is
+ // too far from range of the current time - leading to too big
+ // a nano adjustment. The caller is expected to recover by
+ // computing a more accurate offset and calling this method
+ // again. (For the record 2^32 secs is ~136 years, so that
+ // should rarely happen)
+ //
+ jlong diff = seconds - offset_secs;
+ if (diff >= MAX_DIFF_SECS || diff <= MIN_DIFF_SECS) {
+ return -1; // sentinel value: the offset is too far off the target
+ }
+
+ // return the adjustment. If you compute a time by adding
+ // this number of nanoseconds along with the number of seconds
+ // in the offset you should get the current UTC time.
+ return (diff * (jlong)1000000000) + nanos;
+JVM_END
JVM_ENTRY(void, JVM_ArrayCopy(JNIEnv *env, jclass ignored, jobject src, jint src_pos,
jobject dst, jint dst_pos, jint length))