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diff --git a/js/src/vm/Time.h b/js/src/vm/Time.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef vm_Time_h
+#define vm_Time_h
+
+#include <stddef.h>
+#include <stdint.h>
+
+/*
+ * Broken down form of 64 bit time value.
+ */
+struct PRMJTime {
+    int32_t tm_usec;            /* microseconds of second (0-999999) */
+    int8_t tm_sec;              /* seconds of minute (0-59) */
+    int8_t tm_min;              /* minutes of hour (0-59) */
+    int8_t tm_hour;             /* hour of day (0-23) */
+    int8_t tm_mday;             /* day of month (1-31) */
+    int8_t tm_mon;              /* month of year (0-11) */
+    int8_t tm_wday;             /* 0=sunday, 1=monday, ... */
+    int32_t tm_year;            /* absolute year, AD */
+    int16_t tm_yday;            /* day of year (0 to 365) */
+    int8_t tm_isdst;            /* non-zero if DST in effect */
+};
+
+/* Some handy constants */
+#define PRMJ_USEC_PER_SEC       1000000L
+#define PRMJ_USEC_PER_MSEC      1000L
+
+/* Return the current local time in micro-seconds */
+extern int64_t
+PRMJ_Now();
+
+/* Initialize the resources associated with PRMJ_Now. */
+#if defined(XP_WIN)
+extern void
+PRMJ_NowInit();
+#else
+inline void
+PRMJ_NowInit() {}
+#endif
+
+/* Release the resources associated with PRMJ_Now; don't call PRMJ_Now again */
+#ifdef XP_WIN
+extern void
+PRMJ_NowShutdown();
+#else
+inline void
+PRMJ_NowShutdown() {}
+#endif
+
+/* Format a time value into a buffer. Same semantics as strftime() */
+extern size_t
+PRMJ_FormatTime(char* buf, int buflen, const char* fmt, PRMJTime* tm);
+
+
+/**
+ * Requesting the number of cycles from the CPU.
+ *
+ * `rdtsc`, or Read TimeStamp Cycle, is an instruction provided by
+ * x86-compatible CPUs that lets processes request the number of
+ * cycles spent by the CPU executing instructions since the CPU was
+ * started. It may be used for performance monitoring, but you should
+ * be aware of the following limitations.
+ *
+ *
+ * 1. The value is *not* monotonic.
+ *
+ * The value is reset to 0 whenever a CPU is turned off (e.g. computer
+ * in full hibernation, single CPU going turned off). Moreover, on
+ * multi-core/multi-CPU architectures, the cycles of each core/CPU are
+ * generally not synchronized.  Therefore, is a process or thread is
+ * rescheduled to another core/CPU, the result of `rdtsc` may decrease
+ * arbitrarily.
+ *
+ * The only way to prevent this is to pin your thread to a particular
+ * CPU, which is generally not a good idea.
+ *
+ *
+ *
+ * 2. The value increases independently.
+ *
+ * The value may increase whenever the CPU executes an instruction,
+ * regardless of the process that has issued this
+ * instruction. Moreover, if a process or thread is rescheduled to
+ * another core/CPU, the result of `rdtsc` may increase arbitrarily.
+ *
+ * The only way to prevent this is to ensure that your thread is the
+ * sole owner of the CPU. See [1] for an example. This is also
+ * generally not a good idea.
+ *
+ *
+ *
+ * 3. The value does not measure time.
+ *
+ * On older architectures (pre-Pentium 4), there was no constant mapping
+ * between rdtsc and CPU time.
+ *
+ *
+ * 4. Instructions may be reordered.
+ *
+ * The CPU can reorder instructions. Also, rdtsc does not necessarily
+ * wait until all previous instructions have finished executing before
+ * reading the counter. Similarly, subsequent instructions may begin
+ * execution before the read operation is performed. If you use rdtsc
+ * for micro-benchmarking, you may end up measuring something else
+ * than what you expect. See [1] for a study of countermeasures.
+ *
+ *
+ * ** Performance
+ *
+ * According to unchecked sources on the web, the overhead of rdtsc is
+ * expected to be 150-200 cycles on old architectures, 6-50 on newer
+ * architectures. Agner's instruction tables [2] seem to confirm the latter
+ * results.
+ *
+ *
+ * [1]
+ * http://www.intel.com/content/dam/www/public/us/en/documents/white-papers/ia-32-ia-64-benchmark-code-execution-paper.pdf
+ * [2] http://www.agner.org/optimize/instruction_tables.pdf
+ */
+
+#define MOZ_HAVE_RDTSC 1
+
+#if defined(_WIN32)
+
+#include <intrin.h>
+static __inline uint64_t
+ReadTimestampCounter(void)
+{
+    return __rdtsc();
+}
+
+#elif defined(__i386__)
+
+static __inline__ uint64_t
+ReadTimestampCounter(void)
+{
+    uint64_t x;
+    __asm__ volatile (".byte 0x0f, 0x31" : "=A" (x));
+    return x;
+}
+
+#elif defined(__x86_64__)
+
+static __inline__ uint64_t
+ReadTimestampCounter(void)
+{
+    unsigned hi, lo;
+    __asm__ __volatile__ ("rdtsc" : "=a"(lo), "=d"(hi));
+    return ( (uint64_t)lo)|( ((uint64_t)hi)<<32 );
+}
+
+#else
+
+#undef MOZ_HAVE_RDTSC
+
+#endif
+
+#endif /* vm_Time_h */