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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_DateTime_h
+#define vm_DateTime_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/FloatingPoint.h"
+#include "mozilla/MathAlgorithms.h"
+
+#include <stdint.h>
+
+#include "js/Conversions.h"
+#include "js/Date.h"
+#include "js/Initialization.h"
+#include "js/Value.h"
+
+namespace js {
+
+/* Constants defined by ES5 15.9.1.10. */
+const double HoursPerDay = 24;
+const double MinutesPerHour = 60;
+const double SecondsPerMinute = 60;
+const double msPerSecond = 1000;
+const double msPerMinute = msPerSecond * SecondsPerMinute;
+const double msPerHour = msPerMinute * MinutesPerHour;
+
+/* ES5 15.9.1.2. */
+const double msPerDay = msPerHour * HoursPerDay;
+
+/*
+ * Additional quantities not mentioned in the spec. Be careful using these!
+ * They aren't doubles (and aren't defined in terms of all the other constants
+ * so that they can be used in constexpr scenarios; if you need constants that
+ * trigger floating point semantics, you'll have to manually cast to get it.
+ */
+const unsigned SecondsPerHour = 60 * 60;
+const unsigned SecondsPerDay = SecondsPerHour * 24;
+
+const double StartOfTime = -8.64e15;
+const double EndOfTime = 8.64e15;
+
+/*
+ * Stores date/time information, particularly concerning the current local
+ * time zone, and implements a small cache for daylight saving time offset
+ * computation.
+ *
+ * The basic idea is premised upon this fact: the DST offset never changes more
+ * than once in any thirty-day period. If we know the offset at t_0 is o_0,
+ * the offset at [t_1, t_2] is also o_0, where t_1 + 3_0 days == t_2,
+ * t_1 <= t_0, and t0 <= t2. (In other words, t_0 is always somewhere within a
+ * thirty-day range where the DST offset is constant: DST changes never occur
+ * more than once in any thirty-day period.) Therefore, if we intelligently
+ * retain knowledge of the offset for a range of dates (which may vary over
+ * time), and if requests are usually for dates within that range, we can often
+ * provide a response without repeated offset calculation.
+ *
+ * Our caching strategy is as follows: on the first request at date t_0 compute
+ * the requested offset o_0. Save { start: t_0, end: t_0, offset: o_0 } as the
+ * cache's state. Subsequent requests within that range are straightforwardly
+ * handled. If a request for t_i is far outside the range (more than thirty
+ * days), compute o_i = dstOffset(t_i) and save { start: t_i, end: t_i,
+ * offset: t_i }. Otherwise attempt to *overextend* the range to either
+ * [start - 30d, end] or [start, end + 30d] as appropriate to encompass
+ * t_i. If the offset o_i30 is the same as the cached offset, extend the
+ * range. Otherwise the over-guess crossed a DST change -- compute
+ * o_i = dstOffset(t_i) and either extend the original range (if o_i == offset)
+ * or start a new one beneath/above the current one with o_i30 as the offset.
+ *
+ * This cache strategy results in 0 to 2 DST offset computations. The naive
+ * always-compute strategy is 1 computation, and since cache maintenance is a
+ * handful of integer arithmetic instructions the speed difference between
+ * always-1 and 1-with-cache is negligible. Caching loses if two computations
+ * happen: when the date is within 30 days of the cached range and when that
+ * 30-day range crosses a DST change. This is relatively uncommon. Further,
+ * instances of such are often dominated by in-range hits, so caching is an
+ * overall slight win.
+ *
+ * Why 30 days? For correctness the duration must be smaller than any possible
+ * duration between DST changes. Past that, note that 1) a large duration
+ * increases the likelihood of crossing a DST change while reducing the number
+ * of cache misses, and 2) a small duration decreases the size of the cached
+ * range while producing more misses. Using a month as the interval change is
+ * a balance between these two that tries to optimize for the calendar month at
+ * a time that a site might display. (One could imagine an adaptive duration
+ * that accommodates near-DST-change dates better; we don't believe the
+ * potential win from better caching offsets the loss from extra complexity.)
+ */
+class DateTimeInfo
+{
+ static DateTimeInfo instance;
+
+ // Date/time info is shared across all threads in DateTimeInfo::instance,
+ // for consistency with ICU's handling of its default time zone. Thus we
+ // need something to protect concurrent accesses.
+ //
+ // The spec implicitly assumes DST and time zone adjustment information
+ // never change in the course of a function -- sometimes even across
+ // reentrancy. So make critical sections as narrow as possible, and use a
+ // bog-standard spinlock with busy-waiting in case of contention for
+ // simplicity.
+ class MOZ_RAII AcquireLock
+ {
+ static mozilla::Atomic<bool, mozilla::ReleaseAcquire> spinLock;
+
+ public:
+ AcquireLock() {
+ while (!spinLock.compareExchange(false, true))
+ continue;
+ }
+ ~AcquireLock() {
+ MOZ_ASSERT(spinLock, "spinlock should have been acquired");
+ spinLock = false;
+ }
+ };
+
+ friend const char* JS::detail::InitWithFailureDiagnostic(bool);
+
+ // Initialize global date/time tracking state. This operation occurs
+ // during, and is restricted to, SpiderMonkey initialization.
+ static void init();
+
+ public:
+ /*
+ * Get the DST offset in milliseconds at a UTC time. This is usually
+ * either 0 or |msPerSecond * SecondsPerHour|, but at least one exotic time
+ * zone (Lord Howe Island, Australia) has a fractional-hour offset, just to
+ * keep things interesting.
+ */
+ static int64_t getDSTOffsetMilliseconds(int64_t utcMilliseconds) {
+ AcquireLock lock;
+
+ return DateTimeInfo::instance.internalGetDSTOffsetMilliseconds(utcMilliseconds);
+ }
+
+ /* ES5 15.9.1.7. */
+ static double localTZA() {
+ AcquireLock lock;
+
+ return DateTimeInfo::instance.localTZA_;
+ }
+
+ private:
+ // We don't want anyone accidentally calling *only*
+ // DateTimeInfo::updateTimeZoneAdjustment() to respond to a system time
+ // zone change (missing the necessary poking of ICU as well), so ensure
+ // only JS::ResetTimeZone() can call this via access restrictions.
+ friend void JS::ResetTimeZone();
+
+ static void updateTimeZoneAdjustment() {
+ AcquireLock lock;
+
+ DateTimeInfo::instance.internalUpdateTimeZoneAdjustment();
+ }
+
+ /*
+ * The current local time zone adjustment, cached because retrieving this
+ * dynamically is Slow, and a certain venerable benchmark which shall not
+ * be named depends on it being fast.
+ *
+ * SpiderMonkey occasionally and arbitrarily updates this value from the
+ * system time zone to attempt to keep this reasonably up-to-date. If
+ * temporary inaccuracy can't be tolerated, JSAPI clients may call
+ * JS::ResetTimeZone to forcibly sync this with the system time zone.
+ */
+ double localTZA_;
+
+ /*
+ * Compute the DST offset at the given UTC time in seconds from the epoch.
+ * (getDSTOffsetMilliseconds attempts to return a cached value, but in case
+ * of a cache miss it calls this method. The cache is represented through
+ * the offset* and *{Start,End}Seconds fields below.)
+ */
+ int64_t computeDSTOffsetMilliseconds(int64_t utcSeconds);
+
+ int64_t offsetMilliseconds;
+ int64_t rangeStartSeconds, rangeEndSeconds; // UTC-based
+
+ int64_t oldOffsetMilliseconds;
+ int64_t oldRangeStartSeconds, oldRangeEndSeconds; // UTC-based
+
+ /*
+ * Cached offset in seconds from the current UTC time to the current
+ * local standard time (i.e. not including any offset due to DST).
+ */
+ int32_t utcToLocalStandardOffsetSeconds;
+
+ static const int64_t MaxUnixTimeT = 2145859200; /* time_t 12/31/2037 */
+
+ static const int64_t RangeExpansionAmount = 30 * SecondsPerDay;
+
+ int64_t internalGetDSTOffsetMilliseconds(int64_t utcMilliseconds);
+ void internalUpdateTimeZoneAdjustment();
+
+ void sanityCheck();
+};
+
+/**
+ * ICU's default time zone, used for various date/time formatting operations
+ * that include the local time in the representation, is allowed to go stale
+ * for unfortunate performance reasons. Call this function when an up-to-date
+ * default time zone is required, to resync ICU's default time zone with
+ * reality.
+ */
+extern void
+ResyncICUDefaultTimeZone();
+
+} /* namespace js */
+
+#endif /* vm_DateTime_h */