[Base] Add chrono support

- WinSystemClock is a FILETIME clock without scaling, can convert to
  system_time
- XSystemClock is a FILTETIME clock with scaling applied, can only
  convert to WinSystemClock

src/xenia/base/chrono.h

@@ -0,0 +1,176 @@
+/**
+ ******************************************************************************
+ * Xenia : Xbox 360 Emulator Research Project                                 *
+ ******************************************************************************
+ * Copyright 2022 Ben Vanik. All rights reserved.                             *
+ * Released under the BSD license - see LICENSE in the root for more details. *
+ ******************************************************************************
+ */
+
+#ifndef XENIA_BASE_CHRONO_H_
+#define XENIA_BASE_CHRONO_H_
+
+#include <chrono>
+#include <cstdint>
+
+// https://github.com/HowardHinnant/date/commit/5ba1c1ad8514362dba596f228eb20eb13f63d948#r33275526
+#define HAS_UNCAUGHT_EXCEPTIONS 1
+#include "third_party/date/include/date/tz.h"
+
+#include "xenia/base/clock.h"
+
+namespace xe {
+using hundrednano = std::ratio<1, 10000000>;
+
+namespace chrono {
+
+using hundrednanoseconds = std::chrono::duration<int64_t, hundrednano>;
+
+// https://docs.microsoft.com/en-us/windows/win32/sysinfo/converting-a-time-t-value-to-a-file-time
+//  Don't forget the 89 leap days.
+static constexpr std::chrono::seconds seconds_1601_to_1970 =
+    (396 * 365 + 89) * std::chrono::seconds(60 * 60 * 24);
+
+// TODO(JoelLinn) define xstead_clock xsystem_clock etc.
+
+namespace internal {
+// Trick to reduce code duplication and keep all the chrono template magic
+// working
+enum class Domain {
+  // boring host clock:
+  Host,
+  // adheres to guest scaling (differrent speed, changing clock drift etc):
+  Guest
+};
+
+template <Domain domain_>
+struct NtSystemClock {
+  using rep = int64_t;
+  using period = hundrednano;
+  using duration = hundrednanoseconds;
+  using time_point = std::chrono::time_point<NtSystemClock<domain_>>;
+  // This really depends on the context the clock is used in:
+  // static constexpr bool is_steady = false;
+
+ public:
+  // The delta between std::chrono::system_clock (Jan 1 1970) and NT file
+  // time (Jan 1 1601), in seconds. In the spec std::chrono::system_clock's
+  // epoch is undefined, but C++20 cements it as Jan 1 1970.
+  static constexpr std::chrono::seconds unix_epoch_delta() {
+    using std::chrono::steady_clock;
+    auto filetime_epoch = date::year{1601} / date::month{1} / date::day{1};
+    auto system_clock_epoch = date::year{1970} / date::month{1} / date::day{1};
+    auto fp = static_cast<date::sys_seconds>(
+        static_cast<date::sys_days>(filetime_epoch));
+    auto sp = static_cast<date::sys_seconds>(
+        static_cast<date::sys_days>(system_clock_epoch));
+    return fp.time_since_epoch() - sp.time_since_epoch();
+  }
+
+ public:
+  static constexpr uint64_t to_file_time(time_point const& tp) noexcept {
+    return static_cast<uint64_t>(tp.time_since_epoch().count());
+  }
+
+  static constexpr time_point from_file_time(uint64_t const& tp) noexcept {
+    return time_point{duration{tp}};
+  }
+
+  // To convert XSystemClock to sys, do clock_cast<WinSystemTime>(tp) first
+  // SFINAE hack https://stackoverflow.com/a/58813009
+  template <Domain domain_fresh_ = domain_>
+  static constexpr std::enable_if_t<domain_fresh_ == Domain::Host,
+                                    std::chrono::system_clock::time_point>
+  to_sys(const time_point& tp) {
+    using sys_duration = std::chrono::system_clock::duration;
+    using sys_time = std::chrono::system_clock::time_point;
+
+    auto dp = tp;
+    dp += unix_epoch_delta();
+    auto cdp = std::chrono::time_point_cast<sys_duration>(dp);
+    return sys_time{cdp.time_since_epoch()};
+  }
+
+  template <Domain domain_fresh_ = domain_>
+  static constexpr std::enable_if_t<domain_fresh_ == Domain::Host, time_point>
+  from_sys(const std::chrono::system_clock::time_point& tp) {
+    auto ctp = std::chrono::time_point_cast<duration>(tp);
+    auto dp = time_point{ctp.time_since_epoch()};
+    dp -= unix_epoch_delta();
+    return dp;
+  }
+
+  [[nodiscard]] static time_point now() noexcept {
+    if constexpr (domain_ == Domain::Host) {
+      // QueryHostSystemTime() returns windows epoch times even on POSIX
+      return from_file_time(Clock::QueryHostSystemTime());
+    } else if constexpr (domain_ == Domain::Guest) {
+      return from_file_time(Clock::QueryGuestSystemTime());
+    }
+  }
+};
+}  // namespace internal
+
+// Unscaled system clock which can be used for filetime <-> system_clock
+// conversion
+using WinSystemClock = internal::NtSystemClock<internal::Domain::Host>;
+
+// Guest system clock, scaled
+using XSystemClock = internal::NtSystemClock<internal::Domain::Guest>;
+
+}  // namespace chrono
+}  // namespace xe
+
+namespace date {
+
+template <>
+struct clock_time_conversion<::xe::chrono::WinSystemClock,
+                             ::xe::chrono::XSystemClock> {
+  using WClock_ = ::xe::chrono::WinSystemClock;
+  using XClock_ = ::xe::chrono::XSystemClock;
+
+  template <typename Duration>
+  typename WClock_::time_point operator()(
+      const std::chrono::time_point<XClock_, Duration>& t) const {
+    // Consult chrono_steady_cast.h for explanation on this:
+    std::atomic_thread_fence(std::memory_order_acq_rel);
+    auto w_now = WClock_::now();
+    auto x_now = XClock_::now();
+    std::atomic_thread_fence(std::memory_order_acq_rel);
+
+    auto delta = (t - x_now);
+    if (!::cvars::clock_no_scaling) {
+      delta = std::chrono::floor<WClock_::duration>(
+          delta * xe::Clock::guest_time_scalar());
+    }
+    return w_now + delta;
+  }
+};
+
+template <>
+struct clock_time_conversion<::xe::chrono::XSystemClock,
+                             ::xe::chrono::WinSystemClock> {
+  using WClock_ = ::xe::chrono::WinSystemClock;
+  using XClock_ = ::xe::chrono::XSystemClock;
+
+  template <typename Duration>
+  typename XClock_::time_point operator()(
+      const std::chrono::time_point<WClock_, Duration>& t) const {
+    // Consult chrono_steady_cast.h for explanation on this:
+    std::atomic_thread_fence(std::memory_order_acq_rel);
+    auto w_now = WClock_::now();
+    auto x_now = XClock_::now();
+    std::atomic_thread_fence(std::memory_order_acq_rel);
+
+    xe::chrono::hundrednanoseconds delta = (t - w_now);
+    if (!::cvars::clock_no_scaling) {
+      delta = std::chrono::floor<WClock_::duration>(
+          delta / xe::Clock::guest_time_scalar());
+    }
+    return x_now + delta;
+  }
+};
+
+}  // namespace date
+
+#endif