1 files changed, 104 insertions, 2 deletions

diff --git a/src/common/thread.cpp b/src/common/thread.cpp
index f4bdb3f7c0..49f6f5cf98 100644
--- a/src/common/thread.cpp
+++ b/src/common/thread.cpp
@@ -4,6 +4,8 @@
 // SPDX-FileCopyrightText: 2014 Citra Emulator Project
 // SPDX-License-Identifier: GPL-2.0-or-later
 
+#include <chrono>
+#include <limits>
 #include <string>
 #include <thread>
 
@@ -18,24 +20,35 @@
 #elif defined(_WIN32)
 #include <windows.h>
 #include "common/string_util.h"
+#include "common/windows/timer_resolution.h"
 #else
 #if defined(__FreeBSD__)
 #include <sys/cpuset.h>
 #include <sys/_cpuset.h>
 #include <pthread_np.h>
+// Compatibility with CPUset
+#define cpu_set_t cpuset_t
 #elif defined(__DragonFly__) || defined(__OpenBSD__) || defined(__Bitrig__)
 #include <pthread_np.h>
 #endif
 #include <pthread.h>
 #include <sched.h>
 #endif
+
 #ifndef _WIN32
 #include <unistd.h>
 #endif
 
-#ifdef __FreeBSD__
-#   define cpu_set_t cpuset_t
+#include "common/cpu_features.h"
+#ifdef ARCHITECTURE_x86_64
+#ifdef _MSC_VER
+#include <intrin.h>
+#else
+#include <x86intrin.h>
 #endif
+#include "common/x64/rdtsc.h"
+#endif
+#include "core/core_timing.h"
 
 namespace Common {
 
@@ -144,4 +157,93 @@ void PinCurrentThreadToPerformanceCore(size_t core_id) {
     }
 }
 
+#ifdef ARCHITECTURE_x86_64
+// On Linux and UNIX systems, a futex would nominally be used to cover the costs
+// the idea is that it's intuitivelly cheaper to use a direct instruction as opposed to a full futex call
+// the underlying libc++ implementation uses pthread_cond_timedwait which MAY invoke a futex
+// Let's pretend the OS is too expensive to jump into, and avoid ANY context switches
+// this should *IN THEORY* lower CPU usage while just waiting for stuff effectively
+// For windows the minimal quanta resolution is about 500us, and normal CRT cond var is 1.5ms(?)
+// so may as well avoid that too
+// Let's just give ALL platforms the same mechanisms (almost) for when they have umonitor OR waitpkg
+#ifdef __clang__
+__attribute__((target("waitpkg,mwaitx")))
+#elif defined(__GNUC__)
+#pragma GCC target("waitpkg")
+#pragma GCC target("mwaitx")
+#endif
+bool Event::WaitFor(const std::chrono::nanoseconds time) {
+#ifdef _WIN32
+    auto const start = Common::X64::FencedRDTSC();
+    auto const& caps = Common::g_cpu_caps;
+    [[maybe_unused]] auto const end = start + Common::g_wall_clock.NsToTicks(time);
+    if (caps.monitorx) {
+        while (true) {
+            // Armed monitor, as per manual, MWAITX must be conditional if the condition isn't satisfied
+            // to prevent a race condition.
+            _mm_monitorx(reinterpret_cast<u64*>(std::addressof(is_set)), 0, 0);
+            if (!is_set.load()) {
+                // RDTSC may be fenced here due to atomic load
+                auto const now = _rdtsc();
+                if (end > now) {
+                    u32 const cycles = std::min<u32>((std::numeric_limits<u32>::max)(), s64(end) - s64(now));
+                    // See here: https://github.com/torvalds/linux/blob/948a64995aca6820abefd17f1a4258f5835c5ad9/arch/x86/lib/delay.c#L93
+                    // MWAITX accepts a 32-bit input timer which determines the total number of cycles to wait for
+                    // NOT THE TOTAL ABSOLUTE TSC VALUE, it's just a delta
+                    // BIT[1] = use a timer
+                    // Hint = 0: Use C1 state when sleepy (means slower wakeup but better savings)
+                    _mm_mwaitx(1 << 1, 0u, cycles);
+                    if (!is_set.load())
+                        return false;
+                } else
+                    return false; //timeout
+            }
+            bool expected = true;
+            if (is_set.compare_exchange_weak(expected, false, std::memory_order_release))
+                return true;
+        }
+    } else if (caps.waitpkg) {
+        // #UD If CPUID.7.0:ECX.WAITPKG[bit 5]=0.
+        while (true) {
+            _umonitor(std::addressof(is_set));
+            if (!is_set.load() && !_umwait(0, end)) //umwait is absolute time!!!
+                return false;
+            bool expected = true;
+            if (is_set.compare_exchange_weak(expected, false, std::memory_order_release))
+                return true;
+        }
+    } else {
+        while (!is_set.load() && end > _rdtsc())
+            Common::Windows::SleepForOneTick();
+        if (is_set.load())
+            Reset();
+        return true;
+    }
+#else
+    std::unique_lock lk{mutex};
+    if (!condvar.wait_for(lk, time, [this] { return is_set.load(); }))
+        return false;
+    is_set = false;
+    return true;
+#endif
+}
+#else
+bool Event::WaitFor(const std::chrono::nanoseconds time) {
+#ifdef _WIN32
+    auto const end = Common::g_wall_clock.GetTimeNS() + time;
+    while (!is_set.load() && end > Common::g_wall_clock.GetTimeNS())
+        Common::Windows::SleepForOneTick();
+    if (is_set.load())
+        Reset();
+    return true;
+#else
+    std::unique_lock lk{mutex};
+    if (!condvar.wait_for(lk, time, [this] { return is_set.load(); }))
+        return false;
+    is_set = false;
+    return true;
+#endif
+}
+#endif
+
 } // namespace Common