Libretro: grant threaded emulation one frame after input polling

Park the GL emulation worker between frontend runs rather than letting GPU inflight capacity authorize guest CPU lookahead. Wake frame, pause and quit transitions through the same eventcount predicate protocol. Publish all mapped digital buttons with one masked update. This trades cross-run CPU lookahead for consistent input and audio boundaries while retaining rendering overlap inside a run.

libretro/libretro.cpp

@@ -1336,6 +1336,7 @@ namespace Libretro {
    retro_atomic_int_t emuThreadState{ (int)EmuThreadState::DISABLED };
 
    static Thread emuThread;
+   static retro_atomic_int_t emuFrameRequested{ 0 };
    static void EmuFrame() {
       // Frontend callbacks only publish changes. Files and guest memory are
       // touched here, on the same owner that executes PSP code.
@@ -1399,7 +1400,17 @@ namespace Libretro {
          switch (state)
          {
             case EmuThreadState::RUNNING:
-               EmuFrame();
+               // A libretro run grants exactly one frame after polling input.
+               // GPU buffer availability must not let guest execution run ahead
+               // with the previous run's input or audio timeline.
+               ParkingLotWait(&emuThreadState, [] {
+                  return retro_atomic_load_acquire_int(&emuFrameRequested) != 0 ||
+                     EmuThreadStateGet() != EmuThreadState::RUNNING;
+               });
+               if (EmuThreadStateGet() != EmuThreadState::RUNNING)
+                  break;
+               if (retro_atomic_exchange_int(&emuFrameRequested, 0))
+                  EmuFrame();
                break;
             case EmuThreadState::PAUSE_REQUESTED:
                // CAS so a concurrent QUIT_REQUESTED is not clobbered; the loop re-reads the state.
@@ -1456,6 +1467,7 @@ namespace Libretro {
       while (ctx->ThreadFrame()) {}
 
       emuThread.join();
+      retro_atomic_store_relaxed_int(&emuFrameRequested, 0);
       ctx->ThreadEnd();
    }
 
@@ -1464,6 +1476,7 @@ namespace Libretro {
          return;
 
       EmuThreadStateSet(EmuThreadState::PAUSE_REQUESTED);
+      ParkingLotNotify(&emuThreadState);
 
       // Run queued work, readback syncs included, up to the marker the emu
       // thread queues once it has parked at a frame boundary.
@@ -1608,7 +1621,7 @@ static void retro_input(void) {
    unsigned i;
    int16_t ret = 0;
    // clang-format off
-   static struct
+   static const struct
    {
       u32 retro;
       u32 sceCtrl;
@@ -1639,14 +1652,15 @@ static void retro_input(void) {
             ret |= (1 << i);
    }
 
-   for (i = 0; i < sizeof(map) / sizeof(*map); i++) {
-      bool pressed = ret & (1 << map[i].retro);
-      if (pressed) {
-         __CtrlUpdateButtons(map[i].sceCtrl, 0);
-      } else {
-         __CtrlUpdateButtons(0, map[i].sceCtrl);
-      }
+   u32 buttons = 0;
+   u32 controlledButtons = 0;
+   for (const auto &entry : map) {
+      controlledButtons |= entry.sceCtrl;
+      if (ret & (1 << entry.retro))
+         buttons |= entry.sceCtrl;
    }
+   // Publish one digital snapshot rather than twelve transient combinations.
+   __CtrlUpdateButtons(buttons, controlledButtons & ~buttons);
 
    float x_left = input_state_cb(0, RETRO_DEVICE_ANALOG, RETRO_DEVICE_INDEX_ANALOG_LEFT, RETRO_DEVICE_ID_ANALOG_X) / 32767.0f;
    float y_left = input_state_cb(0, RETRO_DEVICE_ANALOG, RETRO_DEVICE_INDEX_ANALOG_LEFT, RETRO_DEVICE_ID_ANALOG_Y) / -32767.0f;
@@ -1760,6 +1774,8 @@ void retro_run(void) {
       if (EmuThreadStateGet() != EmuThreadState::RUNNING) {
          EmuThreadStart();
       }
+      retro_atomic_store_release_int(&emuFrameRequested, 1);
+      ParkingLotNotify(&emuThreadState);
 
       if (!ctx->ThreadFrame()) {
          // We're done processing the last frame from the emu thread.