Bug 1527181 - Add webgl.auto-flush:true. r=lsalzman

* webgl.auto-flush (default: true) enqueues a runnable to flush pending
  webgl commands after each draw.
* webgl.auto-flush.gl (default: false) causes this auto-flush to also
  call glFlush.

This is probably not useful for pure RAF-based apps, since they run
RAF();Present(); back-to-back. For other cases though, this should
improve latency and may reduce jank.

Differential Revision: https://phabricator.services.mozilla.com/D123031

dom/canvas/ClientWebGLContext.cpp

@@ -360,7 +360,7 @@ void ClientWebGLContext::Present(WebGLFramebufferJS* const xrFb,
   if (!xrFb) {
     mIsCanvasDirty = false;
   }
-
+  CancelAutoFlush();
   Run<RPROC(Present)>(xrFb ? xrFb->mId : 0, type, webvr);
 }
 
@@ -2705,12 +2705,14 @@ void ClientWebGLContext::DepthRange(GLclampf zNear, GLclampf zFar) {
   Run<RPROC(DepthRange)>(zNear, zFar);
 }
 
-void ClientWebGLContext::Flush() {
+void ClientWebGLContext::Flush(const bool flushGl) {
   const FuncScope funcScope(*this, "flush");
   const auto notLost = mNotLost;
   if (IsContextLost()) return;
 
-  Run<RPROC(Flush)>();
+  if (flushGl) {
+    Run<RPROC(Flush)>();
+  }
 
   if (notLost->inProcess) return;
   const auto& child = mNotLost->outOfProcess;

dom/canvas/ClientWebGLContext.h

@@ -15,6 +15,7 @@
 #include "mozilla/dom/WebGLRenderingContextBinding.h"
 #include "mozilla/dom/WebGL2RenderingContextBinding.h"
 #include "mozilla/layers/LayersSurfaces.h"
+#include "mozilla/StaticPrefs_webgl.h"
 #include "WebGLFormats.h"
 #include "WebGLStrongTypes.h"
 #include "WebGLTypes.h"
@@ -1012,12 +1013,43 @@ class ClientWebGLContext final : public nsICanvasRenderingContextInternal,
   void DoReadPixels(const webgl::ReadPixelsDesc&, Range<uint8_t>) const;
   uvec2 DrawingBufferSize();
 
+  // -
+
+  bool mAutoFlushPending = false;
+
+  void AutoEnqueueFlush() {
+    if (MOZ_LIKELY(mAutoFlushPending)) return;
+    mAutoFlushPending = true;
+
+    const auto weak = WeakPtr<ClientWebGLContext>(this);
+    const auto DeferredFlush = [weak]() {
+      const auto strong = RefPtr<ClientWebGLContext>(weak);
+      if (!strong) return;
+      if (!strong->mAutoFlushPending) return;
+      strong->mAutoFlushPending = false;
+
+      if (!StaticPrefs::webgl_auto_flush()) return;
+      const bool flushGl = StaticPrefs::webgl_auto_flush_gl();
+      strong->Flush(flushGl);
+    };
+
+    already_AddRefed<mozilla::Runnable> runnable = NS_NewRunnableFunction(
+        "enqueue Event_webglcontextrestored", DeferredFlush);
+    NS_DispatchToCurrentThread(std::move(runnable));
+  }
+
+  void CancelAutoFlush() { mAutoFlushPending = false; }
+
+  // -
+
   void AfterDrawCall() {
     if (!mNotLost) return;
     const auto& state = State();
     if (!state.mBoundDrawFb) {
       MarkCanvasDirty();
     }
+
+    AutoEnqueueFlush();
   }
 
   // -------------------------------------------------------------------------
@@ -1251,7 +1283,7 @@ class ClientWebGLContext final : public nsICanvasRenderingContextInternal,
 
   void DepthRange(GLclampf zNear, GLclampf zFar);
 
-  void Flush();
+  void Flush(bool flushGl = true);
 
   void Finish();
 

modules/libpref/init/StaticPrefList.yaml

@@ -11431,6 +11431,16 @@
   value: false
   mirror: always
 
+- name: webgl.auto-flush
+  type: RelaxedAtomicBool
+  value: true
+  mirror: always
+
+- name: webgl.auto-flush.gl
+  type: RelaxedAtomicBool
+  value: false
+  mirror: always
+
 - name: webgl.can-lose-context-in-foreground
   type: RelaxedAtomicBool
   value: true