Bug 1083672 - Avoid copying more than needed when drawing a canvas onto itself. r=jrmuizel

dom/canvas/CanvasRenderingContext2D.cpp

@@ -3854,6 +3854,32 @@ bool CanvasRenderingContext2D::IsPointInStroke(const CanvasPath& mPath, double x
   return tempPath->StrokeContainsPoint(strokeOptions, Point(x, y), mTarget->GetTransform());
 }
 
+// Returns a surface that contains only the part needed to draw aSourceRect.
+// On entry, aSourceRect is relative to aSurface, and on return aSourceRect is
+// relative to the returned surface.
+static TemporaryRef<SourceSurface>
+ExtractSubrect(SourceSurface* aSurface, mgfx::Rect* aSourceRect, DrawTarget* aTargetDT)
+{
+  mgfx::Rect roundedOutSourceRect = *aSourceRect;
+  roundedOutSourceRect.RoundOut();
+  mgfx::IntRect roundedOutSourceRectInt;
+  if (!roundedOutSourceRect.ToIntRect(&roundedOutSourceRectInt)) {
+    return aSurface;
+  }
+
+  RefPtr<DrawTarget> subrectDT =
+    aTargetDT->CreateSimilarDrawTarget(roundedOutSourceRectInt.Size(), SurfaceFormat::B8G8R8A8);
+
+  if (!subrectDT) {
+    return aSurface;
+  }
+
+  *aSourceRect -= roundedOutSourceRect.TopLeft();
+
+  subrectDT->CopySurface(aSurface, roundedOutSourceRectInt, IntPoint());
+  return subrectDT->Snapshot();
+}
+
 //
 // image
 //
@@ -3999,10 +4025,19 @@ CanvasRenderingContext2D::DrawImage(const HTMLImageOrCanvasOrVideoElement& image
   }
 
   if (srcSurf) {
+    mgfx::Rect sourceRect(sx, sy, sw, sh);
+    if (element == mCanvasElement) {
+      // srcSurf is a snapshot of mTarget. If we draw to mTarget now, we'll
+      // trigger a COW copy of the whole canvas into srcSurf. That's a huge
+      // waste if sourceRect doesn't cover the whole canvas.
+      // We avoid copying the whole canvas by manually copying just the part
+      // that we need.
+      srcSurf = ExtractSubrect(srcSurf, &sourceRect, mTarget);
+    }
     AdjustedTarget(this, bounds.IsEmpty() ? nullptr : &bounds)->
       DrawSurface(srcSurf,
                   mgfx::Rect(dx, dy, dw, dh),
-                  mgfx::Rect(sx, sy, sw, sh),
+                  sourceRect,
                   DrawSurfaceOptions(filter),
                   DrawOptions(CurrentState().globalAlpha, UsedOperation()));
   } else {