Bug 964794 - Remove extra Lock and minimize DT creation/destruction in D3D11 TextureClient. r=Bas

gfx/layers/d3d11/TextureD3D11.cpp

@@ -146,6 +146,7 @@ TextureClientD3D11::TextureClientD3D11(gfx::SurfaceFormat aFormat, TextureFlags
   : TextureClient(aFlags)
   , mFormat(aFormat)
   , mIsLocked(false)
+  , mNeedsClear(false)
 {}
 
 TextureClientD3D11::~TextureClientD3D11()
@@ -159,6 +160,12 @@ TextureClientD3D11::Lock(OpenMode aMode)
   }
   MOZ_ASSERT(!mIsLocked, "The Texture is already locked!");
   LockD3DTexture(mTexture.get());
+
+  if (mNeedsClear) {
+    mDrawTarget->ClearRect(Rect(0, 0, GetSize().width, GetSize().height));
+    mNeedsClear = false;
+  }
+
   mIsLocked = true;
   return true;
 }
@@ -169,7 +176,6 @@ TextureClientD3D11::Unlock()
   MOZ_ASSERT(mIsLocked, "Unlocked called while the texture is not locked!");
   if (mDrawTarget) {
     mDrawTarget->Flush();
-    mDrawTarget = nullptr;
   }
   UnlockD3DTexture(mTexture.get());
   mIsLocked = false;
@@ -184,6 +190,8 @@ TextureClientD3D11::GetAsDrawTarget()
     return mDrawTarget;
   }
 
+  // The DrawTarget is created only once, and is only usable between calls
+  // to Lock and Unlock.
   mDrawTarget = Factory::CreateDrawTargetForD3D10Texture(mTexture, mFormat);
   return mDrawTarget;
 }
@@ -207,13 +215,8 @@ TextureClientD3D11::AllocateForSurface(gfx::IntSize aSize, TextureAllocationFlag
     return false;
   }
 
-  if (aFlags & ALLOC_CLEAR_BUFFER) {
-    DebugOnly<bool> locked = Lock(OPEN_WRITE_ONLY);
-    MOZ_ASSERT(locked);
-    RefPtr<DrawTarget> dt = GetAsDrawTarget();
-    dt->ClearRect(Rect(0, 0, GetSize().width, GetSize().height));
-    Unlock();
-  }
+  // Defer clearing to the next time we lock to avoid an extra (expensive) lock.
+  mNeedsClear = aFlags & ALLOC_CLEAR_BUFFER;
 
   return true;
 }

gfx/layers/d3d11/TextureD3D11.h

@@ -66,6 +66,7 @@ protected:
   RefPtr<gfx::DrawTarget> mDrawTarget;
   gfx::SurfaceFormat mFormat;
   bool mIsLocked;
+  bool mNeedsClear;
 };
 
 /**