Metal: avoid redundant scissor rect state changes (#8207)

filament/backend/src/metal/MetalContext.h

@@ -161,6 +161,7 @@ struct MetalContext {
     CullModeStateTracker cullModeState;
     WindingStateTracker windingState;
     DepthClampStateTracker depthClampState;
+    ScissorRectStateTracker scissorRectState;
     Handle<HwRenderPrimitive> currentRenderPrimitive;
 
     // State caches.

filament/backend/src/metal/MetalDriver.mm

@@ -1227,6 +1227,7 @@ void MetalDriver::beginRenderPass(Handle<HwRenderTarget> rth,
     mContext->depthStencilState.invalidate();
     mContext->cullModeState.invalidate();
     mContext->windingState.invalidate();
+    mContext->scissorRectState.invalidate();
     mContext->currentPolygonOffset = {0.0f, 0.0f};
 
     mContext->finalizedDescriptorSets.clear();
@@ -1953,7 +1954,11 @@ void MetalDriver::scissor(Viewport scissorBox) {
             .height = static_cast<NSUInteger>(bottom - top)
     };
 
-    [mContext->currentRenderPassEncoder setScissorRect:scissorRect];
+    auto& srs = mContext->scissorRectState;
+    srs.updateState(scissorRect);
+    if (srs.stateChanged()) {
+        [mContext->currentRenderPassEncoder setScissorRect:scissorRect];
+    }
 }
 
 void MetalDriver::beginTimerQuery(Handle<HwTimerQuery> tqh) {

filament/backend/src/metal/MetalState.h

@@ -204,9 +204,8 @@ private:
 // Different kinds of state, like pipeline state, uniform buffer state, etc., are passed to the
 // current Metal command encoder and persist throughout the lifetime of the encoder (a frame).
 // StateTracker is used to prevent calling redundant state change methods.
-template<typename StateType>
+template <typename StateType, typename StateEqual = std::equal_to<StateType>>
 class StateTracker {
-
 public:
 
     // Call to force the state to dirty at the beginning of each frame, as all state must be
@@ -214,7 +213,7 @@ public:
     void invalidate() noexcept { mStateDirty = true; }
 
     void updateState(const StateType& newState) noexcept {
-        if (mCurrentState != newState) {
+        if (!StateEqual()(mCurrentState, newState)) {
             mCurrentState = newState;
             mStateDirty = true;
         }
@@ -235,7 +234,6 @@ private:
 
     bool mStateDirty = true;
     StateType mCurrentState = {};
-
 };
 
 // Pipeline state
@@ -343,6 +341,16 @@ using DepthStencilStateTracker = StateTracker<DepthStencilState>;
 using DepthStencilStateCache = StateCache<DepthStencilState, id<MTLDepthStencilState>,
         DepthStateCreator>;
 
+struct MtlScissorRectEqual {
+    bool operator()(const MTLScissorRect& lhs, const MTLScissorRect& rhs) const {
+        return lhs.height == rhs.height &&
+            lhs.width == rhs.width &&
+            lhs.x == rhs.x &&
+            lhs.y == rhs.y;
+    }
+};
+using ScissorRectStateTracker = StateTracker<MTLScissorRect, MtlScissorRectEqual>;
+
 // Uniform buffers
 
 class MetalBufferObject;