[Impeller] Optimize away intersect clips that cover the entire pass target.

impeller/entity/BUILD.gn

@@ -267,6 +267,7 @@ impeller_component("entity_unittests") {
 
   sources = [
     "contents/checkerboard_contents_unittests.cc",
+    "contents/clip_contents_unittests.cc",
     "contents/content_context_unittests.cc",
     "contents/filters/gaussian_blur_filter_contents_unittests.cc",
     "contents/filters/inputs/filter_input_unittests.cc",

impeller/entity/contents/clip_contents.cc

@@ -89,6 +89,18 @@ bool ClipContents::RenderDepthClip(const ContentContext& renderer,
                                    const Geometry& geometry) const {
   using VS = ClipPipeline::VertexShader;
 
+  if (clip_op == Entity::ClipOperation::kIntersect &&
+      geometry.IsAxisAlignedRect() &&
+      entity.GetTransform().IsTranslationScaleOnly()) {
+    std::optional<Rect> coverage = geometry.GetCoverage(entity.GetTransform());
+    if (coverage.has_value() &&
+        coverage->Contains(Rect::MakeSize(pass.GetRenderTargetSize()))) {
+      // Skip axis-aligned intersect clips that cover the whole render target
+      // since they won't draw anything to the depth buffer.
+      return true;
+    }
+  }
+
   VS::FrameInfo info;
   info.depth = GetShaderClipDepth(entity);
 

impeller/entity/contents/clip_contents_unittests.cc

@@ -0,0 +1,59 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <memory>
+#include <optional>
+
+#include "gtest/gtest.h"
+
+#include "impeller/entity/contents/checkerboard_contents.h"
+#include "impeller/entity/contents/clip_contents.h"
+#include "impeller/entity/contents/content_context.h"
+#include "impeller/entity/contents/contents.h"
+#include "impeller/entity/contents/test/recording_render_pass.h"
+#include "impeller/entity/entity.h"
+#include "impeller/entity/entity_playground.h"
+#include "impeller/renderer/render_target.h"
+
+namespace impeller {
+namespace testing {
+
+using EntityTest = EntityPlayground;
+
+TEST_P(EntityTest, ClipContentsOptimizesFullScreenIntersectClips) {
+  if (!ContentContext::kEnableStencilThenCover) {
+    GTEST_SKIP();
+    return;
+  }
+
+  // Set up mock environment.
+
+  auto content_context = GetContentContext();
+  auto buffer = content_context->GetContext()->CreateCommandBuffer();
+  auto render_target =
+      GetContentContext()->GetRenderTargetCache()->CreateOffscreenMSAA(
+          *content_context->GetContext(), {100, 100},
+          /*mip_count=*/1);
+  auto render_pass = buffer->CreateRenderPass(render_target);
+  auto recording_pass = std::make_shared<RecordingRenderPass>(
+      render_pass, GetContext(), render_target);
+
+  // Set up clip contents.
+
+  auto contents = std::make_shared<ClipContents>();
+  contents->SetClipOperation(Entity::ClipOperation::kIntersect);
+  contents->SetGeometry(Geometry::MakeCover());
+
+  Entity entity;
+  entity.SetContents(std::move(contents));
+
+  // Render the clip contents.
+
+  ASSERT_TRUE(recording_pass->GetCommands().empty());
+  ASSERT_TRUE(entity.Render(*content_context, *recording_pass));
+  ASSERT_FALSE(recording_pass->GetCommands().empty());
+}
+
+}  // namespace testing
+}  // namespace impeller