[Impeller] dont redundantly set stencil reference on vulkan backend. (#164763)

Cache the last stencil reference in RenderPassVK. If the requested
stencil reference is set to the same value, don't update it on the cmd
buffer.


Hypothetical performance improvement, but easy to do.

Author: Jonah Williams

engine/src/flutter/ci/licenses_golden/excluded_files

@@ -202,6 +202,7 @@
 ../../../flutter/impeller/renderer/backend/vulkan/pipeline_cache_data_vk_unittests.cc
 ../../../flutter/impeller/renderer/backend/vulkan/render_pass_builder_vk_unittests.cc
 ../../../flutter/impeller/renderer/backend/vulkan/render_pass_cache_unittests.cc
+../../../flutter/impeller/renderer/backend/vulkan/render_pass_vk_unittests.cc
 ../../../flutter/impeller/renderer/backend/vulkan/resource_manager_vk_unittests.cc
 ../../../flutter/impeller/renderer/backend/vulkan/swapchain/README.md
 ../../../flutter/impeller/renderer/backend/vulkan/swapchain/khr/README.md

engine/src/flutter/impeller/renderer/backend/vulkan/BUILD.gn

@@ -19,6 +19,7 @@ impeller_component("vulkan_unittests") {
     "pipeline_cache_data_vk_unittests.cc",
     "render_pass_builder_vk_unittests.cc",
     "render_pass_cache_unittests.cc",
+    "render_pass_vk_unittests.cc",
     "resource_manager_vk_unittests.cc",
     "test/gpu_tracer_unittests.cc",
     "test/mock_vulkan.cc",

engine/src/flutter/impeller/renderer/backend/vulkan/render_pass_vk.cc

@@ -340,6 +340,10 @@ void RenderPassVK::SetCommandLabel(std::string_view label) {
 
 // |RenderPass|
 void RenderPassVK::SetStencilReference(uint32_t value) {
+  if (current_stencil_ == value) {
+    return;
+  }
+  current_stencil_ = value;
   command_buffer_vk_.setStencilReference(
       vk::StencilFaceFlagBits::eVkStencilFrontAndBack, value);
 }

engine/src/flutter/impeller/renderer/backend/vulkan/render_pass_vk.h

@@ -34,6 +34,7 @@ class RenderPassVK final : public RenderPass {
   vk::CommandBuffer command_buffer_vk_;
   std::shared_ptr<Texture> color_image_vk_;
   std::shared_ptr<Texture> resolve_image_vk_;
+  uint32_t current_stencil_ = 0;
 
   // Per-command state.
   std::array<vk::DescriptorImageInfo, kMaxBindings> image_workspace_;

engine/src/flutter/impeller/renderer/backend/vulkan/render_pass_vk_unittests.cc

@@ -0,0 +1,53 @@
+// 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 "flutter/testing/testing.h"  // IWYU pragma: keep
+#include "gtest/gtest.h"
+#include "impeller/core/formats.h"
+#include "impeller/renderer/backend/vulkan/render_pass_builder_vk.h"
+#include "impeller/renderer/backend/vulkan/render_pass_vk.h"
+#include "impeller/renderer/backend/vulkan/test/mock_vulkan.h"
+#include "impeller/renderer/render_target.h"
+#include "vulkan/vulkan_enums.hpp"
+
+namespace impeller {
+namespace testing {
+
+TEST(RenderPassVK, DoesNotRedundantlySetStencil) {
+  std::shared_ptr<ContextVK> context = MockVulkanContextBuilder().Build();
+  std::shared_ptr<Context> copy = context;
+  auto cmd_buffer = context->CreateCommandBuffer();
+
+  RenderTargetAllocator allocator(context->GetResourceAllocator());
+  RenderTarget target = allocator.CreateOffscreenMSAA(*copy.get(), {1, 1}, 1);
+
+  std::shared_ptr<RenderPass> render_pass =
+      cmd_buffer->CreateRenderPass(target);
+
+  // Stencil reference set once at buffer start.
+  auto called_functions = GetMockVulkanFunctions(context->GetDevice());
+  EXPECT_EQ(std::count(called_functions->begin(), called_functions->end(),
+                       "vkCmdSetStencilReference"),
+            1);
+
+  // Duplicate stencil ref is not replaced.
+  render_pass->SetStencilReference(0);
+  render_pass->SetStencilReference(0);
+  render_pass->SetStencilReference(0);
+
+  called_functions = GetMockVulkanFunctions(context->GetDevice());
+  EXPECT_EQ(std::count(called_functions->begin(), called_functions->end(),
+                       "vkCmdSetStencilReference"),
+            1);
+
+  // Different stencil value is updated.
+  render_pass->SetStencilReference(1);
+  called_functions = GetMockVulkanFunctions(context->GetDevice());
+  EXPECT_EQ(std::count(called_functions->begin(), called_functions->end(),
+                       "vkCmdSetStencilReference"),
+            2);
+}
+
+}  // namespace testing
+}  // namespace impeller