[Impeller] preallocate command buffer to next power of two of entity list.

impeller/entity/entity_pass.cc

@@ -835,8 +835,9 @@ bool EntityPass::OnRender(
   TRACE_EVENT0("impeller", "EntityPass::OnRender");
 
   auto context = renderer.GetContext();
-  InlinePassContext pass_context(
-      context, pass_target, GetTotalPassReads(renderer), collapsed_parent_pass);
+  InlinePassContext pass_context(context, pass_target,
+                                 GetTotalPassReads(renderer), GetElementCount(),
+                                 collapsed_parent_pass);
   if (!pass_context.IsValid()) {
     VALIDATION_LOG << SPrintF("Pass context invalid (Depth=%d)", pass_depth);
     return false;

impeller/entity/inline_pass_context.cc

@@ -18,9 +18,11 @@ InlinePassContext::InlinePassContext(
     std::shared_ptr<Context> context,
     EntityPassTarget& pass_target,
     uint32_t pass_texture_reads,
+    uint32_t entity_count,
     std::optional<RenderPassResult> collapsed_parent_pass)
     : context_(std::move(context)),
       pass_target_(pass_target),
+      entity_count_(entity_count),
       is_collapsed_(collapsed_parent_pass.has_value()) {
   if (collapsed_parent_pass.has_value()) {
     pass_ = collapsed_parent_pass.value().pass;
@@ -149,7 +151,10 @@ InlinePassContext::RenderPassResult InlinePassContext::GetRenderPass(
     VALIDATION_LOG << "Could not create render pass.";
     return {};
   }
-
+  // Commands are fairly large (500B) objects, so re-allocation of the command
+  // buffer while encoding can add a surprising amount of overhead. We make a
+  // conservative npot estimate to avoid this case.
+  pass_->ReserveCommands(Allocation::NextPowerOfTwoSize(entity_count_));
   pass_->SetLabel(
       "EntityPass Render Pass: Depth=" + std::to_string(pass_depth) +
       " Count=" + std::to_string(pass_count_));

impeller/entity/inline_pass_context.h

@@ -24,14 +24,21 @@ class InlinePassContext {
       std::shared_ptr<Context> context,
       EntityPassTarget& pass_target,
       uint32_t pass_texture_reads,
+      uint32_t entity_count,
       std::optional<RenderPassResult> collapsed_parent_pass = std::nullopt);
+
   ~InlinePassContext();
 
   bool IsValid() const;
+
   bool IsActive() const;
+
   std::shared_ptr<Texture> GetTexture();
+
   bool EndPass();
+
   EntityPassTarget& GetPassTarget() const;
+
   uint32_t GetPassCount() const;
 
   RenderPassResult GetRenderPass(uint32_t pass_depth);
@@ -42,6 +49,7 @@ class InlinePassContext {
   std::shared_ptr<CommandBuffer> command_buffer_;
   std::shared_ptr<RenderPass> pass_;
   uint32_t pass_count_ = 0;
+  uint32_t entity_count_ = 0;
 
   // Whether this context is collapsed into a parent entity pass.
   bool is_collapsed_ = false;

impeller/renderer/command.h

@@ -92,10 +92,9 @@ struct Bindings {
 ///             * Specify any stage bindings.
 ///             * (Optional) Specify a debug label.
 ///
-///             Command are very lightweight objects and can be created
-///             frequently and on demand. The resources referenced in commands
-///             views into buffers managed by other allocators and resource
-///             managers.
+///             Command can be created frequently and on demand. The resources
+///             referenced in commands views into buffers managed by other
+///             allocators and resource managers.
 ///
 struct Command : public ResourceBinder {
   //----------------------------------------------------------------------------

impeller/renderer/render_pass.h

@@ -36,6 +36,13 @@ class RenderPass {
 
   void SetLabel(std::string label);
 
+  /// @brief Reserve [command_count] commands in the HAL command buffer.
+  ///
+  /// Note: this is not the native command buffer.
+  void ReserveCommands(size_t command_count) {
+    commands_.reserve(command_count);
+  }
+
   HostBuffer& GetTransientsBuffer();
 
   //----------------------------------------------------------------------------

impeller/renderer/renderer_unittests.cc

@@ -33,6 +33,7 @@
 #include "impeller/renderer/command_buffer.h"
 #include "impeller/renderer/pipeline_builder.h"
 #include "impeller/renderer/pipeline_library.h"
+#include "impeller/renderer/render_target.h"
 #include "impeller/renderer/renderer.h"
 #include "impeller/renderer/sampler_library.h"
 #include "impeller/renderer/surface.h"
@@ -1257,6 +1258,21 @@ TEST_P(RendererTest, StencilMask) {
   OpenPlaygroundHere(callback);
 }
 
+TEST_P(RendererTest, CanPreAllocateCommands) {
+  auto context = GetContext();
+  auto cmd_buffer = context->CreateCommandBuffer();
+  auto render_target_cache = std::make_shared<RenderTargetAllocator>(
+      GetContext()->GetResourceAllocator());
+
+  auto render_target =
+      RenderTarget::CreateOffscreen(*context, *render_target_cache, {100, 100});
+  auto render_pass = cmd_buffer->CreateRenderPass(render_target);
+
+  render_pass->ReserveCommands(100u);
+
+  EXPECT_EQ(render_pass->GetCommands().capacity(), 100u);
+}
+
 }  // namespace testing
 }  // namespace impeller