Background

To improve startup performance in later runs of the app, Impeller can use Vulkan's pipeline cache to cache some shader pipeline data to disk. To enable this feature, the embedder must configure Impeller Vulkan's pipeline cache directory (ContextVK::Settings::cache_directory).

However, this feature is not yet available on the embedder API, so out-of-tree embedders cannot benefit from this optimization.

Work

Add an embedder API to configure Impeller Vulkan's cache directory.

Strawman API:

/// Callback for when a VkImage has been written to and is ready for use by the
/// embedder.
typedef bool (*FlutterVulkanPresentCallback)(
    void* /* user data */,
    const FlutterVulkanImage* /* image */);

typedef struct {
  /// The size of this struct. Must be sizeof(FlutterVulkanRendererConfig).
  size_t struct_size;

  // ...

  /// The path to the Vulkan pipeline cache data.
  /// The string can be collected after the call to `FlutterEngineRun` returns.
  /// The string must be NULL terminated.
  /// If NULL, Impeller will not use the Vulkan pipeline cache.
  const char* cache_path;

} FlutterVulkanRendererConfig;

Resources

• Vulkan's pipeline cache docs: https://docs.vulkan.org/guide/latest/pipeline_cache.html
• Impeller Vulkan's pipeline cache logic: https://github.com/flutter/flutter/blob/496e76f95b64889b03234f7d450aea0fdddf0c8e/engine/src/flutter/impeller/renderer/backend/vulkan/pipeline_cache_vk.cc
• Impeller Vulkan's config for the pipeline cache directory, ContextVK::Settings::cache_directory:

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

  Line 82 in 496e76f

  fml::UniqueFD cache_directory;

• Embedder API's logic to create Impeller Vulkan's settings:

  flutter/engine/src/flutter/shell/platform/embedder/embedder_surface_vulkan_impeller.cc

  Lines 53 to 56 in 496e76f

  	impeller::ContextVK::Settings settings;
  	settings.shader_libraries_data = shader_mappings;
  	settings.proc_address_callback =
  	vulkan_dispatch_table.get_instance_proc_address;