src/third_party/angle/src/libANGLE/renderer/vulkan/doc/FastOpenGLStateTransitions.md - cobalt - Git at Google

 # Fast OpenGL State Transitions

 Typical OpenGL programs issue a few small state change commands between draw call commands. We want
 the typical app's use case to be as fast as possible so this leads to unique performance challenges.

 Vulkan in quite different from OpenGL because it requires a separate compiled
 [VkPipeline][VkPipeline] for each state vector. Compiling VkPipelines is multiple orders of
 magnitude slower than enabling or disabling an OpenGL render state. To speed this up we use three
 levels of caching when transitioning states in the Vulkan back-end.

 The first level is the driver's [VkPipelineCache][VkPipelineCache]. The driver
 cache reduces pipeline recompilation time significantly. But even cached
 pipeline recompilations are orders of manitude slower than OpenGL state changes.

 The second level cache is an ANGLE-owned hash map from OpenGL state vectors to compiled pipelines.
 See [GraphicsPipelineCache][GraphicsPipelineCache] in [vk_cache_utils.h](../vk_cache_utils.h). ANGLE's
 [GraphicsPipelineDesc][GraphicsPipelineDesc] class is a tightly packed 256-byte description of the
 current OpenGL rendering state. We also use a [xxHash](https://github.com/Cyan4973/xxHash) for the
 fastest possible hash computation. The hash map speeds up state changes considerably. But it is
 still significantly slower than OpenGL implementations.

 To get best performance we use a transition table from each OpenGL state vector to neighbouring
 state vectors. The transition table points from GraphicsPipelineCache entries directly to
 neighbouring VkPipeline objects. When the application changes state the state change bits are
 recorded into a compact bit mask that covers the GraphicsPipelineDesc state vector. Then on the next
 draw call we scan the transition bit mask and compare the GraphicsPipelineDesc of the current state
 vector and the state vector of the cached transition. With the hash map we compute a hash over the
 entire state vector and then do a 256-byte `memcmp` to guard against hash collisions. With the
 transition table we will only compare as many bytes as were changed in the transition bit mask. By
 skipping the expensive hashing and `memcmp` we can get as good or faster performance than native
 OpenGL drivers.

 Note that the current design of the transition table stores transitions in an unsorted list. If
 applications map from one state to many this will slow down the transition time. This could be
 improved in the future using a faster look up. For instance we could keep a sorted transition table
 or use a small hash map for transitions.

 [VkPipeline]: https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkPipeline.html
 [VkPipelineCache]: https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkPipelineCache.html
 [GraphicsPipelineCache]: https://chromium.googlesource.com/angle/angle/+/225f08bf85a368f905362cdd1366e4795680452c/src/libANGLE/renderer/vulkan/vk_cache_utils.h#498
 [GraphicsPipelineDesc]: https://chromium.googlesource.com/angle/angle/+/225f08bf85a368f905362cdd1366e4795680452c/src/libANGLE/renderer/vulkan/vk_cache_utils.h#244
	# Fast OpenGL State Transitions

	Typical OpenGL programs issue a few small state change commands between draw call commands. We want
	the typical app's use case to be as fast as possible so this leads to unique performance challenges.

	Vulkan in quite different from OpenGL because it requires a separate compiled
	[VkPipeline][VkPipeline] for each state vector. Compiling VkPipelines is multiple orders of
	magnitude slower than enabling or disabling an OpenGL render state. To speed this up we use three
	levels of caching when transitioning states in the Vulkan back-end.

	The first level is the driver's [VkPipelineCache][VkPipelineCache]. The driver
	cache reduces pipeline recompilation time significantly. But even cached
	pipeline recompilations are orders of manitude slower than OpenGL state changes.

	The second level cache is an ANGLE-owned hash map from OpenGL state vectors to compiled pipelines.
	See [GraphicsPipelineCache][GraphicsPipelineCache] in [vk_cache_utils.h](../vk_cache_utils.h). ANGLE's
	[GraphicsPipelineDesc][GraphicsPipelineDesc] class is a tightly packed 256-byte description of the
	current OpenGL rendering state. We also use a [xxHash](https://github.com/Cyan4973/xxHash) for the
	fastest possible hash computation. The hash map speeds up state changes considerably. But it is
	still significantly slower than OpenGL implementations.

	To get best performance we use a transition table from each OpenGL state vector to neighbouring
	state vectors. The transition table points from GraphicsPipelineCache entries directly to
	neighbouring VkPipeline objects. When the application changes state the state change bits are
	recorded into a compact bit mask that covers the GraphicsPipelineDesc state vector. Then on the next
	draw call we scan the transition bit mask and compare the GraphicsPipelineDesc of the current state
	vector and the state vector of the cached transition. With the hash map we compute a hash over the
	entire state vector and then do a 256-byte `memcmp` to guard against hash collisions. With the
	transition table we will only compare as many bytes as were changed in the transition bit mask. By
	skipping the expensive hashing and `memcmp` we can get as good or faster performance than native
	OpenGL drivers.

	Note that the current design of the transition table stores transitions in an unsorted list. If
	applications map from one state to many this will slow down the transition time. This could be
	improved in the future using a faster look up. For instance we could keep a sorted transition table
	or use a small hash map for transitions.

	[VkPipeline]: https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkPipeline.html
	[VkPipelineCache]: https://www.khronos.org/registry/vulkan/specs/1.1-extensions/man/html/VkPipelineCache.html
	[GraphicsPipelineCache]: https://chromium.googlesource.com/angle/angle/+/225f08bf85a368f905362cdd1366e4795680452c/src/libANGLE/renderer/vulkan/vk_cache_utils.h#498
	[GraphicsPipelineDesc]: https://chromium.googlesource.com/angle/angle/+/225f08bf85a368f905362cdd1366e4795680452c/src/libANGLE/renderer/vulkan/vk_cache_utils.h#244