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112 lines
6.6 KiB
112 lines
6.6 KiB
# Dirty Bits and State Changes
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OpenGL render loops typically involve changing some render states followed by
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a draw call. For instance the app might change a few uniforms and invoke
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`glDrawElements`:
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```
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for (const auto &obj : scene) {
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for (const auto &uni : obj.uniforms) {
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glUniform4fv(uni.loc, uni.data);
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}
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glDrawElements(GL_TRIANGLES, obj.eleCount, GL_UNSIGNED_SHORT, obj.eleOffset);
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}
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```
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Another update loop may change Texture and Vertex Array state before the draw:
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```
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for (const auto &obj : scene) {
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glBindBuffer(GL_ARRAY_BUFFER, obj.arrayBuffer);
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glBufferSubData(GL_ARRAY_BUFFER, obj.bufferOffset, obj.bufferSize, obj.bufferData);
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glVertexAttribPointer(obj.arrayIndex, obj.arraySize, GL_FLOAT, GL_FALSE, 0, nullptr);
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glBindTexture(GL_TEXTURE_2D, obj.texture);
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glDrawElements(GL_TRIANGLES, obj.eleCount, GL_UNSIGNED_SHORT, obj.eleOffset);
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}
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```
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Other update loops may change render states like the blending modes, the depth test, or Framebuffer
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attachments. In each case ANGLE needs to validate, track, and translate these state changes to the
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back-end as efficiently as possible.
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## Dirty Bits
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Each OpenGL Context state value is stored in [`gl::State`](../src/libANGLE/State.h). For instance
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the blending state, depth/stencil state, and current object bindings. Our problem is deciding how to
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notify the back-end when app changes front-end state. We decided to bundle changed state into
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bitsets. Each 1 bit indicates a specific changed state value. We call these bitsets "*dirty bits*".
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See [`gl::State::DirtyBitType`][DirtyBitType].
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Each back-end handles state changes in a `syncState` implementation function that takes a dirty
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bitset. See examples in the [GL back-end][GLSyncState], [D3D11 back-end][D3D11SyncState] and
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[Vulkan back-end][VulkanSyncState].
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Container objects such as Vertex Array Objects and Framebuffers also have their own OpenGL front-end
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state. [VAOs][VAOState] store vertex arrays and array buffer bindings. [Framebuffers][FBOState]
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store attachment state and the active read and draw buffers. These containers also have internal
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dirty bits and `syncState` methods. See [`gl::Framebuffer::DirtyBitType`][FBODirtyBits] and
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[`rx::FramebufferVk::syncState`][FBOVkSyncState] for example.
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Dirty bits allow us to efficiently process groups of state updates. We use fast instrinsic functions
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to scan the bitsets for 1 bits. See [`bitset_utils.h`](../src/common/bitset_utils.h) for more
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information.
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## Cached Validation and State Change Notifications
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To optimize validation we cache many checks. See [`gl::StateCache`][StateCache] for examples. We
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need to refresh cached values on state changes. For instance, enabling a generic vertex array
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changes a cached mask of active vertex arrays. Changes to a texture's images could change a cached
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framebuffer's completeness when the texture is bound as an attachment. And if the draw framebuffer
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becomes incomplete it changes a cached draw call validation check.
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See a below example of a call to `glTexImage2D` that can affect draw call validation:
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<!-- Generated from https://bramp.github.io/js-sequence-diagrams/
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participant App
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participant Context
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participant Framebuffer
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participant Texture
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App->Context: glTexImage2D
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Context->Texture: setImage
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Texture- ->Framebuffer: onSubjectStateChange
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Note over Framebuffer: cache update
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Framebuffer- ->Context: onSubjectStateChange
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Note over Context: cache update
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-->
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We use the [Observer pattern](https://en.wikipedia.org/wiki/Observer_pattern) to implement cache
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invalidation notifications. See [`Observer.h`](../src/libANGLE/Observer.h). In the example the
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`Framebuffer` observes `Texture` attachments via [`angle::ObserverBinding`][ObserverBinding].
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`Framebuffer` implements [`angle::ObserverInterface::onSubjectStateChange`][FBOStateChange] to
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receive a notification to update its completeness cache. The `STORAGE_CHANGED` message triggers a
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call to [`gl::Context::onSubjectStateChange`][ContextStateChange] which in turn calls
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[`gl::StateCache::updateBasicDrawStatesError`][StateCacheUpdate] to re-validate the draw
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framebuffer's completeness. On subsequent draw calls we skip re-validation at minimal cost.
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See the below diagram for the dependency relations between Subjects and Observers.
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## Back-end specific Optimizations
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See [Fast OpenGL State Transitions][FastStateTransitions] in [Vulkan documents][VulkanREADME] for
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additional information for how we implement state change optimization on the Vulkan back-end.
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[DirtyBitType]: https://chromium.googlesource.com/angle/angle/+/5f662c0042703344eb0eef6d1c123e902e3aefbf/src/libANGLE/State.h#483
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[GLSyncState]: https://chromium.googlesource.com/angle/angle/+/5f662c0042703344eb0eef6d1c123e902e3aefbf/src/libANGLE/renderer/gl/StateManagerGL.cpp#1576
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[D3D11SyncState]: https://chromium.googlesource.com/angle/angle/+/5f662c0042703344eb0eef6d1c123e902e3aefbf/src/libANGLE/renderer/d3d/d3d11/StateManager11.cpp#852
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[VulkanSyncState]: https://chromium.googlesource.com/angle/angle/+/5f662c0042703344eb0eef6d1c123e902e3aefbf/src/libANGLE/renderer/vulkan/ContextVk.cpp#642
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[VAOState]: https://chromium.googlesource.com/angle/angle/+/5f662c0042703344eb0eef6d1c123e902e3aefbf/src/libANGLE/VertexArray.h#35
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[FBOState]: https://chromium.googlesource.com/angle/angle/+/5f662c0042703344eb0eef6d1c123e902e3aefbf/src/libANGLE/Framebuffer.h#52
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[FBODirtyBits]: https://chromium.googlesource.com/angle/angle/+/5f662c0042703344eb0eef6d1c123e902e3aefbf/src/libANGLE/Framebuffer.h#319
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[FBOVkSyncState]: https://chromium.googlesource.com/angle/angle/+/5f662c0042703344eb0eef6d1c123e902e3aefbf/src/libANGLE/renderer/vulkan/FramebufferVk.cpp#726
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[StateCache]: https://chromium.googlesource.com/angle/angle/+/5f662c0042703344eb0eef6d1c123e902e3aefbf/src/libANGLE/Context.h#98
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[ObserverBinding]: https://chromium.googlesource.com/angle/angle/+/5f662c0042703344eb0eef6d1c123e902e3aefbf/src/libANGLE/Observer.h#103
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[FBOStateChange]: https://chromium.googlesource.com/angle/angle/+/5f662c0042703344eb0eef6d1c123e902e3aefbf/src/libANGLE/Framebuffer.cpp#1811
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[ContextStateChange]: https://chromium.googlesource.com/angle/angle/+/5f662c0042703344eb0eef6d1c123e902e3aefbf/src/libANGLE/Context.cpp#7981
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[StateCacheUpdate]: https://chromium.googlesource.com/angle/angle/+/5f662c0042703344eb0eef6d1c123e902e3aefbf/src/libANGLE/Context.cpp#8190
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[FastStateTransitions]: ../src/libANGLE/renderer/vulkan/doc/FastOpenGLStateTransitions.md
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[VulkanREADME]: ../src/libANGLE/renderer/vulkan/README.md
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