[impeller] Support partitioned host buffer #174463

eyebrowsoffire · 2025-08-26T20:10:39Z

This change adds support for host buffer that keeps index buffers separate from other general-purpose data. This is a requirement for WebGL: https://registry.khronos.org/webgl/specs/latest/2.0/#BUFFER_OBJECT_BINDING

No current backends use the partitioned implementation, but once I add WebGL support it will use it to avoid #149603

gaaclarke

Introducing polymorphism in HostBuffer adds complication, overhead, and hurts locality. It's not necessary for what we are trying to do.

All you need to do is introduce a new datatype:

struct HostBuffers {
  HostBuffer data;
  std::optional<HostBuffer> indices;
};

engine/src/flutter/impeller/core/partitioned_host_buffer.h

engine/src/flutter/impeller/entity/contents/host_buffer_unittests.cc

eyebrowsoffire · 2025-08-27T17:59:08Z

Introducing polymorphism in HostBuffer adds complication, overhead, and hurts locality. It's not necessary for what we are trying to do.

All you need to do is introduce a new datatype:
struct HostBuffers {

  HostBuffer data;

  std::optional<HostBuffer> indices;

};

The idea here is that most backends still should use a unified host buffer under the hood. Separation of indices into another buffer is pretty much specific to WebGL, so for other backends the specification of the buffer category is basically a no-op. But the backend-agnostic code needs to specify whether it is going to use the buffer for indexes or for other data, so that if the backend needs to separate it it can. Hence the polymorphism, we need different behavior for different backends.

However, it doesn't necessarily have to be dynamic polymorphism, I'm happy to refactor this a different way. But I'm not sure your suggestion on its own solves the problem. We can jump on a call to discuss the details and get aligned on a solution.

gaaclarke

LGTM, thanks, this is a much better approach I think. It might be worth telling the gemini bot to double check it since there is a lot of small refactoring which it's good at checking. I reviewed it too and looked all good.

gaaclarke · 2025-08-28T15:21:59Z

engine/src/flutter/impeller/renderer/renderer_unittests.cc

+  auto data_host_buffer = HostBuffer::Create(
+      GetContext()->GetResourceAllocator(), GetContext()->GetIdleWaiter(),
+      GetContext()->GetCapabilities()->GetMinimumUniformAlignment());
+  auto indexes_host_buffer =
+      GetContext()->GetCapabilities()->NeedsPartitionedHostBuffer()
+          ? HostBuffer::Create(
+                GetContext()->GetResourceAllocator(),
+                GetContext()->GetIdleWaiter(),
+                GetContext()->GetCapabilities()->GetMinimumUniformAlignment())
+          : data_host_buffer;


nit: We can probably pull this into a function since it happens a fair bit.

gaaclarke · 2025-08-28T15:23:59Z

engine/src/flutter/impeller/entity/contents/content_context.h

+  /// @brief Retrieve the current host buffer for transient storage of indexes
+  ///        used for indexed draws.


It's worth noting that GetTransientsIndexesBuffer may return the same value as GetTransientsDataBuffer in certain circumstances.

gaaclarke · 2025-08-28T15:26:44Z

engine/src/flutter/impeller/entity/contents/content_context.cc

+      indexes_host_buffer_(
+          context_->GetCapabilities()->NeedsPartitionedHostBuffer()
+              ? HostBuffer::Create(
+                    context_->GetResourceAllocator(),
+                    context_->GetIdleWaiter(),
+                    context_->GetCapabilities()->GetMinimumUniformAlignment())
+              : data_host_buffer_),


Please move this into the constructor. The order in which initializers are executed is controlled by the order in the header. I think we have a lint that forces them to be in the same order but I think it's safer just to avoid order issues in initializers.

gaaclarke · 2025-08-28T15:29:10Z

fyi the test failure looks apropos, ResetHostBufferBasedOnFrameBoundary

gemini-code-assist

Code Review

This pull request introduces support for partitioned host buffers, a necessary change for upcoming WebGL support. The primary modification involves splitting GetTransientsBuffer() into GetTransientsDataBuffer() and GetTransientsIndexesBuffer(), and updating numerous call sites to use the appropriate buffer for vertex, index, and uniform data. The implementation correctly handles backends that don't require partitioning by using a shared buffer. The changes are extensive but have been applied consistently across the codebase. I've identified one pre-existing issue in a test file that is now surfaced by these changes.

engine/src/flutter/impeller/entity/entity_unittests.cc

auto-submit · 2025-08-28T18:46:33Z

autosubmit label was removed for flutter/flutter/174463, because - The status or check suite Linux mac_clang_tidy has failed. Please fix the issues identified (or deflake) before re-applying this label.

The status or check suite Linux linux_clang_tidy has failed. Please fix the issues identified (or deflake) before re-applying this label.

auto-submit · 2025-08-28T22:05:02Z

autosubmit label was removed for flutter/flutter/174463, because - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.

…74463)

Roll Flutter from da5523a to 6b18740 (49 revisions) flutter/flutter@da5523a...6b18740 2025-08-29 engine-flutter-autoroll@skia.org Roll Dart SDK from 11f6cd99f6b3 to 72cda0f3dc42 (2 revisions) (flutter/flutter#174697) 2025-08-29 sokolovskyi.konstantin@gmail.com Fix empty adaptive text selection toolbars building. 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(flutter/flutter#174530) 2025-08-28 flar@google.com Revert "Check GTK calls are done on the same thread." (flutter/flutter#174604) 2025-08-27 engine-flutter-autoroll@skia.org Roll Skia from 2a12b57fbbf0 to 430d60054d66 (3 revisions) (flutter/flutter#174590) 2025-08-27 ttankkeo112@gmail.com Retry "Implements the Android native stretch effect as a fragment shader (Impeller-only)." (flutter/flutter#173885) 2025-08-27 matanlurey@users.noreply.github.com Use raw `--removal-label "cp: ..."` when removing labels for unmerged PRs (flutter/flutter#174596) 2025-08-27 jakemac@google.com Flutter driver deserialization (flutter/flutter#172927) 2025-08-27 robert.ancell@canonical.com Check GTK calls are done on the same thread. (flutter/flutter#174488) 2025-08-27 matanlurey@users.noreply.github.com Fix broken reference to `PULL_REQUEST_CP_TEMPLATE.md` after refactor (flutter/flutter#174595) ...

This change adds support for host buffer that keeps index buffers separate from other general-purpose data. This is a requirement for WebGL: https://registry.khronos.org/webgl/specs/latest/2.0/#BUFFER_OBJECT_BINDING No current backends use the partitioned implementation, but once I add WebGL support it will use it to avoid flutter#149603

…74463)

This change adds support for host buffer that keeps index buffers separate from other general-purpose data. This is a requirement for WebGL: https://registry.khronos.org/webgl/specs/latest/2.0/#BUFFER_OBJECT_BINDING No current backends use the partitioned implementation, but once I add WebGL support it will use it to avoid flutter#149603

Adjust host buffer interface.

007e11f

github-actions bot added a: text input Entering text in a text field or keyboard related problems engine flutter/engine related. See also e: labels. e: impeller Impeller rendering backend issues and features requests labels Aug 26, 2025

eyebrowsoffire added 2 commits August 26, 2025 14:31

Added partitioned host buffer.

4dd3be0

Add unit tests for simple and partitioned host buffer.

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eyebrowsoffire marked this pull request as ready for review August 26, 2025 22:23

eyebrowsoffire changed the title ~~Support partitioned host buffer~~ [impeller] Support partitioned host buffer Aug 26, 2025

Add some more doc comments and fix clang-tidy.

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eyebrowsoffire added 2 commits August 27, 2025 18:43

Remove polymorphism and split the host buffers at the ContentContext.

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Add unit test for new capability field.

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gaaclarke approved these changes Aug 28, 2025

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Addressed Aaron's comments and fix unit test.

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Fix pre-existing issue pointed out by Gemini.

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[Impeller] OpenGLES backend tries to reuse buffers across different buffer targets, which fails on WebGL. #149603

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		/// @brief Retrieve the current host buffer for transient storage of indexes
		/// used for indexed draws.

[impeller] Support partitioned host buffer #174463

[impeller] Support partitioned host buffer #174463

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