Add GpuArrayBuffer and BatchedUniformBuffer (#8204)

# Objective

- Add a type for uploading a Rust `Vec<T>` to a GPU `array<T>`.
- Makes progress towards https://github.com/bevyengine/bevy/issues/89.

## Solution

- Port @superdump's `BatchedUniformBuffer` to bevy main, as a fallback
for WebGL2, which doesn't support storage buffers.
- Rather than getting an `array<T>` in a shader, you get an `array<T,
N>`, and have to rebind every N elements via dynamic offsets.
- Add `GpuArrayBuffer` to abstract over
`StorageBuffer<Vec<T>>`/`BatchedUniformBuffer`.

## Future Work
Add a shader macro kinda thing to abstract over the following
automatically:
https://github.com/bevyengine/bevy/pull/8204#pullrequestreview-1396911727

---

## Changelog
* Added `GpuArrayBuffer`, `GpuComponentArrayBufferPlugin`,
`GpuArrayBufferable`, and `GpuArrayBufferIndex` types.
* Added `DynamicUniformBuffer::new_with_alignment()`.

---------

Co-authored-by: Robert Swain <robert.swain@gmail.com>
Co-authored-by: François <mockersf@gmail.com>
Co-authored-by: Teodor Tanasoaia <28601907+teoxoy@users.noreply.github.com>
Co-authored-by: IceSentry <IceSentry@users.noreply.github.com>
Co-authored-by: Vincent <9408210+konsolas@users.noreply.github.com>
Co-authored-by: robtfm <50659922+robtfm@users.noreply.github.com>

crates/bevy_render/src/gpu_component_array_buffer.rs

@@ -0,0 +1,55 @@
+use crate::{
+    render_resource::{GpuArrayBuffer, GpuArrayBufferable},
+    renderer::{RenderDevice, RenderQueue},
+    Render, RenderApp, RenderSet,
+};
+use bevy_app::{App, Plugin};
+use bevy_ecs::{
+    prelude::{Component, Entity},
+    schedule::IntoSystemConfigs,
+    system::{Commands, Query, Res, ResMut},
+};
+use std::marker::PhantomData;
+
+/// This plugin prepares the components of the corresponding type for the GPU
+/// by storing them in a [`GpuArrayBuffer`].
+pub struct GpuComponentArrayBufferPlugin<C: Component + GpuArrayBufferable>(PhantomData<C>);
+
+impl<C: Component + GpuArrayBufferable> Plugin for GpuComponentArrayBufferPlugin<C> {
+    fn build(&self, app: &mut App) {
+        if let Ok(render_app) = app.get_sub_app_mut(RenderApp) {
+            render_app
+                .insert_resource(GpuArrayBuffer::<C>::new(
+                    render_app.world.resource::<RenderDevice>(),
+                ))
+                .add_systems(
+                    Render,
+                    prepare_gpu_component_array_buffers::<C>.in_set(RenderSet::Prepare),
+                );
+        }
+    }
+}
+
+impl<C: Component + GpuArrayBufferable> Default for GpuComponentArrayBufferPlugin<C> {
+    fn default() -> Self {
+        Self(PhantomData::<C>)
+    }
+}
+
+fn prepare_gpu_component_array_buffers<C: Component + GpuArrayBufferable>(
+    mut commands: Commands,
+    render_device: Res<RenderDevice>,
+    render_queue: Res<RenderQueue>,
+    mut gpu_array_buffer: ResMut<GpuArrayBuffer<C>>,
+    components: Query<(Entity, &C)>,
+) {
+    gpu_array_buffer.clear();
+
+    let entities = components
+        .iter()
+        .map(|(entity, component)| (entity, gpu_array_buffer.push(component.clone())))
+        .collect::<Vec<_>>();
+    commands.insert_or_spawn_batch(entities);
+
+    gpu_array_buffer.write_buffer(&render_device, &render_queue);
+}

crates/bevy_render/src/lib.rs

@@ -11,6 +11,7 @@ pub mod extract_component;
 mod extract_param;
 pub mod extract_resource;
 pub mod globals;
+pub mod gpu_component_array_buffer;
 pub mod mesh;
 pub mod pipelined_rendering;
 pub mod primitives;

crates/bevy_render/src/render_resource/batched_uniform_buffer.rs

@@ -0,0 +1,152 @@
+use super::{GpuArrayBufferIndex, GpuArrayBufferable};
+use crate::{
+    render_resource::DynamicUniformBuffer,
+    renderer::{RenderDevice, RenderQueue},
+};
+use encase::{
+    private::{ArrayMetadata, BufferMut, Metadata, RuntimeSizedArray, WriteInto, Writer},
+    ShaderType,
+};
+use std::{marker::PhantomData, num::NonZeroU64};
+use wgpu::{BindingResource, Limits};
+
+// 1MB else we will make really large arrays on macOS which reports very large
+// `max_uniform_buffer_binding_size`. On macOS this ends up being the minimum
+// size of the uniform buffer as well as the size of each chunk of data at a
+// dynamic offset.
+#[cfg(any(not(feature = "webgl"), not(target_arch = "wasm32")))]
+const MAX_REASONABLE_UNIFORM_BUFFER_BINDING_SIZE: u32 = 1 << 20;
+
+// WebGL2 quirk: using uniform buffers larger than 4KB will cause extremely
+// long shader compilation times, so the limit needs to be lower on WebGL2.
+// This is due to older shader compilers/GPUs that don't support dynamically
+// indexing uniform buffers, and instead emulate it with large switch statements
+// over buffer indices that take a long time to compile.
+#[cfg(all(feature = "webgl", target_arch = "wasm32"))]
+const MAX_REASONABLE_UNIFORM_BUFFER_BINDING_SIZE: u32 = 1 << 12;
+
+/// Similar to [`DynamicUniformBuffer`], except every N elements (depending on size)
+/// are grouped into a batch as an `array<T, N>` in WGSL.
+///
+/// This reduces the number of rebindings required due to having to pass dynamic
+/// offsets to bind group commands, and if indices into the array can be passed
+/// in via other means, it enables batching of draw commands.
+pub struct BatchedUniformBuffer<T: GpuArrayBufferable> {
+    // Batches of fixed-size arrays of T are written to this buffer so that
+    // each batch in a fixed-size array can be bound at a dynamic offset.
+    uniforms: DynamicUniformBuffer<MaxCapacityArray<Vec<T>>>,
+    // A batch of T are gathered into this `MaxCapacityArray` until it is full,
+    // then it is written into the `DynamicUniformBuffer`, cleared, and new T
+    // are gathered here, and so on for each batch.
+    temp: MaxCapacityArray<Vec<T>>,
+    current_offset: u32,
+    dynamic_offset_alignment: u32,
+}
+
+impl<T: GpuArrayBufferable> BatchedUniformBuffer<T> {
+    pub fn batch_size(limits: &Limits) -> usize {
+        (limits
+            .max_uniform_buffer_binding_size
+            .min(MAX_REASONABLE_UNIFORM_BUFFER_BINDING_SIZE) as u64
+            / T::min_size().get()) as usize
+    }
+
+    pub fn new(limits: &Limits) -> Self {
+        let capacity = Self::batch_size(limits);
+        let alignment = limits.min_uniform_buffer_offset_alignment;
+
+        Self {
+            uniforms: DynamicUniformBuffer::new_with_alignment(alignment as u64),
+            temp: MaxCapacityArray(Vec::with_capacity(capacity), capacity),
+            current_offset: 0,
+            dynamic_offset_alignment: alignment,
+        }
+    }
+
+    #[inline]
+    pub fn size(&self) -> NonZeroU64 {
+        self.temp.size()
+    }
+
+    pub fn clear(&mut self) {
+        self.uniforms.clear();
+        self.current_offset = 0;
+        self.temp.0.clear();
+    }
+
+    pub fn push(&mut self, component: T) -> GpuArrayBufferIndex<T> {
+        let result = GpuArrayBufferIndex {
+            index: self.temp.0.len() as u32,
+            dynamic_offset: Some(self.current_offset),
+            element_type: PhantomData,
+        };
+        self.temp.0.push(component);
+        if self.temp.0.len() == self.temp.1 {
+            self.flush();
+        }
+        result
+    }
+
+    pub fn flush(&mut self) {
+        self.uniforms.push(self.temp.clone());
+
+        self.current_offset +=
+            align_to_next(self.temp.size().get(), self.dynamic_offset_alignment as u64) as u32;
+
+        self.temp.0.clear();
+    }
+
+    pub fn write_buffer(&mut self, device: &RenderDevice, queue: &RenderQueue) {
+        if !self.temp.0.is_empty() {
+            self.flush();
+        }
+        self.uniforms.write_buffer(device, queue);
+    }
+
+    #[inline]
+    pub fn binding(&self) -> Option<BindingResource> {
+        let mut binding = self.uniforms.binding();
+        if let Some(BindingResource::Buffer(binding)) = &mut binding {
+            // MaxCapacityArray is runtime-sized so can't use T::min_size()
+            binding.size = Some(self.size());
+        }
+        binding
+    }
+}
+
+#[inline]
+fn align_to_next(value: u64, alignment: u64) -> u64 {
+    debug_assert!(alignment & (alignment - 1) == 0);
+    ((value - 1) | (alignment - 1)) + 1
+}
+
+// ----------------------------------------------------------------------------
+// MaxCapacityArray was implemented by Teodor Tanasoaia for encase. It was
+// copied here as it was not yet included in an encase release and it is
+// unclear if it is the correct long-term solution for encase.
+
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord)]
+struct MaxCapacityArray<T>(T, usize);
+
+impl<T> ShaderType for MaxCapacityArray<T>
+where
+    T: ShaderType<ExtraMetadata = ArrayMetadata>,
+{
+    type ExtraMetadata = ArrayMetadata;
+
+    const METADATA: Metadata<Self::ExtraMetadata> = T::METADATA;
+
+    fn size(&self) -> ::core::num::NonZeroU64 {
+        Self::METADATA.stride().mul(self.1.max(1) as u64).0
+    }
+}
+
+impl<T> WriteInto for MaxCapacityArray<T>
+where
+    T: WriteInto + RuntimeSizedArray,
+{
+    fn write_into<B: BufferMut>(&self, writer: &mut Writer<B>) {
+        debug_assert!(self.0.len() <= self.1);
+        self.0.write_into(writer);
+    }
+}

crates/bevy_render/src/render_resource/buffer_vec.rs

@@ -21,9 +21,11 @@ use wgpu::BufferUsages;
 /// from system RAM to VRAM.
 ///
 /// Other options for storing GPU-accessible data are:
+/// * [`StorageBuffer`](crate::render_resource::StorageBuffer)
 /// * [`DynamicStorageBuffer`](crate::render_resource::DynamicStorageBuffer)
 /// * [`UniformBuffer`](crate::render_resource::UniformBuffer)
 /// * [`DynamicUniformBuffer`](crate::render_resource::DynamicUniformBuffer)
+/// * [`GpuArrayBuffer`](crate::render_resource::GpuArrayBuffer)
 /// * [`BufferVec`](crate::render_resource::BufferVec)
 /// * [`Texture`](crate::render_resource::Texture)
 pub struct BufferVec<T: Pod> {

crates/bevy_render/src/render_resource/gpu_array_buffer.rs

@@ -0,0 +1,129 @@
+use super::StorageBuffer;
+use crate::{
+    render_resource::batched_uniform_buffer::BatchedUniformBuffer,
+    renderer::{RenderDevice, RenderQueue},
+};
+use bevy_ecs::{prelude::Component, system::Resource};
+use encase::{private::WriteInto, ShaderSize, ShaderType};
+use std::{marker::PhantomData, mem};
+use wgpu::{BindGroupLayoutEntry, BindingResource, BindingType, BufferBindingType, ShaderStages};
+
+/// Trait for types able to go in a [`GpuArrayBuffer`].
+pub trait GpuArrayBufferable: ShaderType + ShaderSize + WriteInto + Clone {}
+impl<T: ShaderType + ShaderSize + WriteInto + Clone> GpuArrayBufferable for T {}
+
+/// Stores an array of elements to be transferred to the GPU and made accessible to shaders as a read-only array.
+///
+/// On platforms that support storage buffers, this is equivalent to [`StorageBuffer<Vec<T>>`].
+/// Otherwise, this falls back to a dynamic offset uniform buffer with the largest
+/// array of T that fits within a uniform buffer binding (within reasonable limits).
+///
+/// Other options for storing GPU-accessible data are:
+/// * [`StorageBuffer`](crate::render_resource::StorageBuffer)
+/// * [`DynamicStorageBuffer`](crate::render_resource::DynamicStorageBuffer)
+/// * [`UniformBuffer`](crate::render_resource::UniformBuffer)
+/// * [`DynamicUniformBuffer`](crate::render_resource::DynamicUniformBuffer)
+/// * [`BufferVec`](crate::render_resource::BufferVec)
+/// * [`Texture`](crate::render_resource::Texture)
+#[derive(Resource)]
+pub enum GpuArrayBuffer<T: GpuArrayBufferable> {
+    Uniform(BatchedUniformBuffer<T>),
+    Storage((StorageBuffer<Vec<T>>, Vec<T>)),
+}
+
+impl<T: GpuArrayBufferable> GpuArrayBuffer<T> {
+    pub fn new(device: &RenderDevice) -> Self {
+        let limits = device.limits();
+        if limits.max_storage_buffers_per_shader_stage == 0 {
+            GpuArrayBuffer::Uniform(BatchedUniformBuffer::new(&limits))
+        } else {
+            GpuArrayBuffer::Storage((StorageBuffer::default(), Vec::new()))
+        }
+    }
+
+    pub fn clear(&mut self) {
+        match self {
+            GpuArrayBuffer::Uniform(buffer) => buffer.clear(),
+            GpuArrayBuffer::Storage((_, buffer)) => buffer.clear(),
+        }
+    }
+
+    pub fn push(&mut self, value: T) -> GpuArrayBufferIndex<T> {
+        match self {
+            GpuArrayBuffer::Uniform(buffer) => buffer.push(value),
+            GpuArrayBuffer::Storage((_, buffer)) => {
+                let index = buffer.len() as u32;
+                buffer.push(value);
+                GpuArrayBufferIndex {
+                    index,
+                    dynamic_offset: None,
+                    element_type: PhantomData,
+                }
+            }
+        }
+    }
+
+    pub fn write_buffer(&mut self, device: &RenderDevice, queue: &RenderQueue) {
+        match self {
+            GpuArrayBuffer::Uniform(buffer) => buffer.write_buffer(device, queue),
+            GpuArrayBuffer::Storage((buffer, vec)) => {
+                buffer.set(mem::take(vec));
+                buffer.write_buffer(device, queue);
+            }
+        }
+    }
+
+    pub fn binding_layout(
+        binding: u32,
+        visibility: ShaderStages,
+        device: &RenderDevice,
+    ) -> BindGroupLayoutEntry {
+        BindGroupLayoutEntry {
+            binding,
+            visibility,
+            ty: if device.limits().max_storage_buffers_per_shader_stage == 0 {
+                BindingType::Buffer {
+                    ty: BufferBindingType::Uniform,
+                    has_dynamic_offset: true,
+                    // BatchedUniformBuffer uses a MaxCapacityArray that is runtime-sized, so we use
+                    // None here and let wgpu figure out the size.
+                    min_binding_size: None,
+                }
+            } else {
+                BindingType::Buffer {
+                    ty: BufferBindingType::Storage { read_only: true },
+                    has_dynamic_offset: false,
+                    min_binding_size: Some(T::min_size()),
+                }
+            },
+            count: None,
+        }
+    }
+
+    pub fn binding(&self) -> Option<BindingResource> {
+        match self {
+            GpuArrayBuffer::Uniform(buffer) => buffer.binding(),
+            GpuArrayBuffer::Storage((buffer, _)) => buffer.binding(),
+        }
+    }
+
+    pub fn batch_size(device: &RenderDevice) -> Option<u32> {
+        let limits = device.limits();
+        if limits.max_storage_buffers_per_shader_stage == 0 {
+            Some(BatchedUniformBuffer::<T>::batch_size(&limits) as u32)
+        } else {
+            None
+        }
+    }
+}
+
+/// An index into a [`GpuArrayBuffer`] for a given element.
+#[derive(Component)]
+pub struct GpuArrayBufferIndex<T: GpuArrayBufferable> {
+    /// The index to use in a shader into the array.
+    pub index: u32,
+    /// The dynamic offset to use when setting the bind group in a pass.
+    /// Only used on platforms that don't support storage buffers.
+    pub dynamic_offset: Option<u32>,
+    pub element_type: PhantomData<T>,
+}

crates/bevy_render/src/render_resource/mod.rs

@@ -1,7 +1,9 @@
+mod batched_uniform_buffer;
 mod bind_group;
 mod bind_group_layout;
 mod buffer;
 mod buffer_vec;
+mod gpu_array_buffer;
 mod pipeline;
 mod pipeline_cache;
 mod pipeline_specializer;
@@ -15,6 +17,7 @@ pub use bind_group::*;
 pub use bind_group_layout::*;
 pub use buffer::*;
 pub use buffer_vec::*;
+pub use gpu_array_buffer::*;
 pub use pipeline::*;
 pub use pipeline_cache::*;
 pub use pipeline_specializer::*;

crates/bevy_render/src/render_resource/storage_buffer.rs

@@ -25,6 +25,7 @@ use wgpu::{util::BufferInitDescriptor, BindingResource, BufferBinding, BufferUsa
 /// * [`DynamicStorageBuffer`](crate::render_resource::DynamicStorageBuffer)
 /// * [`UniformBuffer`](crate::render_resource::UniformBuffer)
 /// * [`DynamicUniformBuffer`](crate::render_resource::DynamicUniformBuffer)
+/// * [`GpuArrayBuffer`](crate::render_resource::GpuArrayBuffer)
 /// * [`BufferVec`](crate::render_resource::BufferVec)
 /// * [`Texture`](crate::render_resource::Texture)
 ///
@@ -154,6 +155,7 @@ impl<T: ShaderType + WriteInto> StorageBuffer<T> {
 /// * [`StorageBuffer`](crate::render_resource::StorageBuffer)
 /// * [`UniformBuffer`](crate::render_resource::UniformBuffer)
 /// * [`DynamicUniformBuffer`](crate::render_resource::DynamicUniformBuffer)
+/// * [`GpuArrayBuffer`](crate::render_resource::GpuArrayBuffer)
 /// * [`BufferVec`](crate::render_resource::BufferVec)
 /// * [`Texture`](crate::render_resource::Texture)
 ///

crates/bevy_render/src/render_resource/uniform_buffer.rs

@@ -22,9 +22,10 @@ use wgpu::{util::BufferInitDescriptor, BindingResource, BufferBinding, BufferUsa
 /// (vectors), or structures with fields that are vectors.
 ///
 /// Other options for storing GPU-accessible data are:
-/// * [`DynamicUniformBuffer`](crate::render_resource::DynamicUniformBuffer)
 /// * [`StorageBuffer`](crate::render_resource::StorageBuffer)
 /// * [`DynamicStorageBuffer`](crate::render_resource::DynamicStorageBuffer)
+/// * [`DynamicUniformBuffer`](crate::render_resource::DynamicUniformBuffer)
+/// * [`GpuArrayBuffer`](crate::render_resource::GpuArrayBuffer)
 /// * [`BufferVec`](crate::render_resource::BufferVec)
 /// * [`Texture`](crate::render_resource::Texture)
 ///
@@ -151,6 +152,8 @@ impl<T: ShaderType + WriteInto> UniformBuffer<T> {
 /// * [`DynamicStorageBuffer`](crate::render_resource::DynamicStorageBuffer)
 /// * [`UniformBuffer`](crate::render_resource::UniformBuffer)
 /// * [`DynamicUniformBuffer`](crate::render_resource::DynamicUniformBuffer)
+/// * [`GpuArrayBuffer`](crate::render_resource::GpuArrayBuffer)
+/// * [`BufferVec`](crate::render_resource::BufferVec)
 /// * [`Texture`](crate::render_resource::Texture)
 ///
 /// [std140 alignment/padding requirements]: https://www.w3.org/TR/WGSL/#address-spaces-uniform
@@ -177,6 +180,17 @@ impl<T: ShaderType> Default for DynamicUniformBuffer<T> {
 }
 
 impl<T: ShaderType + WriteInto> DynamicUniformBuffer<T> {
+    pub fn new_with_alignment(alignment: u64) -> Self {
+        Self {
+            scratch: DynamicUniformBufferWrapper::new_with_alignment(Vec::new(), alignment),
+            buffer: None,
+            label: None,
+            changed: false,
+            buffer_usage: BufferUsages::COPY_DST | BufferUsages::UNIFORM,
+            _marker: PhantomData,
+        }
+    }
+
     #[inline]
     pub fn buffer(&self) -> Option<&Buffer> {
         self.buffer.as_ref()