Meshlet fill cluster buffers rewritten (#15955)

# Objective
- Make the meshlet fill cluster buffers pass slightly faster
- Address #15920 for meshlets
- Added PreviousGlobalTransform as a required meshlet component to avoid
extra archetype moves, slightly alleviating
#14681 for meshlets
- Enforce that MeshletPlugin::cluster_buffer_slots is not greater than
2^25 (glitches will occur otherwise). Technically this field controls
post-lod/culling cluster count, and the issue is on pre-lod/culling
cluster count, but it's still valid now, and in the future this will be
more true.

Needs to be merged after #15846
and #15886

## Solution

- Old pass dispatched a thread per cluster, and did a binary search over
the instances to find which instance the cluster belongs to, and what
meshlet index within the instance it is.
- New pass dispatches a workgroup per instance, and has the workgroup
loop over all meshlets in the instance in order to write out the cluster
data.
- Use a push constant instead of arrayLength to fix the linked bug
- Remap 1d->2d dispatch for software raster only if actually needed to
save on spawning excess workgroups

## Testing

- Did you test these changes? If so, how?
- Ran the meshlet example, and an example with 1041 instances of 32217
meshlets per instance. Profiled the second scene with nsight, went from
0.55ms -> 0.40ms. Small savings. We're pretty much VRAM bandwidth bound
at this point.
- How can other people (reviewers) test your changes? Is there anything
specific they need to know?
  - Run the meshlet example

## Changelog (non-meshlets)
- PreviousGlobalTransform now implements the Default trait

crates/bevy_pbr/src/meshlet/cull_clusters.wgsl

@@ -13,7 +13,7 @@
     meshlet_software_raster_indirect_args,
     meshlet_hardware_raster_indirect_args,
     meshlet_raster_clusters,
-    meshlet_raster_cluster_rightmost_slot,
+    constants,
     MeshletBoundingSphere,
 }
 #import bevy_render::maths::affine3_to_square
@@ -32,7 +32,7 @@ fn cull_clusters(
 ) {
     // Calculate the cluster ID for this thread
     let cluster_id = local_invocation_index + 128u * dot(workgroup_id, vec3(num_workgroups.x * num_workgroups.x, num_workgroups.x, 1u));
-    if cluster_id >= arrayLength(&meshlet_cluster_meshlet_ids) { return; }
+    if cluster_id >= constants.scene_cluster_count { return; }
 
 #ifdef MESHLET_SECOND_CULLING_PASS
     if !cluster_is_second_pass_candidate(cluster_id) { return; }
@@ -138,7 +138,7 @@ fn cull_clusters(
     } else {
         // Append this cluster to the list for hardware rasterization
         buffer_slot = atomicAdd(&meshlet_hardware_raster_indirect_args.instance_count, 1u);
-        buffer_slot = meshlet_raster_cluster_rightmost_slot - buffer_slot;
+        buffer_slot = constants.meshlet_raster_cluster_rightmost_slot - buffer_slot;
     }
     meshlet_raster_clusters[buffer_slot] = cluster_id;
 }

crates/bevy_pbr/src/meshlet/fill_cluster_buffers.wgsl

@@ -1,44 +1,50 @@
 #import bevy_pbr::meshlet_bindings::{
-    cluster_count,
-    meshlet_instance_meshlet_counts_prefix_sum,
+    scene_instance_count,
+    meshlet_global_cluster_count,
+    meshlet_instance_meshlet_counts,
     meshlet_instance_meshlet_slice_starts,
     meshlet_cluster_instance_ids,
     meshlet_cluster_meshlet_ids,
 }
 
 /// Writes out instance_id and meshlet_id to the global buffers for each cluster in the scene.
 
+var<workgroup> cluster_slice_start_workgroup: u32;
+
 @compute
-@workgroup_size(128, 1, 1) // 128 threads per workgroup, 1 cluster per thread
+@workgroup_size(1024, 1, 1) // 1024 threads per workgroup, 1 instance per workgroup
 fn fill_cluster_buffers(
     @builtin(workgroup_id) workgroup_id: vec3<u32>,
     @builtin(num_workgroups) num_workgroups: vec3<u32>,
     @builtin(local_invocation_index) local_invocation_index: u32,
 ) {
-    // Calculate the cluster ID for this thread
-    let cluster_id = local_invocation_index + 128u * dot(workgroup_id, vec3(num_workgroups.x * num_workgroups.x, num_workgroups.x, 1u));
-    if cluster_id >= cluster_count { return; } // TODO: Could be an arrayLength?
-
-    // Binary search to find the instance this cluster belongs to
-    var left = 0u;
-    var right = arrayLength(&meshlet_instance_meshlet_counts_prefix_sum) - 1u;
-    while left <= right {
-        let mid = (left + right) / 2u;
-        if meshlet_instance_meshlet_counts_prefix_sum[mid] <= cluster_id {
-            left = mid + 1u;
-        } else {
-            right = mid - 1u;
-        }
+    // Calculate the instance ID for this workgroup
+    var instance_id = workgroup_id.x + (workgroup_id.y * num_workgroups.x);
+    if instance_id >= scene_instance_count { return; }
+
+    let instance_meshlet_count = meshlet_instance_meshlet_counts[instance_id];
+    let instance_meshlet_slice_start = meshlet_instance_meshlet_slice_starts[instance_id];
+
+    // Reserve cluster slots for the instance and broadcast to the workgroup
+    if local_invocation_index == 0u {
+        cluster_slice_start_workgroup = atomicAdd(&meshlet_global_cluster_count, instance_meshlet_count);
     }
-    let instance_id = right;
+    let cluster_slice_start = workgroupUniformLoad(&cluster_slice_start_workgroup);
 
-    // Find the meshlet ID for this cluster within the instance's MeshletMesh
-    let meshlet_id_local = cluster_id - meshlet_instance_meshlet_counts_prefix_sum[instance_id];
+    // Loop enough times to write out all the meshlets for the instance given that each thread writes 1 meshlet in each iteration
+    for (var clusters_written = 0u; clusters_written < instance_meshlet_count; clusters_written += 1024u) {
+        // Calculate meshlet ID within this instance's MeshletMesh to process for this thread
+        let meshlet_id_local = clusters_written + local_invocation_index;
+        if meshlet_id_local >= instance_meshlet_count { return; }
 
-    // Find the overall meshlet ID in the global meshlet buffer
-    let meshlet_id = meshlet_id_local + meshlet_instance_meshlet_slice_starts[instance_id];
+        // Find the overall cluster ID in the global cluster buffer
+        let cluster_id = cluster_slice_start + meshlet_id_local;
 
-    // Write results to buffers
-    meshlet_cluster_instance_ids[cluster_id] = instance_id;
-    meshlet_cluster_meshlet_ids[cluster_id] = meshlet_id;
+        // Find the overall meshlet ID in the global meshlet buffer
+        let meshlet_id = instance_meshlet_slice_start + meshlet_id_local;
+
+        // Write results to buffers
+        meshlet_cluster_instance_ids[cluster_id] = instance_id;
+        meshlet_cluster_meshlet_ids[cluster_id] = meshlet_id;
+    }
 }

crates/bevy_pbr/src/meshlet/instance_manager.rs

@@ -10,42 +10,46 @@ use bevy_ecs::{
     query::Has,
     system::{Local, Query, Res, ResMut, Resource, SystemState},
 };
-use bevy_render::sync_world::MainEntity;
-use bevy_render::{render_resource::StorageBuffer, view::RenderLayers, MainWorld};
+use bevy_render::{
+    render_resource::StorageBuffer, sync_world::MainEntity, view::RenderLayers, MainWorld,
+};
 use bevy_transform::components::GlobalTransform;
 use bevy_utils::{HashMap, HashSet};
 use core::ops::{DerefMut, Range};
 
 /// Manages data for each entity with a [`MeshletMesh`].
 #[derive(Resource)]
 pub struct InstanceManager {
-    /// Amount of clusters in the scene (sum of all meshlet counts across all instances)
+    /// Amount of instances in the scene.
+    pub scene_instance_count: u32,
+    /// Amount of clusters in the scene.
     pub scene_cluster_count: u32,
 
-    /// Per-instance [`MainEntity`], [`RenderLayers`], and [`NotShadowCaster`]
+    /// Per-instance [`MainEntity`], [`RenderLayers`], and [`NotShadowCaster`].
     pub instances: Vec<(MainEntity, RenderLayers, bool)>,
-    /// Per-instance [`MeshUniform`]
+    /// Per-instance [`MeshUniform`].
     pub instance_uniforms: StorageBuffer<Vec<MeshUniform>>,
-    /// Per-instance material ID
+    /// Per-instance material ID.
     pub instance_material_ids: StorageBuffer<Vec<u32>>,
-    /// Prefix-sum of meshlet counts per instance
-    pub instance_meshlet_counts_prefix_sum: StorageBuffer<Vec<u32>>,
-    /// Per-instance index to the start of the instance's slice of the meshlets buffer
+    /// Per-instance count of meshlets in the instance's [`MeshletMesh`].
+    pub instance_meshlet_counts: StorageBuffer<Vec<u32>>,
+    /// Per-instance index to the start of the instance's slice of the meshlets buffer.
     pub instance_meshlet_slice_starts: StorageBuffer<Vec<u32>>,
     /// Per-view per-instance visibility bit. Used for [`RenderLayers`] and [`NotShadowCaster`] support.
     pub view_instance_visibility: EntityHashMap<StorageBuffer<Vec<u32>>>,
 
-    /// Next material ID available for a [`Material`]
+    /// Next material ID available for a [`Material`].
     next_material_id: u32,
-    /// Map of [`Material`] to material ID
+    /// Map of [`Material`] to material ID.
     material_id_lookup: HashMap<UntypedAssetId, u32>,
-    /// Set of material IDs used in the scene
+    /// Set of material IDs used in the scene.
     material_ids_present_in_scene: HashSet<u32>,
 }
 
 impl InstanceManager {
     pub fn new() -> Self {
         Self {
+            scene_instance_count: 0,
             scene_cluster_count: 0,
 
             instances: Vec::new(),
@@ -59,9 +63,9 @@ impl InstanceManager {
                 buffer.set_label(Some("meshlet_instance_material_ids"));
                 buffer
             },
-            instance_meshlet_counts_prefix_sum: {
+            instance_meshlet_counts: {
                 let mut buffer = StorageBuffer::default();
-                buffer.set_label(Some("meshlet_instance_meshlet_counts_prefix_sum"));
+                buffer.set_label(Some("meshlet_instance_meshlet_counts"));
                 buffer
             },
             instance_meshlet_slice_starts: {
@@ -80,7 +84,7 @@ impl InstanceManager {
     #[allow(clippy::too_many_arguments)]
     pub fn add_instance(
         &mut self,
-        instance: Entity,
+        instance: MainEntity,
         meshlets_slice: Range<u32>,
         transform: &GlobalTransform,
         previous_transform: Option<&PreviousGlobalTransform>,
@@ -108,20 +112,21 @@ impl InstanceManager {
 
         // Append instance data
         self.instances.push((
-            instance.into(),
+            instance,
             render_layers.cloned().unwrap_or(RenderLayers::default()),
             not_shadow_caster,
         ));
         self.instance_uniforms.get_mut().push(mesh_uniform);
         self.instance_material_ids.get_mut().push(0);
-        self.instance_meshlet_counts_prefix_sum
+        self.instance_meshlet_counts
             .get_mut()
-            .push(self.scene_cluster_count);
+            .push(meshlets_slice.len() as u32);
         self.instance_meshlet_slice_starts
             .get_mut()
             .push(meshlets_slice.start);
 
-        self.scene_cluster_count += meshlets_slice.end - meshlets_slice.start;
+        self.scene_instance_count += 1;
+        self.scene_cluster_count += meshlets_slice.len() as u32;
     }
 
     /// Get the material ID for a [`crate::Material`].
@@ -140,12 +145,13 @@ impl InstanceManager {
     }
 
     pub fn reset(&mut self, entities: &Entities) {
+        self.scene_instance_count = 0;
         self.scene_cluster_count = 0;
 
         self.instances.clear();
         self.instance_uniforms.get_mut().clear();
         self.instance_material_ids.get_mut().clear();
-        self.instance_meshlet_counts_prefix_sum.get_mut().clear();
+        self.instance_meshlet_counts.get_mut().clear();
         self.instance_meshlet_slice_starts.get_mut().clear();
         self.view_instance_visibility
             .retain(|view_entity, _| entities.contains(*view_entity));
@@ -227,7 +233,7 @@ pub fn extract_meshlet_mesh_entities(
 
         // Add the instance's data to the instance manager
         instance_manager.add_instance(
-            instance,
+            instance.into(),
             meshlets_slice,
             transform,
             previous_transform,

crates/bevy_pbr/src/meshlet/meshlet_bindings.wgsl

@@ -51,25 +51,27 @@ struct DrawIndirectArgs {
 const CENTIMETERS_PER_METER = 100.0;
 
 #ifdef MESHLET_FILL_CLUSTER_BUFFERS_PASS
-var<push_constant> cluster_count: u32;
-@group(0) @binding(0) var<storage, read> meshlet_instance_meshlet_counts_prefix_sum: array<u32>; // Per entity instance
+var<push_constant> scene_instance_count: u32;
+@group(0) @binding(0) var<storage, read> meshlet_instance_meshlet_counts: array<u32>; // Per entity instance
 @group(0) @binding(1) var<storage, read> meshlet_instance_meshlet_slice_starts: array<u32>; // Per entity instance
 @group(0) @binding(2) var<storage, read_write> meshlet_cluster_instance_ids: array<u32>; // Per cluster
 @group(0) @binding(3) var<storage, read_write> meshlet_cluster_meshlet_ids: array<u32>; // Per cluster
+@group(0) @binding(4) var<storage, read_write> meshlet_global_cluster_count: atomic<u32>; // Single object shared between all workgroups
 #endif
 
 #ifdef MESHLET_CULLING_PASS
-var<push_constant> meshlet_raster_cluster_rightmost_slot: u32;
+struct Constants { scene_cluster_count: u32, meshlet_raster_cluster_rightmost_slot: u32 }
+var<push_constant> constants: Constants;
 @group(0) @binding(0) var<storage, read> meshlet_cluster_meshlet_ids: array<u32>; // Per cluster
 @group(0) @binding(1) var<storage, read> meshlet_bounding_spheres: array<MeshletBoundingSpheres>; // Per meshlet
 @group(0) @binding(2) var<storage, read> meshlet_simplification_errors: array<u32>; // Per meshlet
 @group(0) @binding(3) var<storage, read> meshlet_cluster_instance_ids: array<u32>; // Per cluster
 @group(0) @binding(4) var<storage, read> meshlet_instance_uniforms: array<Mesh>; // Per entity instance
 @group(0) @binding(5) var<storage, read> meshlet_view_instance_visibility: array<u32>; // 1 bit per entity instance, packed as a bitmask
 @group(0) @binding(6) var<storage, read_write> meshlet_second_pass_candidates: array<atomic<u32>>; // 1 bit per cluster , packed as a bitmask
-@group(0) @binding(7) var<storage, read_write> meshlet_software_raster_indirect_args: DispatchIndirectArgs; // Single object shared between all workgroups/clusters/triangles
-@group(0) @binding(8) var<storage, read_write> meshlet_hardware_raster_indirect_args: DrawIndirectArgs; // Single object shared between all workgroups/clusters/triangles
-@group(0) @binding(9) var<storage, read_write> meshlet_raster_clusters: array<u32>; // Single object shared between all workgroups/clusters/triangles
+@group(0) @binding(7) var<storage, read_write> meshlet_software_raster_indirect_args: DispatchIndirectArgs; // Single object shared between all workgroups
+@group(0) @binding(8) var<storage, read_write> meshlet_hardware_raster_indirect_args: DrawIndirectArgs; // Single object shared between all workgroups
+@group(0) @binding(9) var<storage, read_write> meshlet_raster_clusters: array<u32>; // Single object shared between all workgroups
 @group(0) @binding(10) var depth_pyramid: texture_2d<f32>; // From the end of the last frame for the first culling pass, and from the first raster pass for the second culling pass
 @group(0) @binding(11) var<uniform> view: View;
 @group(0) @binding(12) var<uniform> previous_view: PreviousViewUniforms;
@@ -95,7 +97,7 @@ fn cluster_is_second_pass_candidate(cluster_id: u32) -> bool {
 @group(0) @binding(3) var<storage, read> meshlet_vertex_positions: array<u32>; // Many per meshlet
 @group(0) @binding(4) var<storage, read> meshlet_cluster_instance_ids: array<u32>; // Per cluster
 @group(0) @binding(5) var<storage, read> meshlet_instance_uniforms: array<Mesh>; // Per entity instance
-@group(0) @binding(6) var<storage, read> meshlet_raster_clusters: array<u32>; // Single object shared between all workgroups/clusters/triangles
+@group(0) @binding(6) var<storage, read> meshlet_raster_clusters: array<u32>; // Single object shared between all workgroups
 @group(0) @binding(7) var<storage, read> meshlet_software_raster_cluster_count: u32;
 #ifdef MESHLET_VISIBILITY_BUFFER_RASTER_PASS_OUTPUT
 @group(0) @binding(8) var<storage, read_write> meshlet_visibility_buffer: array<atomic<u64>>; // Per pixel

crates/bevy_pbr/src/meshlet/mod.rs

@@ -56,7 +56,7 @@ use self::{
     },
     visibility_buffer_raster_node::MeshletVisibilityBufferRasterPassNode,
 };
-use crate::{graph::NodePbr, Material, MeshMaterial3d};
+use crate::{graph::NodePbr, Material, MeshMaterial3d, PreviousGlobalTransform};
 use bevy_app::{App, Plugin, PostUpdate};
 use bevy_asset::{load_internal_asset, AssetApp, AssetId, Handle};
 use bevy_core_pipeline::{
@@ -129,6 +129,8 @@ pub struct MeshletPlugin {
     /// If this number is too low, you'll see rendering artifacts like missing or blinking meshes.
     ///
     /// Each cluster slot costs 4 bytes of VRAM.
+    ///
+    /// Must not be greater than 2^25.
     pub cluster_buffer_slots: u32,
 }
 
@@ -147,6 +149,11 @@ impl Plugin for MeshletPlugin {
         #[cfg(target_endian = "big")]
         compile_error!("MeshletPlugin is only supported on little-endian processors.");
 
+        if self.cluster_buffer_slots > 2_u32.pow(25) {
+            error!("MeshletPlugin::cluster_buffer_slots must not be greater than 2^25.");
+            std::process::exit(1);
+        }
+
         load_internal_asset!(
             app,
             MESHLET_BINDINGS_SHADER_HANDLE,
@@ -293,7 +300,7 @@ impl Plugin for MeshletPlugin {
 /// The meshlet mesh equivalent of [`bevy_render::mesh::Mesh3d`].
 #[derive(Component, Clone, Debug, Default, Deref, DerefMut, Reflect, PartialEq, Eq, From)]
 #[reflect(Component, Default)]
-#[require(Transform, Visibility)]
+#[require(Transform, PreviousGlobalTransform, Visibility)]
 pub struct MeshletMesh3d(pub Handle<MeshletMesh>);
 
 impl From<MeshletMesh3d> for AssetId<MeshletMesh> {

crates/bevy_pbr/src/meshlet/pipelines.rs

@@ -84,7 +84,7 @@ impl FromWorld for MeshletPipelines {
                 layout: vec![cull_layout.clone()],
                 push_constant_ranges: vec![PushConstantRange {
                     stages: ShaderStages::COMPUTE,
-                    range: 0..4,
+                    range: 0..8,
                 }],
                 shader: MESHLET_CULLING_SHADER_HANDLE,
                 shader_defs: vec![
@@ -99,7 +99,7 @@ impl FromWorld for MeshletPipelines {
                 layout: vec![cull_layout],
                 push_constant_ranges: vec![PushConstantRange {
                     stages: ShaderStages::COMPUTE,
-                    range: 0..4,
+                    range: 0..8,
                 }],
                 shader: MESHLET_CULLING_SHADER_HANDLE,
                 shader_defs: vec![
@@ -441,7 +441,10 @@ impl FromWorld for MeshletPipelines {
                 pipeline_cache.queue_compute_pipeline(ComputePipelineDescriptor {
                     label: Some("meshlet_remap_1d_to_2d_dispatch_pipeline".into()),
                     layout: vec![layout],
-                    push_constant_ranges: vec![],
+                    push_constant_ranges: vec![PushConstantRange {
+                        stages: ShaderStages::COMPUTE,
+                        range: 0..4,
+                    }],
                     shader: MESHLET_REMAP_1D_TO_2D_DISPATCH_SHADER_HANDLE,
                     shader_defs: vec![],
                     entry_point: "remap_dispatch".into(),

crates/bevy_pbr/src/meshlet/remap_1d_to_2d_dispatch.wgsl

@@ -8,13 +8,16 @@ struct DispatchIndirectArgs {
 
 @group(0) @binding(0) var<storage, read_write> meshlet_software_raster_indirect_args: DispatchIndirectArgs;
 @group(0) @binding(1) var<storage, read_write> meshlet_software_raster_cluster_count: u32;
+var<push_constant> max_compute_workgroups_per_dimension: u32;
 
 @compute
 @workgroup_size(1, 1, 1)
 fn remap_dispatch() {
     meshlet_software_raster_cluster_count = meshlet_software_raster_indirect_args.x;
 
-    let n = u32(ceil(sqrt(f32(meshlet_software_raster_indirect_args.x))));
-    meshlet_software_raster_indirect_args.x = n;
-    meshlet_software_raster_indirect_args.y = n;
+    if meshlet_software_raster_cluster_count > max_compute_workgroups_per_dimension {
+        let n = u32(ceil(sqrt(f32(meshlet_software_raster_cluster_count))));
+        meshlet_software_raster_indirect_args.x = n;
+        meshlet_software_raster_indirect_args.y = n;
+    }
 }