reduce tricky unsafety and simplify table structure (bevyengine#2221)

I've noticed that we are overusing interior mutability of the Table data, where in many cases we already own a unique reference to it. That prompted a slight refactor aiming to reduce number of safety constraints that must be manually upheld. Now the majority of those are just about avoiding bound checking, which is relatively easy to prove right.

Another aspect is reducing the complexity of Table struct. Notably, we don't ever use archetypes stored there, so this whole thing goes away. Capacity and grow amount were mostly superficial, as we are already using Vecs inside anyway, so I've got rid of those too. Now the overall table capacity is being driven by the internal entity Vec capacity. This has a side effect of automatically implementing exponential growth pattern for BitVecs reallocations inside Table, which to my measurements slightly improves performance in tests that are heavy on inserts. YMMV, but I hope that those tests were at least remotely correct.

crates/bevy_ecs/src/bundle.rs

@@ -130,7 +130,7 @@ impl BundleInfo {
         &self,
         sparse_sets: &mut SparseSets,
         entity: Entity,
-        table: &Table,
+        table: &mut Table,
         table_row: usize,
         bundle_status: &[ComponentStatus],
         bundle: T,
@@ -145,8 +145,8 @@ impl BundleInfo {
             let component_status = bundle_status.get_unchecked(bundle_component);
             match self.storage_types[bundle_component] {
                 StorageType::Table => {
-                    let column = table.get_column(component_id).unwrap();
-                    column.set_unchecked(table_row, component_ptr);
+                    let column = table.get_column_mut(component_id).unwrap();
+                    column.set_data_unchecked(table_row, component_ptr);
                     let column_status = column.get_ticks_unchecked_mut(table_row);
                     match component_status {
                         ComponentStatus::Added => {

crates/bevy_ecs/src/storage/blob_vec.rs

@@ -35,7 +35,7 @@ impl BlobVec {
                 item_layout,
                 drop,
             };
-            blob_vec.reserve(capacity);
+            blob_vec.reserve_exact(capacity);
             blob_vec
         }
     }
@@ -55,14 +55,14 @@ impl BlobVec {
         self.capacity
     }
 
-    pub fn reserve(&mut self, amount: usize) {
+    pub fn reserve_exact(&mut self, additional: usize) {
         let available_space = self.capacity - self.len;
-        if available_space < amount {
-            self.grow(amount - available_space);
+        if available_space < additional {
+            self.grow_exact(additional - available_space);
         }
     }
 
-    fn grow(&mut self, increment: usize) {
+    fn grow_exact(&mut self, increment: usize) {
         debug_assert!(self.item_layout.size() != 0);
 
         let new_capacity = self.capacity + increment;
@@ -102,7 +102,7 @@ impl BlobVec {
     /// the newly allocated space must be immediately populated with a valid value
     #[inline]
     pub unsafe fn push_uninit(&mut self) -> usize {
-        self.reserve(1);
+        self.reserve_exact(1);
         let index = self.len;
         self.len += 1;
         index

crates/bevy_ecs/src/storage/table.rs

@@ -1,5 +1,4 @@
 use crate::{
-    archetype::ArchetypeId,
     component::{ComponentId, ComponentInfo, ComponentTicks, Components},
     entity::Entity,
     storage::{BlobVec, SparseSet},
@@ -48,12 +47,24 @@ impl Column {
         }
     }
 
+    #[inline]
+    fn ticks_mut(&mut self) -> &mut Vec<ComponentTicks> {
+        self.ticks.get_mut()
+    }
+
+    /// # Safety
+    /// Assumes data has already been allocated for the given row.
+    #[inline]
+    pub unsafe fn set_unchecked(&mut self, row: usize, data: *mut u8, ticks: ComponentTicks) {
+        self.set_data_unchecked(row, data);
+        *self.ticks_mut().get_unchecked_mut(row) = ticks;
+    }
+
     /// # Safety
-    /// Assumes data has already been allocated for the given row/column.
-    /// Allows aliased mutable accesses to the data at the given `row`. Caller must ensure that this
-    /// does not happen.
+    /// Assumes data has already been allocated for the given row.
     #[inline]
-    pub unsafe fn set_unchecked(&self, row: usize, data: *mut u8) {
+    pub unsafe fn set_data_unchecked(&mut self, row: usize, data: *mut u8) {
+        debug_assert!(row < self.len());
         self.data.set_unchecked(row, data);
     }
 
@@ -68,20 +79,19 @@ impl Column {
     }
 
     /// # Safety
-    /// Assumes data has already been allocated for the given row/column.
-    /// Allows aliased mutable accesses to the row's [ComponentTicks].
-    /// Caller must ensure that this does not happen.
+    /// Assumes data has already been allocated for the given row.
     #[inline]
-    #[allow(clippy::mut_from_ref)]
-    pub unsafe fn get_ticks_unchecked_mut(&self, row: usize) -> &mut ComponentTicks {
+    pub unsafe fn get_ticks_unchecked_mut(&mut self, row: usize) -> &mut ComponentTicks {
         debug_assert!(row < self.len());
-        (*self.ticks.get()).get_unchecked_mut(row)
+        self.ticks_mut().get_unchecked_mut(row)
     }
 
+    /// # Safety
+    /// index must be in-bounds
     #[inline]
     pub(crate) unsafe fn swap_remove_unchecked(&mut self, row: usize) {
         self.data.swap_remove_and_drop_unchecked(row);
-        (*self.ticks.get()).swap_remove(row);
+        self.ticks_mut().swap_remove(row);
     }
 
     #[inline]
@@ -90,26 +100,22 @@ impl Column {
         row: usize,
     ) -> (*mut u8, ComponentTicks) {
         let data = self.data.swap_remove_and_forget_unchecked(row);
-        let ticks = (*self.ticks.get()).swap_remove(row);
+        let ticks = self.ticks_mut().swap_remove(row);
         (data, ticks)
     }
 
-    /// # Safety
-    /// allocated value must be immediately set at the returned row
-    pub(crate) unsafe fn push_uninit(&mut self) -> usize {
+    // # Safety
+    // - ptr must point to valid data of this column's component type
+    pub(crate) unsafe fn push(&mut self, ptr: *mut u8, ticks: ComponentTicks) {
         let row = self.data.push_uninit();
-        (*self.ticks.get()).push(ComponentTicks::new(0));
-        row
+        self.data.set_unchecked(row, ptr);
+        self.ticks_mut().push(ticks);
     }
 
     #[inline]
-    pub(crate) fn reserve(&mut self, additional: usize) {
-        self.data.reserve(additional);
-        // SAFE: unique access to self
-        unsafe {
-            let ticks = &mut (*self.ticks.get());
-            ticks.reserve(additional);
-        }
+    pub(crate) fn reserve_exact(&mut self, additional: usize) {
+        self.data.reserve_exact(additional);
+        self.ticks_mut().reserve_exact(additional);
     }
 
     /// # Safety
@@ -144,8 +150,7 @@ impl Column {
 
     #[inline]
     pub(crate) fn check_change_ticks(&mut self, change_tick: u32) {
-        let ticks = unsafe { (*self.ticks.get()).iter_mut() };
-        for component_ticks in ticks {
+        for component_ticks in self.ticks_mut() {
             component_ticks.check_ticks(change_tick);
         }
     }
@@ -154,29 +159,20 @@ impl Column {
 pub struct Table {
     columns: SparseSet<ComponentId, Column>,
     entities: Vec<Entity>,
-    archetypes: Vec<ArchetypeId>,
-    grow_amount: usize,
-    capacity: usize,
 }
 
 impl Table {
-    pub const fn new(grow_amount: usize) -> Table {
+    pub const fn new() -> Table {
         Self {
             columns: SparseSet::new(),
             entities: Vec::new(),
-            archetypes: Vec::new(),
-            grow_amount,
-            capacity: 0,
         }
     }
 
-    pub fn with_capacity(capacity: usize, column_capacity: usize, grow_amount: usize) -> Table {
+    pub fn with_capacity(capacity: usize, column_capacity: usize) -> Table {
         Self {
             columns: SparseSet::with_capacity(column_capacity),
             entities: Vec::with_capacity(capacity),
-            archetypes: Vec::new(),
-            grow_amount,
-            capacity,
         }
     }
 
@@ -185,14 +181,10 @@ impl Table {
         &self.entities
     }
 
-    pub fn add_archetype(&mut self, archetype_id: ArchetypeId) {
-        self.archetypes.push(archetype_id);
-    }
-
     pub fn add_column(&mut self, component_info: &ComponentInfo) {
         self.columns.insert(
             component_info.id(),
-            Column::with_capacity(component_info, self.capacity()),
+            Column::with_capacity(component_info, self.entities.capacity()),
         )
     }
 
@@ -232,8 +224,7 @@ impl Table {
         for column in self.columns.values_mut() {
             let (data, ticks) = column.swap_remove_and_forget_unchecked(row);
             if let Some(new_column) = new_table.get_column_mut(column.component_id) {
-                new_column.set_unchecked(new_row, data);
-                *new_column.get_ticks_unchecked_mut(new_row) = ticks;
+                new_column.set_unchecked(new_row, data, ticks);
             }
         }
         TableMoveResult {
@@ -263,8 +254,7 @@ impl Table {
         for column in self.columns.values_mut() {
             if let Some(new_column) = new_table.get_column_mut(column.component_id) {
                 let (data, ticks) = column.swap_remove_and_forget_unchecked(row);
-                new_column.set_unchecked(new_row, data);
-                *new_column.get_ticks_unchecked_mut(new_row) = ticks;
+                new_column.set_unchecked(new_row, data, ticks);
             } else {
                 column.swap_remove_unchecked(row);
             }
@@ -296,8 +286,7 @@ impl Table {
         for column in self.columns.values_mut() {
             let new_column = new_table.get_column_mut(column.component_id).unwrap();
             let (data, ticks) = column.swap_remove_and_forget_unchecked(row);
-            new_column.set_unchecked(new_row, data);
-            *new_column.get_ticks_unchecked_mut(new_row) = ticks;
+            new_column.set_unchecked(new_row, data, ticks);
         }
         TableMoveResult {
             new_row,
@@ -324,20 +313,16 @@ impl Table {
         self.columns.contains(component_id)
     }
 
-    pub fn reserve(&mut self, amount: usize) {
-        let available_space = self.capacity - self.len();
-        if available_space < amount {
-            let min_capacity = self.len() + amount;
-            // normally we would check if min_capacity is 0 for the below calculation, but amount >
-            // available_space and available_space > 0, so min_capacity > 1
-            let new_capacity =
-                ((min_capacity + self.grow_amount - 1) / self.grow_amount) * self.grow_amount;
-            let reserve_amount = new_capacity - self.len();
+    pub fn reserve(&mut self, additional: usize) {
+        if self.entities.capacity() - self.entities.len() < additional {
+            self.entities.reserve(additional);
+
+            // use entities vector capacity as driving capacity for all related allocations
+            let new_capacity = self.entities.capacity();
+
             for column in self.columns.values_mut() {
-                column.reserve(reserve_amount);
+                column.reserve_exact(new_capacity - column.len());
             }
-            self.entities.reserve(reserve_amount);
-            self.capacity = new_capacity;
         }
     }
 
@@ -358,7 +343,7 @@ impl Table {
 
     #[inline]
     pub fn capacity(&self) -> usize {
-        self.capacity
+        self.entities.capacity()
     }
 
     #[inline]
@@ -389,7 +374,7 @@ pub struct Tables {
 
 impl Default for Tables {
     fn default() -> Self {
-        let empty_table = Table::with_capacity(0, 0, 64);
+        let empty_table = Table::with_capacity(0, 0);
         Tables {
             tables: vec![empty_table],
             table_ids: HashMap::default(),
@@ -446,7 +431,7 @@ impl Tables {
         let hash = hasher.finish();
         let tables = &mut self.tables;
         *self.table_ids.entry(hash).or_insert_with(move || {
-            let mut table = Table::with_capacity(0, component_ids.len(), 64);
+            let mut table = Table::with_capacity(0, component_ids.len());
             for component_id in component_ids.iter() {
                 table.add_column(components.get_info_unchecked(*component_id));
             }
@@ -496,18 +481,19 @@ mod tests {
         let type_info = TypeInfo::of::<usize>();
         let component_id = components.get_or_insert_with(type_info.type_id(), || type_info);
         let columns = &[component_id];
-        let mut table = Table::with_capacity(0, columns.len(), 64);
+        let mut table = Table::with_capacity(0, columns.len());
         table.add_column(components.get_info(component_id).unwrap());
         let entities = (0..200).map(Entity::new).collect::<Vec<_>>();
-        for (row, entity) in entities.iter().cloned().enumerate() {
+        for entity in entities.iter() {
+            // SAFE: we allocate and immediately set data afterwards
             unsafe {
-                table.allocate(entity);
+                let row = table.allocate(*entity);
                 let mut value = row;
                 let value_ptr = ((&mut value) as *mut usize).cast::<u8>();
                 table
-                    .get_column(component_id)
+                    .get_column_mut(component_id)
                     .unwrap()
-                    .set_unchecked(row, value_ptr);
+                    .set_data_unchecked(row, value_ptr);
             };
         }
 

crates/bevy_ecs/src/world/entity_ref.rs

@@ -275,7 +275,7 @@ impl<'w> EntityMut<'w> {
         };
         self.location = new_location;
 
-        let table = &storages.tables[archetype.table_id()];
+        let table = &mut storages.tables[archetype.table_id()];
         let table_row = archetype.entity_table_row(new_location.index);
         // SAFE: table row is valid
         unsafe {

crates/bevy_ecs/src/world/mod.rs

@@ -718,12 +718,10 @@ impl World {
         let column = unique_components
             .get_mut(component_id)
             .unwrap_or_else(|| panic!("resource does not exist: {}", std::any::type_name::<T>()));
-        // SAFE: new location is immediately written to below
-        let row = unsafe { column.push_uninit() };
-        // SAFE: row was just allocated above
-        unsafe { column.set_unchecked(row, ptr) };
-        // SAFE: row was just allocated above
-        unsafe { *column.get_ticks_unchecked_mut(row) = ticks };
+        unsafe {
+            // SAFE: pointer is of type T
+            column.push(ptr, ticks);
+        }
         result
     }
 
@@ -787,13 +785,8 @@ impl World {
         if column.is_empty() {
             // SAFE: column is of type T and has been allocated above
             let data = (&mut value as *mut T).cast::<u8>();
-            // SAFE: new location is immediately written to below
-            let row = column.push_uninit();
-            // SAFE: index was just allocated above
-            column.set_unchecked(row, data);
             std::mem::forget(value);
-            // SAFE: index was just allocated above
-            *column.get_ticks_unchecked_mut(row) = ComponentTicks::new(change_tick);
+            column.push(data, ComponentTicks::new(change_tick));
         } else {
             // SAFE: column is of type T and has already been allocated
             *column.get_unchecked(0).cast::<T>() = value;