From a861b14abec47c216cdc0fe610e558aa7419479d Mon Sep 17 00:00:00 2001
From: Lukas Wirth <lukastw97@gmail.com>
Date: Fri, 13 Dec 2024 15:47:09 +0100
Subject: [PATCH] Replace `Page` data Vec with boxed fixed array

---
 src/table.rs | 30 +++++++++++++-----------------
 1 file changed, 13 insertions(+), 17 deletions(-)

diff --git a/src/table.rs b/src/table.rs
index 56a37696..c761ebb7 100644
--- a/src/table.rs
+++ b/src/table.rs
@@ -1,7 +1,9 @@
 use std::{
     any::{Any, TypeId},
     cell::UnsafeCell,
+    mem::MaybeUninit,
     panic::RefUnwindSafe,
+    ptr, slice,
 };
 
 use append_only_vec::AppendOnlyVec;
@@ -60,9 +62,8 @@ pub(crate) struct Page<T: Slot> {
     /// that the data is initialized).
     allocation_lock: Mutex<()>,
 
-    /// Vector with data. This is always created with the capacity/length of `PAGE_LEN`
-    /// and uninitialized data. As we initialize new entries, we increment `allocated`.
-    data: Vec<UnsafeCell<T>>,
+    /// The potentially uninitialized data of this page. As we initialize new entries, we increment `allocated`.
+    data: Box<[UnsafeCell<MaybeUninit<T>>; PAGE_LEN]>,
 }
 
 pub(crate) trait Slot: Any + Send + Sync {
@@ -169,15 +170,11 @@ impl Table {
 impl<T: Slot> Page<T> {
     #[allow(clippy::uninit_vec)]
     fn new(ingredient: IngredientIndex) -> Self {
-        let mut data = Vec::with_capacity(PAGE_LEN);
-        unsafe {
-            data.set_len(PAGE_LEN);
-        }
         Self {
             ingredient,
             allocated: Default::default(),
             allocation_lock: Default::default(),
-            data,
+            data: Box::new([const { UnsafeCell::new(MaybeUninit::uninit()) }; PAGE_LEN]),
         }
     }
 
@@ -196,7 +193,7 @@ impl<T: Slot> Page<T> {
     /// If slot is out of bounds
     pub(crate) fn get(&self, slot: SlotIndex) -> &T {
         self.check_bounds(slot);
-        unsafe { &*self.data[slot.0].get() }
+        unsafe { (*self.data[slot.0].get()).assume_init_ref() }
     }
 
     /// Returns a raw pointer to the given slot.
@@ -211,7 +208,7 @@ impl<T: Slot> Page<T> {
     /// properly with calls to [`get`](`Self::get`) and [`get_mut`](`Self::get_mut`).
     pub(crate) fn get_raw(&self, slot: SlotIndex) -> *mut T {
         self.check_bounds(slot);
-        self.data[slot.0].get()
+        self.data[slot.0].get().cast()
     }
 
     pub(crate) fn allocate<V>(&self, page: PageIndex, value: V) -> Result<Id, V>
@@ -226,7 +223,7 @@ impl<T: Slot> Page<T> {
 
         // Initialize entry `index`
         let data = &self.data[index];
-        unsafe { std::ptr::write(data.get(), value()) };
+        unsafe { (*data.get()).write(value()) };
 
         // Update the length (this must be done after initialization!)
         self.allocated.store(index + 1);
@@ -252,14 +249,13 @@ impl<T: Slot> TablePage for Page<T> {
 
 impl<T: Slot> Drop for Page<T> {
     fn drop(&mut self) {
-        // Free `self.data` and the data within: to do this, we swap it out with an empty vector
-        // and then convert it from a `Vec<UnsafeCell<T>>` with partially uninitialized values
-        // to a `Vec<T>` with the correct length. This way the `Vec` drop impl can do its job.
-        let mut data = std::mem::take(&mut self.data);
+        // Execute destructors for all initialized elements
         let len = self.allocated.load();
+        // SAFETY: self.data is initialized for T's up to len
         unsafe {
-            data.set_len(len);
-            drop(std::mem::transmute::<Vec<UnsafeCell<T>>, Vec<T>>(data));
+            // FIXME: Should be ptr::from_raw_parts_mut but that is unstable
+            let to_drop = slice::from_raw_parts_mut(self.data.as_mut_ptr().cast::<T>(), len);
+            ptr::drop_in_place(to_drop)
         }
     }
 }