perf: increase min buckets on very small types

Consider `HashSet<u8>` on x86_64 with SSE:

| buckets | capacity | allocated bytes |
| ------- | -------- | --------------- |
|       4 |        3 |              36 |
|       8 |        7 |              40 |
|      16 |       14 |              48 |

Quadroupling the number of buckets from 4 to 16 does not even increase
the final allocation size by 50% (48/36=1.333). This is an edge case
due to the padding of the control bytes.

This platform isn't the only one with edges. Here's aarch64 on an M1
for the same `HashSet<u8>`:

| buckets | capacity | allocated bytes |
| ------- | -------- | --------------- |
|       4 |        3 |              20 |
|       8 |        7 |              24 |
|      16 |       14 |              40 |

Notice 4 -> 8 buckets leading to only 4 more bytes (20 -> 24) instead
of roughly doubling.

Generalized, `buckets * table_layout.size` needs to be at least as big
as `table_layout.ctrl_align`. For the cases I listed above, we'd get
these new minimum bucket sizes:

 - x86_64 with SSE: 16
 - aarch64: 8

This is a niche optimization. However, it also removes possible
undefined behavior edge case in resize operations. In addition, it
may be a useful property to utilize over-sized allocations (see
rust-lang#523).

src/raw/mod.rs

@@ -192,16 +192,23 @@ impl ProbeSeq {
 // Workaround for emscripten bug emscripten-core/emscripten-fastcomp#258
 #[cfg_attr(target_os = "emscripten", inline(never))]
 #[cfg_attr(not(target_os = "emscripten"), inline)]
-fn capacity_to_buckets(cap: usize) -> Option<usize> {
+fn capacity_to_buckets(cap: usize, table_layout: TableLayout) -> Option<usize> {
     debug_assert_ne!(cap, 0);
 
     // For small tables we require at least 1 empty bucket so that lookups are
-    // guaranteed to terminate if an element doesn't exist in the table.
-    if cap < 8 {
+    // guaranteed to terminate if an element doesn't exist in the table
+    if cap < 8.max(table_layout.ctrl_align) {
         // We don't bother with a table size of 2 buckets since that can only
-        // hold a single element. Instead we skip directly to a 4 bucket table
+        // hold a single element. Instead, skip directly to a 4 bucket table
         // which can hold 3 elements.
-        return Some(if cap < 4 { 4 } else { 8 });
+        let min_buckets = if cap < 4 { 4 } else { 8 };
+
+        // Also ensure that buckets * table_layout.size is at least as big as
+        // table_layout.ctrl_align. If it's not, then relatively quite a few
+        // bytes are wasted padding to the alignment.
+        let denominator = table_layout.size.max(1);
+        let ratio = table_layout.ctrl_align / denominator;
+        return Some(ratio.max(min_buckets));
     }
 
     // Otherwise require 1/8 buckets to be empty (87.5% load)
@@ -1126,7 +1133,7 @@ impl<T, A: Allocator> RawTable<T, A> {
         // elements. If the calculation overflows then the requested bucket
         // count must be larger than what we have right and nothing needs to be
         // done.
-        let min_buckets = match capacity_to_buckets(min_size) {
+        let min_buckets = match capacity_to_buckets(min_size, Self::TABLE_LAYOUT) {
             Some(buckets) => buckets,
             None => return,
         };
@@ -1257,14 +1264,8 @@ impl<T, A: Allocator> RawTable<T, A> {
     /// * If `self.table.items != 0`, calling of this function with `capacity`
     ///   equal to 0 (`capacity == 0`) results in [`undefined behavior`].
     ///
-    /// * If `capacity_to_buckets(capacity) < Group::WIDTH` and
-    ///   `self.table.items > capacity_to_buckets(capacity)`
-    ///   calling this function results in [`undefined behavior`].
-    ///
-    /// * If `capacity_to_buckets(capacity) >= Group::WIDTH` and
-    ///   `self.table.items > capacity_to_buckets(capacity)`
-    ///   calling this function are never return (will go into an
-    ///   infinite loop).
+    /// * If `self.table.items > capacity_to_buckets(capacity, Self::TABLE_LAYOUT)`
+    ///   calling this function are never return (will loop infinitely).
     ///
     /// See [`RawTableInner::find_insert_slot`] for more information.
     ///
@@ -1782,8 +1783,8 @@ impl RawTableInner {
             // SAFETY: We checked that we could successfully allocate the new table, and then
             // initialized all control bytes with the constant `EMPTY` byte.
             unsafe {
-                let buckets =
-                    capacity_to_buckets(capacity).ok_or_else(|| fallibility.capacity_overflow())?;
+                let buckets = capacity_to_buckets(capacity, table_layout)
+                    .ok_or_else(|| fallibility.capacity_overflow())?;
 
                 let result = Self::new_uninitialized(alloc, table_layout, buckets, fallibility)?;
                 // SAFETY: We checked that the table is allocated and therefore the table already has
@@ -4566,6 +4567,26 @@ impl<T, A: Allocator> RawExtractIf<'_, T, A> {
 mod test_map {
     use super::*;
 
+    #[test]
+    fn test_minimum_capacity_for_small_types() {
+        #[track_caller]
+        fn test_t<T>() {
+            let raw_table: RawTable<T> = RawTable::with_capacity(1);
+            let actual_buckets = raw_table.buckets();
+            let min_buckets = Group::WIDTH / core::mem::size_of::<T>();
+            assert!(
+                actual_buckets >= min_buckets,
+                "expected at least {min_buckets} buckets, got {actual_buckets} buckets"
+            );
+        }
+
+        test_t::<u8>();
+
+        // This is only "small" for some platforms, like x86_64 with SSE, but
+        // there's no harm in running it on other platforms.
+        test_t::<u16>();
+    }
+
     fn rehash_in_place<T>(table: &mut RawTable<T>, hasher: impl Fn(&T) -> u64) {
         unsafe {
             table.table.rehash_in_place(