Fast log2 ceiling (#24301)

* typo

* a fast way to compute log2 ceiling

* rename and assert

* clippy

* fix test return 0 for empty slice

* add test for empty slice

bucket_map/src/index_entry.rs

@@ -1,4 +1,5 @@
 #![allow(dead_code)]
+
 use {
     crate::{
         bucket::Bucket,
@@ -57,8 +58,18 @@ impl IndexEntry {
             .expect("New storage offset must fit into 7 bytes!")
     }
 
+    /// return closest bucket index fit for the slot slice.
+    /// Since bucket size is 2^index, the return value is
+    ///     min index, such that 2^index >= num_slots
+    ///     index = ceiling(log2(num_slots))
+    /// special case, when slot slice empty, return 0th index.
     pub fn data_bucket_from_num_slots(num_slots: Slot) -> u64 {
-        (num_slots as f64).log2().ceil() as u64 // use int log here?
+        // Compute the ceiling of log2 for integer
+        if num_slots == 0 {
+            0
+        } else {
+            (Slot::BITS - (num_slots - 1).leading_zeros()) as u64
+        }
     }
 
     pub fn data_bucket_ix(&self) -> u64 {
@@ -153,4 +164,23 @@ mod tests {
         let mut index = IndexEntry::new(Pubkey::new_unique());
         index.set_storage_offset(too_big);
     }
+
+    #[test]
+    fn test_data_bucket_from_num_slots() {
+        for n in 0..512 {
+            assert_eq!(
+                IndexEntry::data_bucket_from_num_slots(n),
+                (n as f64).log2().ceil() as u64
+            );
+        }
+        assert_eq!(IndexEntry::data_bucket_from_num_slots(u32::MAX as u64), 32);
+        assert_eq!(
+            IndexEntry::data_bucket_from_num_slots(u32::MAX as u64 + 1),
+            32
+        );
+        assert_eq!(
+            IndexEntry::data_bucket_from_num_slots(u32::MAX as u64 + 2),
+            33
+        );
+    }
 }