AcctIdx: move zero lamport out of accounts index (solana-labs#21526)

runtime/src/account_info.rs

@@ -1,3 +1,7 @@
+//! AccountInfo represents a reference to AccountSharedData in either an AppendVec or the write cache.
+//! AccountInfo is not persisted anywhere between program runs.
+//! AccountInfo is purely runtime state.
+//! Note that AccountInfo is saved to disk buckets during runtime, but disk buckets are recreated at startup.
 use crate::{
     accounts_db::{AppendVecId, CACHE_VIRTUAL_OFFSET, CACHE_VIRTUAL_STORAGE_ID},
     accounts_index::{IsCached, ZeroLamport},
@@ -22,16 +26,16 @@ pub struct AccountInfo {
 
     /// needed to track shrink candidacy in bytes. Used to update the number
     /// of alive bytes in an AppendVec as newer slots purge outdated entries
+    /// Note that highest bit is used for ZERO_LAMPORT_BIT
     stored_size: usize,
-
-    /// lamports in the account used when squashing kept for optimization
-    /// purposes to remove accounts with zero balance.
-    lamports: u64,
 }
 
+/// presence of this bit in stored_size indicates this account info references an account with zero lamports
+const ZERO_LAMPORT_BIT: usize = 1 << (usize::BITS - 1);
+
 impl ZeroLamport for AccountInfo {
     fn is_zero_lamport(&self) -> bool {
-        self.lamports == 0
+        self.stored_size & ZERO_LAMPORT_BIT == ZERO_LAMPORT_BIT
     }
 }
 
@@ -42,16 +46,19 @@ impl IsCached for AccountInfo {
 }
 
 impl AccountInfo {
-    pub fn new(storage_location: StorageLocation, stored_size: usize, lamports: u64) -> Self {
+    pub fn new(storage_location: StorageLocation, mut stored_size: usize, lamports: u64) -> Self {
         let (store_id, offset) = match storage_location {
             StorageLocation::AppendVec(store_id, offset) => (store_id, offset),
             StorageLocation::Cached => (CACHE_VIRTUAL_STORAGE_ID, CACHE_VIRTUAL_OFFSET),
         };
+        assert!(stored_size < ZERO_LAMPORT_BIT);
+        if lamports == 0 {
+            stored_size |= ZERO_LAMPORT_BIT;
+        }
         Self {
             store_id,
             offset,
             stored_size,
-            lamports,
         }
     }
 
@@ -64,6 +71,7 @@ impl AccountInfo {
     }
 
     pub fn stored_size(&self) -> usize {
-        self.stored_size
+        // elminate the special bit that indicates the info references an account with zero lamports
+        self.stored_size & !ZERO_LAMPORT_BIT
     }
 }