diff --git a/vm/src/dictdatatype.rs b/vm/src/dictdatatype.rs
index 3838f84e8..487207e88 100644
--- a/vm/src/dictdatatype.rs
+++ b/vm/src/dictdatatype.rs
@@ -37,34 +37,26 @@ impl<T> fmt::Debug for Dict<T> {
     }
 }
 
-#[derive(Debug, Copy, Clone)]
-enum IndexEntry {
-    Dummy,
-    Free,
-    Index(usize),
-}
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+#[repr(transparent)]
+struct IndexEntry(i64);
 
 impl IndexEntry {
-    const FREE: i64 = -1;
-    const DUMMY: i64 = -2;
-}
+    const FREE: Self = Self(-1);
+    const DUMMY: Self = Self(-2);
 
-impl From<i64> for IndexEntry {
-    fn from(idx: i64) -> Self {
-        match idx {
-            IndexEntry::FREE => IndexEntry::Free,
-            IndexEntry::DUMMY => IndexEntry::Dummy,
-            x => IndexEntry::Index(x as usize),
-        }
+    /// # Safety
+    /// idx must not be one of FREE or DUMMY
+    unsafe fn from_index_unchecked(idx: usize) -> Self {
+        debug_assert!((idx as isize) >= 0);
+        Self(idx as i64)
     }
-}
 
-impl From<IndexEntry> for i64 {
-    fn from(idx: IndexEntry) -> Self {
-        match idx {
-            IndexEntry::Free => IndexEntry::FREE,
-            IndexEntry::Dummy => IndexEntry::DUMMY,
-            IndexEntry::Index(i) => i as i64,
+    fn index(self) -> Option<usize> {
+        if self.0 >= 0 {
+            Some(self.0 as usize)
+        } else {
+            None
         }
     }
 }
@@ -73,10 +65,8 @@ impl From<IndexEntry> for i64 {
 struct DictInner<T> {
     used: usize,
     filled: usize,
-    indices: Vec<i64>,
+    indices: Vec<IndexEntry>,
     entries: Vec<Option<DictEntry<T>>>,
-    // index to new inserted element should be in entries
-    next_new_entry_idx: usize,
 }
 
 impl<T: Clone> Clone for Dict<T> {
@@ -95,7 +85,6 @@ impl<T> Default for Dict<T> {
                 filled: 0,
                 indices: vec![IndexEntry::FREE; 8],
                 entries: Vec::new(),
-                next_new_entry_idx: 0,
             }),
         }
     }
@@ -108,6 +97,7 @@ struct DictEntry<T> {
     index: IndexIndex,
     value: T,
 }
+static_assertions::assert_eq_size!(DictEntry<PyObjectRef>, Option<DictEntry<PyObjectRef>>);
 
 impl<T: Clone> DictEntry<T> {
     pub(crate) fn as_tuple(&self) -> (PyObjectRef, T) {
@@ -175,11 +165,16 @@ impl<T> DictInner<T> {
                 let mut idxs = GenIndexes::new(entry.hash, mask);
                 loop {
                     let index_index = idxs.next();
-                    let idx = &mut self.indices[index_index];
-                    if *idx == IndexEntry::FREE {
-                        *idx = entry_idx as i64;
-                        entry.index = index_index;
-                        break;
+                    unsafe {
+                        // Safety: index is always valid here
+                        // index_index is generated by idxs
+                        // entry_idx is saved one
+                        let idx = self.indices.get_unchecked_mut(index_index);
+                        if *idx == IndexEntry::FREE {
+                            *idx = IndexEntry::from_index_unchecked(entry_idx);
+                            entry.index = index_index;
+                            break;
+                        }
                     }
                 }
             } else {
@@ -187,7 +182,6 @@ impl<T> DictInner<T> {
             }
         }
         self.filled = self.used;
-        self.next_new_entry_idx = self.entries.len();
     }
 
     fn unchecked_push(
@@ -204,16 +198,15 @@ impl<T> DictInner<T> {
             value,
             index,
         };
-        let entry_index = self.next_new_entry_idx;
-        if self.entries.len() == entry_index {
-            self.entries.push(Some(entry));
-        } else {
-            self.entries[entry_index] = Some(entry);
-        }
-        self.indices[index] = entry_index as i64;
+        let entry_index = self.entries.len();
+        self.entries.push(Some(entry));
+        self.indices[index] = unsafe {
+            // SAFETY: entry_index is self.entries.len(). it never can
+            // grow to `usize-2` because hash tables cannot full its index
+            IndexEntry::from_index_unchecked(entry_index)
+        };
         self.used += 1;
-        self.next_new_entry_idx += 1;
-        if let IndexEntry::Free = index_entry {
+        if let IndexEntry::FREE = index_entry {
             self.filled += 1;
             if let Some(new_size) = self.should_resize() {
                 self.resize(new_size)
@@ -260,7 +253,7 @@ impl<T: Clone> Dict<T> {
         let _removed = loop {
             let (entry_index, index_index) = self.lookup(vm, key, hash, None)?;
             let mut inner = self.write();
-            if let IndexEntry::Index(index) = entry_index {
+            if let Some(index) = entry_index.index() {
                 // Update existing key
                 if let Some(entry) = inner.entries.get_mut(index) {
                     let entry = entry
@@ -287,7 +280,7 @@ impl<T: Clone> Dict<T> {
 
     pub fn contains<K: DictKey + ?Sized>(&self, vm: &VirtualMachine, key: &K) -> PyResult<bool> {
         let (entry, _) = self.lookup(vm, key, key.key_hash(vm)?, None)?;
-        Ok(matches!(entry, IndexEntry::Index(_)))
+        Ok(entry.index().is_some())
     }
 
     /// Retrieve a key
@@ -305,7 +298,7 @@ impl<T: Clone> Dict<T> {
     ) -> PyResult<Option<T>> {
         let ret = loop {
             let (entry, index_index) = self.lookup(vm, key, hash, None)?;
-            if let IndexEntry::Index(index) = entry {
+            if let Some(index) = entry.index() {
                 let inner = self.read();
                 if let Some(entry) = inner.entries.get(index) {
                     let entry = extract_dict_entry(entry);
@@ -346,7 +339,6 @@ impl<T: Clone> Dict<T> {
             inner.indices.resize(8, IndexEntry::FREE);
             inner.used = 0;
             inner.filled = 0;
-            inner.next_new_entry_idx = 0;
             // defer dec rc
             std::mem::take(&mut inner.entries)
         };
@@ -394,7 +386,7 @@ impl<T: Clone> Dict<T> {
         let _removed = loop {
             let lookup = self.lookup(vm, key, hash, None)?;
             let (entry, index_index) = lookup;
-            if let IndexEntry::Index(_) = entry {
+            if entry.index().is_some() {
                 match self.pop_inner(lookup) {
                     ControlFlow::Break(Some(entry)) => break Some(entry),
                     _ => continue,
@@ -416,8 +408,8 @@ impl<T: Clone> Dict<T> {
         let hash = key.key_hash(vm)?;
         let res = loop {
             let lookup = self.lookup(vm, key, hash, None)?;
-            let (entry, index_index) = lookup;
-            if let IndexEntry::Index(index) = entry {
+            let (index_entry, index_index) = lookup;
+            if let Some(index) = index_entry.index() {
                 let inner = self.read();
                 if let Some(entry) = inner.entries.get(index) {
                     let entry = extract_dict_entry(entry);
@@ -433,7 +425,13 @@ impl<T: Clone> Dict<T> {
             } else {
                 let value = default();
                 let mut inner = self.write();
-                inner.unchecked_push(index_index, hash, key.to_pyobject(vm), value.clone(), entry);
+                inner.unchecked_push(
+                    index_index,
+                    hash,
+                    key.to_pyobject(vm),
+                    value.clone(),
+                    index_entry,
+                );
                 break value;
             }
         };
@@ -454,8 +452,8 @@ impl<T: Clone> Dict<T> {
         let hash = key.key_hash(vm)?;
         let res = loop {
             let lookup = self.lookup(vm, key, hash, None)?;
-            let (entry, index_index) = lookup;
-            if let IndexEntry::Index(index) = entry {
+            let (index_entry, index_index) = lookup;
+            if let Some(index) = index_entry.index() {
                 let inner = self.read();
                 if let Some(entry) = inner.entries.get(index) {
                     let entry = extract_dict_entry(entry);
@@ -473,7 +471,7 @@ impl<T: Clone> Dict<T> {
                 let key = key.to_pyobject(vm);
                 let mut inner = self.write();
                 let ret = (key.clone(), value.clone());
-                inner.unchecked_push(index_index, hash, key, value, entry);
+                inner.unchecked_push(index_index, hash, key, value, index_entry);
                 break ret;
             }
         };
@@ -541,7 +539,7 @@ impl<T: Clone> Dict<T> {
         mut lock: Option<PyRwLockReadGuard<DictInner<T>>>,
     ) -> PyResult<LookupResult> {
         let mut idxs = None;
-        let mut freeslot = None;
+        let mut free_slot = None;
         let ret = 'outer: loop {
             let (entry_key, ret) = {
                 let inner = lock.take().unwrap_or_else(|| self.read());
@@ -550,22 +548,31 @@ impl<T: Clone> Dict<T> {
                 });
                 loop {
                     let index_index = idxs.next();
-                    match IndexEntry::from(inner.indices[index_index]) {
-                        IndexEntry::Dummy => {
-                            if freeslot.is_none() {
-                                freeslot = Some(index_index);
+                    let index_entry = *unsafe {
+                        // Safety: index_index is generated
+                        inner.indices.get_unchecked(index_index)
+                    };
+                    match index_entry {
+                        IndexEntry::DUMMY => {
+                            if free_slot.is_none() {
+                                free_slot = Some(index_index);
                             }
                         }
-                        IndexEntry::Free => {
-                            let idxs = match freeslot {
-                                Some(free) => (IndexEntry::Dummy, free),
-                                None => (IndexEntry::Free, index_index),
+                        IndexEntry::FREE => {
+                            let idxs = match free_slot {
+                                Some(free) => (IndexEntry::DUMMY, free),
+                                None => (IndexEntry::FREE, index_index),
                             };
                             return Ok(idxs);
                         }
-                        IndexEntry::Index(i) => {
-                            let entry = &inner.entries[i].as_ref().unwrap();
-                            let ret = (IndexEntry::Index(i), index_index);
+                        idx => {
+                            let entry = unsafe {
+                                // Safety: DUMMY and FREE are already handled above.
+                                // i is always valid and entry always exists.
+                                let i = idx.index().unwrap_unchecked();
+                                inner.entries.get_unchecked(i).as_ref().unwrap_unchecked()
+                            };
+                            let ret = (idx, index_index);
                             if key.key_is(&entry.key) {
                                 break 'outer ret;
                             } else if entry.hash == hash_value {
@@ -602,7 +609,7 @@ impl<T: Clone> Dict<T> {
         pred: impl Fn(&T) -> Result<bool, E>,
     ) -> Result<PopInnerResult<T>, E> {
         let (entry_index, index_index) = lookup;
-        let entry_index = if let IndexEntry::Index(entry_index) = entry_index {
+        let entry_index = if let Some(entry_index) = entry_index.index() {
             entry_index
         } else {
             return Ok(ControlFlow::Break(None));
@@ -623,7 +630,10 @@ impl<T: Clone> Dict<T> {
             // The dict was changed since we did lookup. Let's try again.
             _ => return Ok(ControlFlow::Continue(())),
         }
-        inner.indices[index_index] = IndexEntry::DUMMY;
+        *unsafe {
+            // index_index is result of lookup
+            inner.indices.get_unchecked_mut(index_index)
+        } = IndexEntry::DUMMY;
         inner.used -= 1;
         let removed = slot.take();
         Ok(ControlFlow::Break(removed))
@@ -644,14 +654,17 @@ impl<T: Clone> Dict<T> {
 
     pub fn pop_back(&self) -> Option<(PyObjectRef, T)> {
         let mut inner = &mut *self.write();
-        let (entry_idx, entry) = inner.entries[..inner.next_new_entry_idx]
-            .iter_mut()
-            .enumerate()
-            .rev()
-            .find_map(|(i, entry)| entry.take().map(|e| (i, e)))?;
+        let entry = loop {
+            let entry = inner.entries.pop()?;
+            if let Some(entry) = entry {
+                break entry;
+            }
+        };
         inner.used -= 1;
-        inner.indices[entry.index] = IndexEntry::DUMMY;
-        inner.next_new_entry_idx = entry_idx;
+        *unsafe {
+            // entry.index always refers valid index
+            inner.indices.get_unchecked_mut(entry.index)
+        } = IndexEntry::DUMMY;
         Some((entry.key, entry.value))
     }