diff --git a/ruby_atomic.h b/ruby_atomic.h
index 085c307ac6..5c9049e001 100644
--- a/ruby_atomic.h
+++ b/ruby_atomic.h
@@ -36,4 +36,36 @@ rbimpl_atomic_load_relaxed(rb_atomic_t *ptr)
 }
 #define ATOMIC_LOAD_RELAXED(var) rbimpl_atomic_load_relaxed(&(var))
 
+static inline uint64_t
+rbimpl_atomic_u64_load_relaxed(const uint64_t *value)
+{
+#if defined(HAVE_GCC_ATOMIC_BUILTINS)
+    return __atomic_load_n(value, __ATOMIC_RELAXED);
+#elif defined(_WIN32)
+    uint64_t val = *value;
+    return InterlockedCompareExchange64(value, val, val);
+#elif defined(__sun) && defined(HAVE_ATOMIC_H) && (defined(_LP64) || defined(_I32LPx))
+    uint64_t val = *value;
+    return atomic_cas_64(value, val, val);
+#else
+    return *value;
+#endif
+}
+#define ATOMIC_U64_LOAD_RELAXED(var) rbimpl_atomic_u64_load_relaxed(&(var))
+
+static inline void
+rbimpl_atomic_u64_set_relaxed(uint64_t *address, uint64_t value)
+{
+#if defined(HAVE_GCC_ATOMIC_BUILTINS)
+    __atomic_store_n(address, value, __ATOMIC_RELAXED);
+#elif defined(_WIN32)
+    InterlockedExchange64(address, value);
+#elif defined(__sun) && defined(HAVE_ATOMIC_H) && (defined(_LP64) || defined(_I32LPx))
+    atomic_swap_64(address, value);
+#else
+    *address = value;
+#endif
+}
+#define ATOMIC_U64_SET_RELAXED(var, val) rbimpl_atomic_u64_set_relaxed(&(var), val)
+
 #endif
diff --git a/shape.h b/shape.h
index b9809c4010..50b39062ec 100644
--- a/shape.h
+++ b/shape.h
@@ -3,34 +3,22 @@
 
 #include "internal/gc.h"
 
-#if (SIZEOF_UINT64_T <= SIZEOF_VALUE)
-
 #define SIZEOF_SHAPE_T 4
-typedef uint32_t attr_index_t;
-typedef uint32_t shape_id_t;
-# define SHAPE_ID_NUM_BITS 32
-
-#else
-
-#define SIZEOF_SHAPE_T 2
 typedef uint16_t attr_index_t;
-typedef uint16_t shape_id_t;
-# define SHAPE_ID_NUM_BITS 16
-
-#endif
+typedef uint32_t shape_id_t;
+#define SHAPE_ID_NUM_BITS 32
 
 typedef uint32_t redblack_id_t;
 
 #define SHAPE_MAX_FIELDS (attr_index_t)(-1)
 
-# define SHAPE_FLAG_MASK (((VALUE)-1) >> SHAPE_ID_NUM_BITS)
+#define SHAPE_FLAG_MASK (((VALUE)-1) >> SHAPE_ID_NUM_BITS)
+#define SHAPE_FLAG_SHIFT ((SIZEOF_VALUE * 8) - SHAPE_ID_NUM_BITS)
 
-# define SHAPE_FLAG_SHIFT ((SIZEOF_VALUE * 8) - SHAPE_ID_NUM_BITS)
+#define SHAPE_MAX_VARIATIONS 8
 
-# define SHAPE_MAX_VARIATIONS 8
-
-# define INVALID_SHAPE_ID (((uintptr_t)1 << SHAPE_ID_NUM_BITS) - 1)
-#define ATTR_INDEX_NOT_SET (attr_index_t)-1
+#define INVALID_SHAPE_ID ((shape_id_t)-1)
+#define ATTR_INDEX_NOT_SET ((attr_index_t)-1)
 
 #define ROOT_SHAPE_ID               0x0
 #define SPECIAL_CONST_SHAPE_ID      0x1
@@ -44,13 +32,13 @@ typedef struct redblack_node redblack_node_t;
 struct rb_shape {
     VALUE edges; // id_table from ID (ivar) to next shape
     ID edge_name; // ID (ivar) for transition from parent to rb_shape
+    redblack_node_t *ancestor_index;
+    shape_id_t parent_id;
     attr_index_t next_field_index; // Fields are either ivars or internal properties like `object_id`
     attr_index_t capacity; // Total capacity of the object with this shape
     uint8_t type;
     uint8_t heap_index;
     uint8_t flags;
-    shape_id_t parent_id;
-    redblack_node_t *ancestor_index;
 };
 
 typedef struct rb_shape rb_shape_t;
@@ -82,6 +70,14 @@ typedef struct {
 } rb_shape_tree_t;
 RUBY_EXTERN rb_shape_tree_t *rb_shape_tree_ptr;
 
+union rb_attr_index_cache {
+    uint64_t pack;
+    struct {
+        shape_id_t shape_id;
+        attr_index_t index;
+    } unpack;
+};
+
 static inline rb_shape_tree_t *
 rb_current_shape_tree(void)
 {
diff --git a/vm_callinfo.h b/vm_callinfo.h
index 6813c1cc94..d4dc3b655e 100644
--- a/vm_callinfo.h
+++ b/vm_callinfo.h
@@ -289,7 +289,7 @@ struct rb_callcache {
 
     union {
         struct {
-          uintptr_t value; // Shape ID in upper bits, index in lower bits
+          uint64_t value; // Shape ID in upper bits, index in lower bits
         } attr;
         const enum method_missing_reason method_missing_reason; /* used by method_missing */
         VALUE v;
@@ -416,24 +416,25 @@ vm_cc_call(const struct rb_callcache *cc)
 }
 
 static inline void
-vm_unpack_shape_and_index(uintptr_t cache_value, shape_id_t *shape_id, attr_index_t *index)
+vm_unpack_shape_and_index(const uint64_t cache_value, shape_id_t *shape_id, attr_index_t *index)
 {
-    *shape_id = (shape_id_t)(cache_value >> SHAPE_FLAG_SHIFT);
-    *index = (attr_index_t)(cache_value & SHAPE_FLAG_MASK) - 1;
+    union rb_attr_index_cache cache = {
+        .pack = cache_value,
+    };
+    *shape_id = cache.unpack.shape_id;
+    *index = cache.unpack.index - 1;
 }
 
 static inline void
 vm_cc_atomic_shape_and_index(const struct rb_callcache *cc, shape_id_t *shape_id, attr_index_t *index)
 {
-    // Atomically read uintptr_t
-    vm_unpack_shape_and_index(cc->aux_.attr.value, shape_id, index);
+    vm_unpack_shape_and_index(ATOMIC_U64_LOAD_RELAXED(cc->aux_.attr.value), shape_id, index);
 }
 
 static inline void
 vm_ic_atomic_shape_and_index(const struct iseq_inline_iv_cache_entry *ic, shape_id_t *shape_id, attr_index_t *index)
 {
-    // Atomically read uintptr_t
-    vm_unpack_shape_and_index(ic->value, shape_id, index);
+    vm_unpack_shape_and_index(ATOMIC_U64_LOAD_RELAXED(ic->value), shape_id, index);
 }
 
 static inline unsigned int
@@ -470,16 +471,22 @@ set_vm_cc_ivar(const struct rb_callcache *cc)
     *(VALUE *)&cc->flags |= VM_CALLCACHE_IVAR;
 }
 
-static inline uintptr_t
+static inline uint64_t
 vm_pack_shape_and_index(shape_id_t shape_id, attr_index_t index)
 {
-    return (attr_index_t)(index + 1) | ((uintptr_t)(shape_id) << SHAPE_FLAG_SHIFT);
+    union rb_attr_index_cache cache = {
+        .unpack = {
+            .shape_id = shape_id,
+            .index = index + 1,
+        }
+    };
+    return cache.pack;
 }
 
 static inline void
 vm_cc_attr_index_set(const struct rb_callcache *cc, attr_index_t index, shape_id_t dest_shape_id)
 {
-    uintptr_t *attr_value = (uintptr_t *)&cc->aux_.attr.value;
+    uint64_t *attr_value = (uint64_t *)&cc->aux_.attr.value;
     if (!vm_cc_markable(cc)) {
         *attr_value = vm_pack_shape_and_index(INVALID_SHAPE_ID, ATTR_INDEX_NOT_SET);
         return;
@@ -497,15 +504,15 @@ vm_cc_ivar_p(const struct rb_callcache *cc)
 }
 
 static inline void
-vm_ic_attr_index_set(const rb_iseq_t *iseq, const struct iseq_inline_iv_cache_entry *ic, attr_index_t index, shape_id_t dest_shape_id)
+vm_ic_attr_index_set(const rb_iseq_t *iseq, struct iseq_inline_iv_cache_entry *ic, attr_index_t index, shape_id_t dest_shape_id)
 {
-    *(uintptr_t *)&ic->value = vm_pack_shape_and_index(dest_shape_id, index);
+    ATOMIC_U64_SET_RELAXED(ic->value, vm_pack_shape_and_index(dest_shape_id, index));
 }
 
 static inline void
-vm_ic_attr_index_initialize(const struct iseq_inline_iv_cache_entry *ic, shape_id_t shape_id)
+vm_ic_attr_index_initialize(struct iseq_inline_iv_cache_entry *ic, shape_id_t shape_id)
 {
-    *(uintptr_t *)&ic->value = vm_pack_shape_and_index(shape_id, ATTR_INDEX_NOT_SET);
+    ATOMIC_U64_SET_RELAXED(ic->value, vm_pack_shape_and_index(shape_id, ATTR_INDEX_NOT_SET));
 }
 
 static inline void
diff --git a/vm_core.h b/vm_core.h
index e4aea59e3f..af2a4f85e3 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -288,7 +288,7 @@ struct iseq_inline_constant_cache {
 };
 
 struct iseq_inline_iv_cache_entry {
-    uintptr_t value; // attr_index in lower bits, dest_shape_id in upper bits
+    uint64_t value; // attr_index in lower bits, dest_shape_id in upper bits
     ID iv_set_name;
 };
 
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index e638ac1e82..78d845405a 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -4774,7 +4774,7 @@ vm_call_method_each_type(rb_execution_context_t *ec, rb_control_frame_t *cfp, st
                     .call_ = cc->call_,
                     .aux_  = {
                         .attr = {
-                            .value = INVALID_SHAPE_ID << SHAPE_FLAG_SHIFT,
+                            .value = vm_pack_shape_and_index(INVALID_SHAPE_ID, ATTR_INDEX_NOT_SET),
                         }
                     },
             });
diff --git a/yjit/src/codegen.rs b/yjit/src/codegen.rs
index 0f6385bada..0925cdb993 100644
--- a/yjit/src/codegen.rs
+++ b/yjit/src/codegen.rs
@@ -2904,7 +2904,7 @@ fn gen_get_ivar(
 
     let ivar_index = unsafe {
         let shape_id = comptime_receiver.shape_id_of();
-        let mut ivar_index: u32 = 0;
+        let mut ivar_index: u16 = 0;
         if rb_shape_get_iv_index(shape_id, ivar_name, &mut ivar_index) {
             Some(ivar_index as usize)
         } else {
@@ -3106,7 +3106,7 @@ fn gen_set_ivar(
     let shape_too_complex = comptime_receiver.shape_too_complex();
     let ivar_index = if !shape_too_complex {
         let shape_id = comptime_receiver.shape_id_of();
-        let mut ivar_index: u32 = 0;
+        let mut ivar_index: u16 = 0;
         if unsafe { rb_shape_get_iv_index(shape_id, ivar_name, &mut ivar_index) } {
             Some(ivar_index as usize)
         } else {
@@ -3395,7 +3395,7 @@ fn gen_definedivar(
 
     let shape_id = comptime_receiver.shape_id_of();
     let ivar_exists = unsafe {
-        let mut ivar_index: u32 = 0;
+        let mut ivar_index: u16 = 0;
         rb_shape_get_iv_index(shape_id, ivar_name, &mut ivar_index)
     };
 
diff --git a/yjit/src/cruby_bindings.inc.rs b/yjit/src/cruby_bindings.inc.rs
index 0829317cff..004864d75b 100644
--- a/yjit/src/cruby_bindings.inc.rs
+++ b/yjit/src/cruby_bindings.inc.rs
@@ -515,7 +515,7 @@ pub struct iseq_inline_constant_cache {
 #[repr(C)]
 #[derive(Debug, Copy, Clone)]
 pub struct iseq_inline_iv_cache_entry {
-    pub value: usize,
+    pub value: u64,
     pub iv_set_name: ID,
 }
 #[repr(C)]
@@ -685,7 +685,7 @@ pub const VM_ENV_FLAG_ESCAPED: vm_frame_env_flags = 4;
 pub const VM_ENV_FLAG_WB_REQUIRED: vm_frame_env_flags = 8;
 pub const VM_ENV_FLAG_ISOLATED: vm_frame_env_flags = 16;
 pub type vm_frame_env_flags = u32;
-pub type attr_index_t = u32;
+pub type attr_index_t = u16;
 pub type shape_id_t = u32;
 pub type redblack_id_t = u32;
 pub type redblack_node_t = redblack_node;
@@ -693,13 +693,13 @@ pub type redblack_node_t = redblack_node;
 pub struct rb_shape {
     pub edges: VALUE,
     pub edge_name: ID,
+    pub ancestor_index: *mut redblack_node_t,
+    pub parent_id: shape_id_t,
     pub next_field_index: attr_index_t,
     pub capacity: attr_index_t,
     pub type_: u8,
     pub heap_index: u8,
     pub flags: u8,
-    pub parent_id: shape_id_t,
-    pub ancestor_index: *mut redblack_node_t,
 }
 pub type rb_shape_t = rb_shape;
 #[repr(C)]
diff --git a/zjit/src/cruby_bindings.inc.rs b/zjit/src/cruby_bindings.inc.rs
index e8fc3d3759..95fb8c5213 100644
--- a/zjit/src/cruby_bindings.inc.rs
+++ b/zjit/src/cruby_bindings.inc.rs
@@ -317,7 +317,7 @@ pub struct iseq_inline_constant_cache {
 }
 #[repr(C)]
 pub struct iseq_inline_iv_cache_entry {
-    pub value: usize,
+    pub value: u64,
     pub iv_set_name: ID,
 }
 #[repr(C)]
@@ -393,7 +393,7 @@ pub const VM_ENV_FLAG_ESCAPED: vm_frame_env_flags = 4;
 pub const VM_ENV_FLAG_WB_REQUIRED: vm_frame_env_flags = 8;
 pub const VM_ENV_FLAG_ISOLATED: vm_frame_env_flags = 16;
 pub type vm_frame_env_flags = u32;
-pub type attr_index_t = u32;
+pub type attr_index_t = u16;
 pub type shape_id_t = u32;
 pub type redblack_id_t = u32;
 pub type redblack_node_t = redblack_node;
@@ -401,13 +401,13 @@ pub type redblack_node_t = redblack_node;
 pub struct rb_shape {
     pub edges: VALUE,
     pub edge_name: ID,
+    pub ancestor_index: *mut redblack_node_t,
+    pub parent_id: shape_id_t,
     pub next_field_index: attr_index_t,
     pub capacity: attr_index_t,
     pub type_: u8,
     pub heap_index: u8,
     pub flags: u8,
-    pub parent_id: shape_id_t,
-    pub ancestor_index: *mut redblack_node_t,
 }
 pub type rb_shape_t = rb_shape;
 #[repr(C)]