diff --git a/ast.c b/ast.c
index 04f2d1384c5edc..bc2adeacd630a4 100644
--- a/ast.c
+++ b/ast.c
@@ -866,6 +866,11 @@ node_locations(VALUE ast_value, const NODE *node)
                                     location_new(&RNODE_IF(node)->if_keyword_loc),
                                     location_new(&RNODE_IF(node)->then_keyword_loc),
                                     location_new(&RNODE_IF(node)->end_keyword_loc));
+      case NODE_MODULE:
+        return rb_ary_new_from_args(3,
+                                    location_new(nd_code_loc(node)),
+                                    location_new(&RNODE_MODULE(node)->module_keyword_loc),
+                                    location_new(&RNODE_MODULE(node)->end_keyword_loc));
       case NODE_NEXT:
         return rb_ary_new_from_args(2,
                                     location_new(nd_code_loc(node)),
diff --git a/autogen.sh b/autogen.sh
index f11a471a08efa5..6cbc5dddabe65e 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -10,7 +10,7 @@ case "$0" in
     *   )  srcdir="";; # Otherwise
 esac
 
-# If install-only is explicitly requested, disbale symlink flags
+# If install-only is explicitly requested, disable symlink flags
 case " $* " in
     *" -i "* | *" --install"* ) symlink_flags="" ;;
     *                         ) symlink_flags="--install --symlink" ;;
diff --git a/benchmark/struct_accessor.yml b/benchmark/struct_accessor.yml
index 61176cfdd48d3a..d95240e2dd8e45 100644
--- a/benchmark/struct_accessor.yml
+++ b/benchmark/struct_accessor.yml
@@ -1,5 +1,12 @@
 prelude: |
   C = Struct.new(:x) do
+    def initialize(...)
+      super
+      @ivar = 42
+    end
+
+    attr_accessor :ivar
+
     class_eval <<-END
       def r
         #{'x;'*256}
@@ -15,11 +22,16 @@ prelude: |
         m = method(:x=)
         #{'m.call(nil);'*256}
       end
+      def r_ivar
+        #{'ivar;'*256}
+      end
     END
   end
+  C.new(nil) # ensure common shape is known
   obj = C.new(nil)
 benchmark:
   member_reader: "obj.r"
   member_writer: "obj.w"
   member_reader_method: "obj.rm"
   member_writer_method: "obj.wm"
+  ivar_reader: "obj.r_ivar"
diff --git a/depend b/depend
index ec8c2771c92104..ecaf33b1c7710c 100644
--- a/depend
+++ b/depend
@@ -6065,6 +6065,7 @@ hash.$(OBJEXT): $(top_srcdir)/internal/set_table.h
 hash.$(OBJEXT): $(top_srcdir)/internal/st.h
 hash.$(OBJEXT): $(top_srcdir)/internal/static_assert.h
 hash.$(OBJEXT): $(top_srcdir)/internal/string.h
+hash.$(OBJEXT): $(top_srcdir)/internal/struct.h
 hash.$(OBJEXT): $(top_srcdir)/internal/symbol.h
 hash.$(OBJEXT): $(top_srcdir)/internal/thread.h
 hash.$(OBJEXT): $(top_srcdir)/internal/time.h
@@ -6288,6 +6289,7 @@ hash.$(OBJEXT): {$(VPATH)}symbol.h
 hash.$(OBJEXT): {$(VPATH)}thread_$(THREAD_MODEL).h
 hash.$(OBJEXT): {$(VPATH)}thread_native.h
 hash.$(OBJEXT): {$(VPATH)}util.h
+hash.$(OBJEXT): {$(VPATH)}variable.h
 hash.$(OBJEXT): {$(VPATH)}vm_core.h
 hash.$(OBJEXT): {$(VPATH)}vm_debug.h
 hash.$(OBJEXT): {$(VPATH)}vm_opts.h
@@ -12926,6 +12928,7 @@ range.$(OBJEXT): $(top_srcdir)/internal/enumerator.h
 range.$(OBJEXT): $(top_srcdir)/internal/error.h
 range.$(OBJEXT): $(top_srcdir)/internal/fixnum.h
 range.$(OBJEXT): $(top_srcdir)/internal/gc.h
+range.$(OBJEXT): $(top_srcdir)/internal/imemo.h
 range.$(OBJEXT): $(top_srcdir)/internal/numeric.h
 range.$(OBJEXT): $(top_srcdir)/internal/range.h
 range.$(OBJEXT): $(top_srcdir)/internal/serial.h
@@ -12948,6 +12951,7 @@ range.$(OBJEXT): {$(VPATH)}config.h
 range.$(OBJEXT): {$(VPATH)}defines.h
 range.$(OBJEXT): {$(VPATH)}encoding.h
 range.$(OBJEXT): {$(VPATH)}id.h
+range.$(OBJEXT): {$(VPATH)}id_table.h
 range.$(OBJEXT): {$(VPATH)}intern.h
 range.$(OBJEXT): {$(VPATH)}internal.h
 range.$(OBJEXT): {$(VPATH)}internal/abi.h
@@ -15580,6 +15584,7 @@ shape.$(OBJEXT): $(top_srcdir)/internal/serial.h
 shape.$(OBJEXT): $(top_srcdir)/internal/set_table.h
 shape.$(OBJEXT): $(top_srcdir)/internal/static_assert.h
 shape.$(OBJEXT): $(top_srcdir)/internal/string.h
+shape.$(OBJEXT): $(top_srcdir)/internal/struct.h
 shape.$(OBJEXT): $(top_srcdir)/internal/symbol.h
 shape.$(OBJEXT): $(top_srcdir)/internal/variable.h
 shape.$(OBJEXT): $(top_srcdir)/internal/vm.h
@@ -16569,6 +16574,7 @@ string.$(OBJEXT): $(top_srcdir)/internal/serial.h
 string.$(OBJEXT): $(top_srcdir)/internal/set_table.h
 string.$(OBJEXT): $(top_srcdir)/internal/static_assert.h
 string.$(OBJEXT): $(top_srcdir)/internal/string.h
+string.$(OBJEXT): $(top_srcdir)/internal/struct.h
 string.$(OBJEXT): $(top_srcdir)/internal/transcode.h
 string.$(OBJEXT): $(top_srcdir)/internal/variable.h
 string.$(OBJEXT): $(top_srcdir)/internal/vm.h
@@ -16766,6 +16772,7 @@ string.$(OBJEXT): {$(VPATH)}thread.h
 string.$(OBJEXT): {$(VPATH)}thread_$(THREAD_MODEL).h
 string.$(OBJEXT): {$(VPATH)}thread_native.h
 string.$(OBJEXT): {$(VPATH)}util.h
+string.$(OBJEXT): {$(VPATH)}variable.h
 string.$(OBJEXT): {$(VPATH)}vm_core.h
 string.$(OBJEXT): {$(VPATH)}vm_debug.h
 string.$(OBJEXT): {$(VPATH)}vm_opts.h
@@ -18103,6 +18110,7 @@ variable.$(OBJEXT): $(top_srcdir)/internal/serial.h
 variable.$(OBJEXT): $(top_srcdir)/internal/set_table.h
 variable.$(OBJEXT): $(top_srcdir)/internal/static_assert.h
 variable.$(OBJEXT): $(top_srcdir)/internal/string.h
+variable.$(OBJEXT): $(top_srcdir)/internal/struct.h
 variable.$(OBJEXT): $(top_srcdir)/internal/symbol.h
 variable.$(OBJEXT): $(top_srcdir)/internal/thread.h
 variable.$(OBJEXT): $(top_srcdir)/internal/variable.h
diff --git a/gc.c b/gc.c
index 4af43edcc4cb4b..64a22cd1b78fff 100644
--- a/gc.c
+++ b/gc.c
@@ -3260,6 +3260,10 @@ rb_gc_mark_children(void *objspace, VALUE obj)
             gc_mark_internal(ptr[i]);
         }
 
+        if (!FL_TEST_RAW(obj, RSTRUCT_GEN_FIELDS)) {
+            gc_mark_internal(RSTRUCT_FIELDS_OBJ(obj));
+        }
+
         break;
       }
 
@@ -4188,6 +4192,15 @@ rb_gc_update_object_references(void *objspace, VALUE obj)
             for (i = 0; i < len; i++) {
                 UPDATE_IF_MOVED(objspace, ptr[i]);
             }
+
+            if (RSTRUCT_EMBED_LEN(obj)) {
+                if (!FL_TEST_RAW(obj, RSTRUCT_GEN_FIELDS)) {
+                    UPDATE_IF_MOVED(objspace, ptr[len]);
+                }
+            }
+            else {
+                UPDATE_IF_MOVED(objspace, RSTRUCT(obj)->as.heap.fields_obj);
+            }
         }
         break;
       default:
diff --git a/internal/struct.h b/internal/struct.h
index a8c773b730300e..337f96a336d796 100644
--- a/internal/struct.h
+++ b/internal/struct.h
@@ -11,10 +11,23 @@
 #include "ruby/internal/stdbool.h"     /* for bool */
 #include "ruby/ruby.h"          /* for struct RBasic */
 
+/* Flags of RStruct
+ *
+ * 1-7: RSTRUCT_EMBED_LEN
+ *          If non-zero, the struct is embedded (its contents follow the
+ *          header, rather than being on a separately allocated buffer) and
+ *          these bits are the length of the Struct.
+ * 8:   RSTRUCT_GEN_FIELDS
+ *          The struct is embedded and has no space left to store the
+ *          IMEMO/fields reference. Any ivar this struct may have will be in
+ *          the generic_fields_tbl. This flag doesn't imply the struct has
+ *          ivars.
+ */
 enum {
     RSTRUCT_EMBED_LEN_MASK = RUBY_FL_USER7 | RUBY_FL_USER6 | RUBY_FL_USER5 | RUBY_FL_USER4 |
                                  RUBY_FL_USER3 | RUBY_FL_USER2 | RUBY_FL_USER1,
     RSTRUCT_EMBED_LEN_SHIFT = (RUBY_FL_USHIFT+1),
+    RSTRUCT_GEN_FIELDS = RUBY_FL_USER8,
 };
 
 struct RStruct {
@@ -23,6 +36,7 @@ struct RStruct {
         struct {
             long len;
             const VALUE *ptr;
+            VALUE fields_obj;
         } heap;
         /* This is a length 1 array because:
          *   1. GCC has a bug that does not optimize C flexible array members
@@ -116,4 +130,31 @@ RSTRUCT_GET(VALUE st, long k)
     return RSTRUCT_CONST_PTR(st)[k];
 }
 
+static inline VALUE
+RSTRUCT_FIELDS_OBJ(VALUE st)
+{
+    const long embed_len = RSTRUCT_EMBED_LEN(st);
+    VALUE fields_obj;
+    if (embed_len) {
+        RUBY_ASSERT(!FL_TEST_RAW(st, RSTRUCT_GEN_FIELDS));
+        fields_obj = RSTRUCT_GET(st, embed_len);
+    }
+    else {
+        fields_obj = RSTRUCT(st)->as.heap.fields_obj;
+    }
+    return fields_obj;
+}
+
+static inline void
+RSTRUCT_SET_FIELDS_OBJ(VALUE st, VALUE fields_obj)
+{
+    const long embed_len = RSTRUCT_EMBED_LEN(st);
+    if (embed_len) {
+        RUBY_ASSERT(!FL_TEST_RAW(st, RSTRUCT_GEN_FIELDS));
+        RSTRUCT_SET(st, embed_len, fields_obj);
+    }
+    else {
+        RB_OBJ_WRITE(st, &RSTRUCT(st)->as.heap.fields_obj, fields_obj);
+    }
+}
 #endif /* INTERNAL_STRUCT_H */
diff --git a/node_dump.c b/node_dump.c
index 9822ae5fc25a3a..c318baeeede009 100644
--- a/node_dump.c
+++ b/node_dump.c
@@ -1009,8 +1009,10 @@ dump_node(VALUE buf, VALUE indent, int comment, const NODE * node)
         ANN("format: module [nd_cpath]; [nd_body]; end");
         ANN("example: module M; ..; end");
         F_NODE(nd_cpath, RNODE_MODULE, "module path");
-        LAST_NODE;
         F_NODE(nd_body, RNODE_MODULE, "module definition");
+        F_LOC(module_keyword_loc, RNODE_MODULE);
+        LAST_NODE;
+        F_LOC(end_keyword_loc, RNODE_MODULE);
         return;
 
       case NODE_SCLASS:
diff --git a/parse.y b/parse.y
index dbe21332b4dd83..e77dc790bcd1cd 100644
--- a/parse.y
+++ b/parse.y
@@ -1145,7 +1145,7 @@ static rb_node_alias_t *rb_node_alias_new(struct parser_params *p, NODE *nd_1st,
 static rb_node_valias_t *rb_node_valias_new(struct parser_params *p, ID nd_alias, ID nd_orig, const YYLTYPE *loc, const YYLTYPE *keyword_loc);
 static rb_node_undef_t *rb_node_undef_new(struct parser_params *p, NODE *nd_undef, const YYLTYPE *loc);
 static rb_node_class_t *rb_node_class_new(struct parser_params *p, NODE *nd_cpath, NODE *nd_body, NODE *nd_super, const YYLTYPE *loc, const YYLTYPE *class_keyword_loc, const YYLTYPE *inheritance_operator_loc, const YYLTYPE *end_keyword_loc);
-static rb_node_module_t *rb_node_module_new(struct parser_params *p, NODE *nd_cpath, NODE *nd_body, const YYLTYPE *loc);
+static rb_node_module_t *rb_node_module_new(struct parser_params *p, NODE *nd_cpath, NODE *nd_body, const YYLTYPE *loc, const YYLTYPE *module_keyword_loc, const YYLTYPE *end_keyword_loc);
 static rb_node_sclass_t *rb_node_sclass_new(struct parser_params *p, NODE *nd_recv, NODE *nd_body, const YYLTYPE *loc);
 static rb_node_colon2_t *rb_node_colon2_new(struct parser_params *p, NODE *nd_head, ID nd_mid, const YYLTYPE *loc, const YYLTYPE *delimiter_loc, const YYLTYPE *name_loc);
 static rb_node_colon3_t *rb_node_colon3_new(struct parser_params *p, ID nd_mid, const YYLTYPE *loc, const YYLTYPE *delimiter_loc, const YYLTYPE *name_loc);
@@ -1253,7 +1253,7 @@ static rb_node_error_t *rb_node_error_new(struct parser_params *p, const YYLTYPE
 #define NEW_VALIAS(n,o,loc,k_loc) (NODE *)rb_node_valias_new(p,n,o,loc,k_loc)
 #define NEW_UNDEF(i,loc) (NODE *)rb_node_undef_new(p,i,loc)
 #define NEW_CLASS(n,b,s,loc,ck_loc,io_loc,ek_loc) (NODE *)rb_node_class_new(p,n,b,s,loc,ck_loc,io_loc,ek_loc)
-#define NEW_MODULE(n,b,loc) (NODE *)rb_node_module_new(p,n,b,loc)
+#define NEW_MODULE(n,b,loc,mk_loc,ek_loc) (NODE *)rb_node_module_new(p,n,b,loc,mk_loc,ek_loc)
 #define NEW_SCLASS(r,b,loc) (NODE *)rb_node_sclass_new(p,r,b,loc)
 #define NEW_COLON2(c,i,loc,d_loc,n_loc) (NODE *)rb_node_colon2_new(p,c,i,loc,d_loc,n_loc)
 #define NEW_COLON3(i,loc,d_loc,n_loc) (NODE *)rb_node_colon3_new(p,i,loc,d_loc,n_loc)
@@ -4621,7 +4621,7 @@ primary		: inline_primary
               bodystmt
               k_end
                 {
-                    $$ = NEW_MODULE($cpath, $bodystmt, &@$);
+                    $$ = NEW_MODULE($cpath, $bodystmt, &@$, &@k_module, &@k_end);
                     nd_set_line(RNODE_MODULE($$)->nd_body, @k_end.end_pos.lineno);
                     set_line_body($bodystmt, @cpath.end_pos.lineno);
                     nd_set_line($$, @cpath.end_pos.lineno);
@@ -11438,13 +11438,15 @@ rb_node_sclass_new(struct parser_params *p, NODE *nd_recv, NODE *nd_body, const
 }
 
 static rb_node_module_t *
-rb_node_module_new(struct parser_params *p, NODE *nd_cpath, NODE *nd_body, const YYLTYPE *loc)
+rb_node_module_new(struct parser_params *p, NODE *nd_cpath, NODE *nd_body, const YYLTYPE *loc, const YYLTYPE *module_keyword_loc, const YYLTYPE *end_keyword_loc)
 {
     /* Keep the order of node creation */
     NODE *scope = NEW_SCOPE(0, nd_body, loc);
     rb_node_module_t *n = NODE_NEWNODE(NODE_MODULE, rb_node_module_t, loc);
     n->nd_cpath = nd_cpath;
     n->nd_body = scope;
+    n->module_keyword_loc = *module_keyword_loc;
+    n->end_keyword_loc = *end_keyword_loc;
 
     return n;
 }
diff --git a/ractor.c b/ractor.c
index 91542b940b6104..dc83ccc1b408c8 100644
--- a/ractor.c
+++ b/ractor.c
@@ -2302,7 +2302,7 @@ static const rb_data_type_t cross_ractor_require_data_type = {
         NULL, // memsize
         NULL, // compact
     },
-    0, 0, RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED | RUBY_TYPED_DECL_MARKING
+    0, 0, RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED | RUBY_TYPED_DECL_MARKING | RUBY_TYPED_EMBEDDABLE
 };
 
 static VALUE
diff --git a/ractor_sync.c b/ractor_sync.c
index a3ed38295b29c6..057448c5f58bfa 100644
--- a/ractor_sync.c
+++ b/ractor_sync.c
@@ -1273,7 +1273,7 @@ static const rb_data_type_t ractor_selector_data_type = {
         ractor_selector_memsize,
         NULL, // update
     },
-    0, 0, RUBY_TYPED_FREE_IMMEDIATELY,
+    0, 0, RUBY_TYPED_FREE_IMMEDIATELY | RUBY_TYPED_WB_PROTECTED,
 };
 
 static struct ractor_selector *
@@ -1318,6 +1318,8 @@ ractor_selector_add(VALUE selv, VALUE rpv)
     }
 
     st_insert(s->ports, (st_data_t)rpv, (st_data_t)rp);
+    RB_OBJ_WRITTEN(selv, Qundef, rpv);
+
     return selv;
 }
 
diff --git a/rubyparser.h b/rubyparser.h
index 9fd6906ca60c08..e436d1c404b2a2 100644
--- a/rubyparser.h
+++ b/rubyparser.h
@@ -901,6 +901,8 @@ typedef struct RNode_MODULE {
 
     struct RNode *nd_cpath;
     struct RNode *nd_body;
+    rb_code_location_t module_keyword_loc;
+    rb_code_location_t end_keyword_loc;
 } rb_node_module_t;
 
 typedef struct RNode_SCLASS {
diff --git a/struct.c b/struct.c
index 74ca9369a61c82..c53e68b3da7f67 100644
--- a/struct.c
+++ b/struct.c
@@ -811,13 +811,22 @@ struct_alloc(VALUE klass)
 {
     long n = num_members(klass);
     size_t embedded_size = offsetof(struct RStruct, as.ary) + (sizeof(VALUE) * n);
+    if (RCLASS_MAX_IV_COUNT(klass) > 0) {
+        embedded_size += sizeof(VALUE);
+    }
+
     VALUE flags = T_STRUCT | (RGENGC_WB_PROTECTED_STRUCT ? FL_WB_PROTECTED : 0);
 
     if (n > 0 && rb_gc_size_allocatable_p(embedded_size)) {
         flags |= n << RSTRUCT_EMBED_LEN_SHIFT;
 
         NEWOBJ_OF(st, struct RStruct, klass, flags, embedded_size, 0);
-
+        if (RCLASS_MAX_IV_COUNT(klass) == 0 && embedded_size == rb_gc_obj_slot_size((VALUE)st)) {
+            FL_SET_RAW((VALUE)st, RSTRUCT_GEN_FIELDS);
+        }
+        else {
+            RSTRUCT_SET_FIELDS_OBJ((VALUE)st, 0);
+        }
         rb_mem_clear((VALUE *)st->as.ary, n);
 
         return (VALUE)st;
diff --git a/symbol.c b/symbol.c
index abb2c76dc2f758..c337cc288ecded 100644
--- a/symbol.c
+++ b/symbol.c
@@ -99,7 +99,9 @@ typedef struct {
     VALUE ids;
 } rb_symbols_t;
 
-rb_symbols_t ruby_global_symbols = {tNEXT_ID-1};
+rb_symbols_t ruby_global_symbols = {
+    .next_id = tNEXT_ID,
+};
 
 struct sym_set_static_sym_entry {
     VALUE sym;
diff --git a/test/ruby/test_ast.rb b/test/ruby/test_ast.rb
index 5524fa7146e035..6372b0d34e5e71 100644
--- a/test/ruby/test_ast.rb
+++ b/test/ruby/test_ast.rb
@@ -1491,6 +1491,11 @@ def test_lambda_locations
       assert_locations(node.children[-1].locations, [[1, 0, 1, 20], [1, 0, 1, 2], [1, 10, 1, 12], [1, 17, 1, 20]])
     end
 
+    def test_module_locations
+      node = ast_parse('module A end')
+      assert_locations(node.children[-1].locations, [[1, 0, 1, 12], [1, 0, 1, 6], [1, 9, 1, 12]])
+    end
+
     def test_if_locations
       node = ast_parse("if cond then 1 else 2 end")
       assert_locations(node.children[-1].locations, [[1, 0, 1, 25], [1, 0, 1, 2], [1, 8, 1, 12], [1, 22, 1, 25]])
diff --git a/test/ruby/test_object_id.rb b/test/ruby/test_object_id.rb
index 24434f8aba0ab8..adb819febce57c 100644
--- a/test/ruby/test_object_id.rb
+++ b/test/ruby/test_object_id.rb
@@ -252,3 +252,52 @@ def initialize
     end;
   end
 end
+
+class TestObjectIdStruct < TestObjectId
+  EmbeddedStruct = Struct.new(:embedded_field)
+
+  def setup
+    @obj = EmbeddedStruct.new
+  end
+end
+
+class TestObjectIdStructGenIvar < TestObjectId
+  GenIvarStruct = Struct.new(:a, :b, :c)
+
+  def setup
+    @obj = GenIvarStruct.new
+  end
+end
+
+class TestObjectIdStructNotEmbed < TestObjectId
+  MANY_IVS = 80
+
+  StructNotEmbed = Struct.new(*MANY_IVS.times.map { |i| :"field_#{i}" })
+
+  def setup
+    @obj = StructNotEmbed.new
+  end
+end
+
+class TestObjectIdStructTooComplex < TestObjectId
+  StructTooComplex = Struct.new(:a) do
+    def initialize
+      @too_complex_obj_id_test = 1
+    end
+  end
+
+  def setup
+    if defined?(RubyVM::Shape::SHAPE_MAX_VARIATIONS)
+      assert_equal 8, RubyVM::Shape::SHAPE_MAX_VARIATIONS
+    end
+    8.times do |i|
+      StructTooComplex.new.instance_variable_set("@TestObjectIdStructTooComplex#{i}", 1)
+    end
+    @obj = StructTooComplex.new
+    @obj.instance_variable_set("@a#{rand(10_000)}", 1)
+
+    if defined?(RubyVM::Shape)
+      assert_predicate(RubyVM::Shape.of(@obj), :too_complex?)
+    end
+  end
+end
diff --git a/test/ruby/test_ractor.rb b/test/ruby/test_ractor.rb
index 97af7e7413f44e..0a456a1d0f5e1f 100644
--- a/test/ruby/test_ractor.rb
+++ b/test/ruby/test_ractor.rb
@@ -99,6 +99,24 @@ class TestClass
     RUBY
   end
 
+  def test_struct_instance_variables
+    assert_ractor(<<~'RUBY')
+      StructIvar = Struct.new(:member) do
+        def initialize(*)
+          super
+          @ivar = "ivar"
+        end
+        attr_reader :ivar
+      end
+      obj = StructIvar.new("member")
+      obj_copy = Ractor.new { Ractor.receive }.send(obj).value
+      assert_equal obj.ivar, obj_copy.ivar
+      refute_same obj.ivar, obj_copy.ivar
+      assert_equal obj.member, obj_copy.member
+      refute_same obj.member, obj_copy.member
+    RUBY
+  end
+
   def test_fork_raise_isolation_error
     assert_ractor(<<~'RUBY')
       ractor = Ractor.new do
diff --git a/test/ruby/test_zjit.rb b/test/ruby/test_zjit.rb
index 6db57e18ba729c..e3ef6f65698ad6 100644
--- a/test/ruby/test_zjit.rb
+++ b/test/ruby/test_zjit.rb
@@ -283,6 +283,14 @@ def test(a, b) = a == b
     }, insns: [:opt_eq], call_threshold: 2
   end
 
+  def test_opt_eq_with_minus_one
+    assert_compiles '[false, true]', %q{
+      def test(a) = a == -1
+      test(1) # profile opt_eq
+      [test(0), test(-1)]
+    }, insns: [:opt_eq], call_threshold: 2
+  end
+
   def test_opt_neq_dynamic
     # TODO(max): Don't split this test; instead, run all tests with and without
     # profiling.
diff --git a/variable.c b/variable.c
index f504cc57f57fc4..76b16b04cb3968 100644
--- a/variable.c
+++ b/variable.c
@@ -29,6 +29,7 @@
 #include "internal/object.h"
 #include "internal/gc.h"
 #include "internal/re.h"
+#include "internal/struct.h"
 #include "internal/symbol.h"
 #include "internal/thread.h"
 #include "internal/variable.h"
@@ -1228,10 +1229,19 @@ rb_obj_fields(VALUE obj, ID field_name)
     ivar_ractor_check(obj, field_name);
 
     VALUE fields_obj = 0;
-    if (rb_obj_exivar_p(obj)) {
-        RB_VM_LOCKING() {
-            if (!st_lookup(generic_fields_tbl_, (st_data_t)obj, (st_data_t *)&fields_obj)) {
-                rb_bug("Object is missing entry in generic_fields_tbl");
+    if (rb_shape_obj_has_fields(obj)) {
+        switch (BUILTIN_TYPE(obj)) {
+          case T_STRUCT:
+            if (LIKELY(!FL_TEST_RAW(obj, RSTRUCT_GEN_FIELDS))) {
+                fields_obj = RSTRUCT_FIELDS_OBJ(obj);
+                break;
+            }
+            // fall through
+          default:
+            RB_VM_LOCKING() {
+                if (!st_lookup(generic_fields_tbl_, (st_data_t)obj, (st_data_t *)&fields_obj)) {
+                    rb_bug("Object is missing entry in generic_fields_tbl");
+                }
             }
         }
     }
@@ -1243,11 +1253,19 @@ rb_free_generic_ivar(VALUE obj)
 {
     if (rb_obj_exivar_p(obj)) {
         st_data_t key = (st_data_t)obj, value;
-
-        RB_VM_LOCKING() {
-            st_delete(generic_fields_tbl_no_ractor_check(), &key, &value);
-            RBASIC_SET_SHAPE_ID(obj, ROOT_SHAPE_ID);
+        switch (BUILTIN_TYPE(obj)) {
+          case T_STRUCT:
+            if (LIKELY(!FL_TEST_RAW(obj, RSTRUCT_GEN_FIELDS))) {
+                RSTRUCT_SET_FIELDS_OBJ(obj, 0);
+                break;
+            }
+            // fall through
+          default:
+            RB_VM_LOCKING() {
+                st_delete(generic_fields_tbl_no_ractor_check(), &key, &value);
+            }
         }
+        RBASIC_SET_SHAPE_ID(obj, ROOT_SHAPE_ID);
     }
 }
 
@@ -1260,12 +1278,20 @@ rb_obj_set_fields(VALUE obj, VALUE fields_obj, ID field_name, VALUE original_fie
     RUBY_ASSERT(!original_fields_obj || IMEMO_TYPE_P(original_fields_obj, imemo_fields));
 
     if (fields_obj != original_fields_obj) {
-        RB_VM_LOCKING() {
-            st_insert(generic_fields_tbl_, (st_data_t)obj, (st_data_t)fields_obj);
+        switch (BUILTIN_TYPE(obj)) {
+          case T_STRUCT:
+            if (LIKELY(!FL_TEST_RAW(obj, RSTRUCT_GEN_FIELDS))) {
+                RSTRUCT_SET_FIELDS_OBJ(obj, fields_obj);
+                break;
+            }
+            // fall through
+          default:
+            RB_VM_LOCKING() {
+                st_insert(generic_fields_tbl_, (st_data_t)obj, (st_data_t)fields_obj);
+            }
+            RB_OBJ_WRITTEN(obj, original_fields_obj, fields_obj);
         }
 
-        RB_OBJ_WRITTEN(obj, original_fields_obj, fields_obj);
-
         if (original_fields_obj) {
             // Clear root shape to avoid triggering cleanup such as free_object_id.
             rb_imemo_fields_clear(original_fields_obj);
@@ -1276,11 +1302,11 @@ rb_obj_set_fields(VALUE obj, VALUE fields_obj, ID field_name, VALUE original_fie
 }
 
 void
-rb_obj_replace_fields(VALUE obj, VALUE fields_obj, ID field_name)
+rb_obj_replace_fields(VALUE obj, VALUE fields_obj)
 {
     RB_VM_LOCKING() {
-        VALUE original_fields_obj = rb_obj_fields(obj, field_name);
-        rb_obj_set_fields(obj, fields_obj, field_name, original_fields_obj);
+        VALUE original_fields_obj = rb_obj_fields_no_ractor_check(obj);
+        rb_obj_set_fields(obj, fields_obj, 0, original_fields_obj);
     }
 }
 
@@ -1608,7 +1634,7 @@ obj_transition_too_complex(VALUE obj, st_table *table)
         {
             VALUE fields_obj = rb_imemo_fields_new_complex_tbl(rb_obj_class(obj), table);
             RBASIC_SET_SHAPE_ID(fields_obj, shape_id);
-            rb_obj_replace_fields(obj, fields_obj, 0);
+            rb_obj_replace_fields(obj, fields_obj);
         }
     }
 
@@ -2299,12 +2325,7 @@ rb_copy_generic_ivar(VALUE dest, VALUE obj)
         rb_shape_copy_fields(new_fields_obj, dest_buf, dest_shape_id, src_buf, src_shape_id);
         RBASIC_SET_SHAPE_ID(new_fields_obj, dest_shape_id);
 
-        RB_VM_LOCKING() {
-            st_insert(generic_fields_tbl_no_ractor_check(), (st_data_t)dest, (st_data_t)new_fields_obj);
-            RB_OBJ_WRITTEN(dest, Qundef, new_fields_obj);
-        }
-
-        RBASIC_SET_SHAPE_ID(dest, dest_shape_id);
+        rb_obj_replace_fields(dest, new_fields_obj);
     }
     return;
 
diff --git a/zjit/src/backend/x86_64/mod.rs b/zjit/src/backend/x86_64/mod.rs
index d21c7ee09c82d2..f7aa1acc4dae63 100644
--- a/zjit/src/backend/x86_64/mod.rs
+++ b/zjit/src/backend/x86_64/mod.rs
@@ -381,7 +381,7 @@ impl Assembler
                         mov(cb, Assembler::SCRATCH0, opnd.into());
                         Assembler::SCRATCH0
                     } else {
-                        opnd.into()
+                        imm_opnd(*value as i64)
                     }
                 },
                 _ => opnd.into()
@@ -963,7 +963,9 @@ mod tests {
         asm.cmp(Opnd::Reg(RAX_REG), Opnd::UImm(0xFF));
         asm.compile_with_num_regs(&mut cb, 0);
 
-        assert_eq!(format!("{:x}", cb), "4881f8ff000000");
+        assert_disasm!(cb, "4881f8ff000000", "
+            0x0: cmp rax, 0xff
+        ");
     }
 
     #[test]
@@ -973,7 +975,22 @@ mod tests {
         asm.cmp(Opnd::Reg(RAX_REG), Opnd::UImm(0xFFFF_FFFF_FFFF));
         asm.compile_with_num_regs(&mut cb, 0);
 
-        assert_eq!(format!("{:x}", cb), "49bbffffffffffff00004c39d8");
+        assert_disasm!(cb, "49bbffffffffffff00004c39d8", "
+            0x0: movabs r11, 0xffffffffffff
+            0xa: cmp rax, r11
+        ");
+    }
+
+    #[test]
+    fn test_emit_cmp_64_bits() {
+        let (mut asm, mut cb) = setup_asm();
+
+        asm.cmp(Opnd::Reg(RAX_REG), Opnd::UImm(0xFFFF_FFFF_FFFF_FFFF));
+        asm.compile_with_num_regs(&mut cb, 0);
+
+        assert_disasm!(cb, "4883f8ff", "
+            0x0: cmp rax, -1
+        ");
     }
 
     #[test]
@@ -1051,7 +1068,9 @@ mod tests {
         asm.test(Opnd::Reg(RAX_REG), Opnd::UImm(0xFF));
         asm.compile_with_num_regs(&mut cb, 0);
 
-        assert_eq!(format!("{:x}", cb), "f6c0ff");
+        assert_disasm!(cb, "48f7c0ff000000", "
+            0x0: test rax, 0xff
+        ");
     }
 
     #[test]