ruby · byroot · Jun 13, 2025 · Jun 13, 2025
diff --git a/internal/string.h b/internal/string.h
@@ -30,6 +30,7 @@ enum ruby_rstring_private_flags {
 #endif
 
 /* string.c */
+VALUE rb_str_dup_m(VALUE str);
 VALUE rb_fstring(VALUE);
 VALUE rb_fstring_cstr(const char *str);
 VALUE rb_fstring_enc_new(const char *ptr, long len, rb_encoding *enc);

diff --git a/shape.c b/shape.c
@@ -1234,6 +1234,23 @@ rb_shape_verify_consistency(VALUE obj, shape_id_t shape_id)
         }
     }
 
+    // Make sure SHAPE_ID_HAS_IVAR_MASK is valid.
+    if (rb_shape_too_complex_p(shape_id)) {
+        RUBY_ASSERT(shape_id & SHAPE_ID_HAS_IVAR_MASK);
+    }
+    else {
+        attr_index_t ivar_count = RSHAPE_LEN(shape_id);
+        if (has_object_id) {
+            ivar_count--;
+        }
+        if (ivar_count) {
+            RUBY_ASSERT(shape_id & SHAPE_ID_HAS_IVAR_MASK);
+        }
+        else {
+            RUBY_ASSERT(!(shape_id & SHAPE_ID_HAS_IVAR_MASK));
+        }
+    }
+
     uint8_t flags_heap_index = rb_shape_heap_index(shape_id);
     if (RB_TYPE_P(obj, T_OBJECT)) {
         size_t shape_id_slot_size = rb_shape_tree.capacities[flags_heap_index - 1] * sizeof(VALUE) + sizeof(struct RBasic);
@@ -1524,14 +1541,15 @@ Init_default_shapes(void)
     root->type = SHAPE_ROOT;
     rb_shape_tree.root_shape = root;
     RUBY_ASSERT(raw_shape_id(rb_shape_tree.root_shape) == ROOT_SHAPE_ID);
+    RUBY_ASSERT(!(raw_shape_id(rb_shape_tree.root_shape) & SHAPE_ID_HAS_IVAR_MASK));
 
     bool dontcare;
     rb_shape_t *root_with_obj_id = get_next_shape_internal(root, id_object_id, SHAPE_OBJ_ID, &dontcare, true);
     RUBY_ASSERT(raw_shape_id(root_with_obj_id) == ROOT_SHAPE_WITH_OBJ_ID);
     RUBY_ASSERT(root_with_obj_id->type == SHAPE_OBJ_ID);
     RUBY_ASSERT(root_with_obj_id->edge_name == id_object_id);
     RUBY_ASSERT(root_with_obj_id->next_field_index == 1);
-    (void)root_with_obj_id;
+    RUBY_ASSERT(!(raw_shape_id(root_with_obj_id) & SHAPE_ID_HAS_IVAR_MASK));
 }
 
 void

diff --git a/shape.h b/shape.h
@@ -23,6 +23,10 @@ STATIC_ASSERT(shape_id_num_bits, SHAPE_ID_NUM_BITS == sizeof(shape_id_t) * CHAR_
 #define SHAPE_ID_HEAP_INDEX_MAX ((1 << SHAPE_ID_HEAP_INDEX_BITS) - 1)
 #define SHAPE_ID_HEAP_INDEX_MASK (SHAPE_ID_HEAP_INDEX_MAX << SHAPE_ID_HEAP_INDEX_OFFSET)
 
+// This masks allows to check if a shape_id contains any ivar.
+// It rely on ROOT_SHAPE_WITH_OBJ_ID==1.
+#define SHAPE_ID_HAS_IVAR_MASK (SHAPE_ID_FL_TOO_COMPLEX | (SHAPE_ID_OFFSET_MASK - 1))
+
 // The interpreter doesn't care about frozen status or slot size when reading ivars.
 // So we normalize shape_id by clearing these bits to improve cache hits.
 // JITs however might care about it.
@@ -327,6 +331,18 @@ rb_shape_obj_has_id(VALUE obj)
     return rb_shape_has_object_id(RBASIC_SHAPE_ID(obj));
 }
 
+static inline bool
+rb_shape_has_ivars(shape_id_t shape_id)
+{
+    return shape_id & SHAPE_ID_HAS_IVAR_MASK;
+}
+
+static inline bool
+rb_shape_obj_has_ivars(VALUE obj)
+{
+    return rb_shape_has_ivars(RBASIC_SHAPE_ID(obj));
+}
+
 // For ext/objspace
 RUBY_SYMBOL_EXPORT_BEGIN
 typedef void each_shape_callback(shape_id_t shape_id, void *data);

diff --git a/string.c b/string.c
@@ -388,12 +388,7 @@ fstring_hash(VALUE str)
 static inline bool
 BARE_STRING_P(VALUE str)
 {
-    if (RBASIC_CLASS(str) != rb_cString) return false;
-
-    if (FL_TEST_RAW(str, FL_EXIVAR)) {
-        return rb_ivar_count(str) == 0;
-    }
-    return true;
+    return RBASIC_CLASS(str) == rb_cString && !rb_shape_obj_has_ivars(str);
 }
 
 static inline st_index_t
@@ -2316,7 +2311,7 @@ VALUE
 rb_str_dup_m(VALUE str)
 {
     if (LIKELY(BARE_STRING_P(str))) {
-        return str_duplicate(rb_obj_class(str), str);
+        return str_duplicate(rb_cString, str);
     }
     else {
         return rb_obj_dup(str);

diff --git a/yjit/bindgen/src/main.rs b/yjit/bindgen/src/main.rs
@@ -228,6 +228,7 @@ fn main() {
         .allowlist_function("rb_obj_as_string_result")
         .allowlist_function("rb_str_byte_substr")
         .allowlist_function("rb_str_substr_two_fixnums")
+        .allowlist_function("rb_str_dup_m")
 
         // From include/ruby/internal/intern/parse.h
         .allowlist_function("rb_backref_get")

diff --git a/yjit/src/codegen.rs b/yjit/src/codegen.rs
@@ -6275,16 +6275,12 @@ fn jit_rb_str_dup(
 
     jit_prepare_call_with_gc(jit, asm);
 
-    // Check !FL_ANY_RAW(str, FL_EXIVAR), which is part of BARE_STRING_P.
     let recv_opnd = asm.stack_pop(1);
     let recv_opnd = asm.load(recv_opnd);
-    let flags_opnd = Opnd::mem(64, recv_opnd, RUBY_OFFSET_RBASIC_FLAGS);
-    asm.test(flags_opnd, Opnd::Imm(RUBY_FL_EXIVAR as i64));
-    asm.jnz(Target::side_exit(Counter::send_str_dup_exivar));
 
     // Call rb_str_dup
     let stack_ret = asm.stack_push(Type::CString);
-    let ret_opnd = asm.ccall(rb_str_dup as *const u8, vec![recv_opnd]);
+    let ret_opnd = asm.ccall(rb_str_dup_m as *const u8, vec![recv_opnd]);
     asm.mov(stack_ret, ret_opnd);
 
     true

diff --git a/yjit/src/cruby_bindings.inc.rs b/yjit/src/cruby_bindings.inc.rs