Handle non-material empty singleton class properly

As an optimization, multiple objects could share the same singleton
class. The optimization introduced in 6698e433934d810b16ee3636b63974c0a75c07f0
wasn't handling this correctly so was generating guards that never pass
for the inputs we defer compilation to wait for. After generating
identical code multiple times and failing, the call site is falsely
recognized as megamorphic and it side exits. See disassembly for the
following before this commit:

    def foo(obj)
      obj.itself
    end

    o = Object.new.singleton_class
    foo(o)
    puts YJIT.disasm(method(:foo))

See also: comment in rb_singleton_class_clone_and_attach().

bootstraptest/test_yjit.rb

@@ -1189,6 +1189,20 @@ assert_equal '123', %q{
   foo(obj)
 }
 
+# Call method on an object that has a non-material
+# singleton class.
+# TODO: assert that it takes no side exits? This
+# test case revealed that we were taking exits unnecessarily.
+assert_normal_exit %q{
+  def foo(obj)
+    obj.itself
+  end
+
+  o = Object.new.singleton_class
+  foo(o)
+  foo(o)
+}
+
 # Call to singleton class
 assert_equal '123', %q{
   class Foo

yjit_codegen.c

@@ -167,7 +167,7 @@ jit_save_sp(jitstate_t* jit, ctx_t* ctx)
     }
 }
 
-static bool jit_guard_known_klass(jitstate_t *jit, ctx_t* ctx, VALUE known_klass, insn_opnd_t insn_opnd, const int max_chain_depth, uint8_t *side_exit);
+static bool jit_guard_known_klass(jitstate_t *jit, ctx_t* ctx, VALUE known_klass, insn_opnd_t insn_opnd, VALUE sample_instance, const int max_chain_depth, uint8_t *side_exit);
 
 #if RUBY_DEBUG
 
@@ -1271,7 +1271,7 @@ gen_getinstancevariable(jitstate_t *jit, ctx_t *ctx)
     mov(cb, REG0, member_opnd(REG_CFP, rb_control_frame_t, self));
     guard_self_is_heap(cb, REG0, COUNTED_EXIT(side_exit, getivar_se_self_not_heap), ctx);
 
-    jit_guard_known_klass(jit, ctx, comptime_val_klass, OPND_SELF, GETIVAR_MAX_DEPTH, side_exit);
+    jit_guard_known_klass(jit, ctx, comptime_val_klass, OPND_SELF, comptime_val, GETIVAR_MAX_DEPTH, side_exit);
 
     return gen_get_ivar(jit, ctx, GETIVAR_MAX_DEPTH, comptime_val, ivar_name, OPND_SELF, side_exit);
 }
@@ -2170,10 +2170,9 @@ Guard that a stack operand has the same class as known_klass.
 Recompile as contingency if possible, or take side exit a last resort.
 */
 static bool
-jit_guard_known_klass(jitstate_t *jit, ctx_t* ctx, VALUE known_klass, insn_opnd_t insn_opnd, const int max_chain_depth, uint8_t *side_exit)
+jit_guard_known_klass(jitstate_t *jit, ctx_t *ctx, VALUE known_klass, insn_opnd_t insn_opnd, VALUE sample_instance, const int max_chain_depth, uint8_t *side_exit)
 {
     val_type_t val_type = ctx_get_opnd_type(ctx, insn_opnd);
-    bool singleton_klass = FL_TEST(known_klass, FL_SINGLETON);
 
     if (known_klass == rb_cNilClass) {
         if (val_type.type != ETYPE_NIL) {
@@ -2206,19 +2205,25 @@ jit_guard_known_klass(jitstate_t *jit, ctx_t* ctx, VALUE known_klass, insn_opnd_
     else if (known_klass == rb_cInteger ||
             known_klass == rb_cSymbol ||
             known_klass == rb_cFloat) {
-        // Can't guard for for these classes because some of they are sometimes
-        // immediate (special const). Can remove this by adding appropriate
-        // dynamic checks.
+        // Can't guard for for these classes because they can have both
+        // immediate (special const) instances and instances on the heap. Can
+        // remove this by adding appropriate dynamic checks.
         return false;
     }
-    else if (singleton_klass) {
+    else if (FL_TEST(known_klass, FL_SINGLETON) && sample_instance == rb_attr_get(known_klass, id__attached__)) {
         // Singleton classes are attached to one specific object, so we can
         // avoid one memory access (and potentially the is_heap check) by
         // looking for the expected object directly.
+        // Note that in case the sample instance has a singleton class that
+        // doesn't attach to the sample instance, it means the sample instance
+        // has an empty singleton class that hasn't been materialized yet. In
+        // this case, comparing against the sample instance doesn't gurantee
+        // that its singleton class is empty, so we can't avoid the memory
+        // access. As an example, `Object.new.singleton_class` is an object in
+        // this situation.
         ADD_COMMENT(cb, "guard known object with singleton class");
-        VALUE known_obj = rb_attr_get(known_klass, id__attached__);
         // TODO: jit_mov_gc_ptr keeps a strong reference, which leaks the object.
-        jit_mov_gc_ptr(jit, cb, REG1, known_obj);
+        jit_mov_gc_ptr(jit, cb, REG1, sample_instance);
         cmp(cb, REG0, REG1);
         jit_chain_guard(JCC_JNE, jit, ctx, max_chain_depth, side_exit);
     }
@@ -2868,7 +2873,7 @@ gen_send_general(jitstate_t *jit, ctx_t *ctx, struct rb_call_data *cd, rb_iseq_t
     x86opnd_t recv = ctx_stack_opnd(ctx, argc);
     insn_opnd_t recv_opnd = OPND_STACK(argc);
     mov(cb, REG0, recv);
-    if (!jit_guard_known_klass(jit, ctx, comptime_recv_klass, recv_opnd, SEND_MAX_DEPTH, side_exit)) {
+    if (!jit_guard_known_klass(jit, ctx, comptime_recv_klass, recv_opnd, comptime_recv, SEND_MAX_DEPTH, side_exit)) {
         return YJIT_CANT_COMPILE;
     }
 
@@ -3075,7 +3080,7 @@ gen_invokesuper(jitstate_t *jit, ctx_t *ctx)
     insn_opnd_t recv_opnd = OPND_STACK(argc);
     mov(cb, REG0, recv);
 
-    if (!jit_guard_known_klass(jit, ctx, comptime_recv_klass, recv_opnd, SEND_MAX_DEPTH, side_exit)) {
+    if (!jit_guard_known_klass(jit, ctx, comptime_recv_klass, recv_opnd, comptime_recv, SEND_MAX_DEPTH, side_exit)) {
         return YJIT_CANT_COMPILE;
     }