tailcall optimization again (#2528)

This is follow up of r67315.

test/ruby/test_optimization.rb

@@ -451,6 +451,19 @@ class TestRubyOptimization < Test::Unit::TestCase
     }
   end
 
+  def test_tailcall_not_to_grow_stack
+    bug16161 = '[ruby-core:94881]'
+
+    tailcall("#{<<-"begin;"}\n#{<<~"end;"}")
+    begin;
+      def sum(n, s = 0)
+        return s if n < 1
+        sum(n - 1, n + s)
+      end
+    end;
+    assert_equal(0, sum(1_000_000).to_s.size, message(bug16161) {disasm(:fact_helper)})
+  end
+
   class Bug10557
     def [](_)
       block_given?

vm_insnhelper.c

@@ -1872,6 +1872,30 @@ vm_call_iseq_setup_normal_opt_start(rb_execution_context_t *ec, rb_control_frame
     return vm_call_iseq_setup_normal(ec, cfp, calling, cc->me, opt_pc, param - delta, local);
 }
 
+static VALUE
+vm_call_iseq_setup_tailcall_opt_start(rb_execution_context_t *ec, rb_control_frame_t *cfp,
+                                      struct rb_calling_info *calling,
+                                      const struct rb_call_info *ci, struct rb_call_cache *cc)
+{
+    const rb_iseq_t *iseq = def_iseq_ptr(cc->me->def);
+    const int lead_num = iseq->body->param.lead_num;
+    const int opt = calling->argc - lead_num;
+    const int opt_pc = (int)iseq->body->param.opt_table[opt];
+
+    RB_DEBUG_COUNTER_INC(ccf_iseq_opt);
+
+#if USE_OPT_HIST
+    if (opt_pc < OPT_HIST_MAX) {
+        opt_hist[opt]++;
+    }
+    else {
+        opt_hist[OPT_HIST_MAX]++;
+    }
+#endif
+
+    return vm_call_iseq_setup_tailcall(ec, cfp, calling, ci, cc, opt_pc);
+}
+
 static void
 args_setup_kw_parameters(rb_execution_context_t *const ec, const rb_iseq_t *const iseq,
                          VALUE *const passed_values, const int passed_keyword_len, const VALUE *const passed_keywords,
@@ -1958,9 +1982,16 @@ vm_callee_setup_arg(rb_execution_context_t *ec, struct rb_calling_info *calling,
                 argument_arity_error(ec, iseq, argc, lead_num, lead_num + opt_num);
             }
 
-            CC_SET_FASTPATH(cc, vm_call_iseq_setup_normal_opt_start,
-                            !IS_ARGS_SPLAT(ci) && !IS_ARGS_KEYWORD(ci) &&
-                            !(METHOD_ENTRY_VISI(cc->me) == METHOD_VISI_PROTECTED));
+            if (LIKELY(!(ci->flag & VM_CALL_TAILCALL))) {
+                CC_SET_FASTPATH(cc, vm_call_iseq_setup_normal_opt_start,
+                                !IS_ARGS_SPLAT(ci) && !IS_ARGS_KEYWORD(ci) &&
+                                !(METHOD_ENTRY_VISI(cc->me) == METHOD_VISI_PROTECTED));
+            }
+            else {
+                CC_SET_FASTPATH(cc, vm_call_iseq_setup_tailcall_opt_start,
+                                !IS_ARGS_SPLAT(ci) && !IS_ARGS_KEYWORD(ci) &&
+                                !(METHOD_ENTRY_VISI(cc->me) == METHOD_VISI_PROTECTED));
+            }
 
             /* initialize opt vars for self-references */
             VM_ASSERT((int)iseq->body->param.size == lead_num + opt_num);