https://git.ruby-lang.org/ruby.git/commit/?id=ddf5020e4f

From ddf5020e4fcae5ed28a064af10124a032590452f Mon Sep 17 00:00:00 2001
From: Koichi Sasada <ko1@a...>
Date: Sun, 6 Oct 2019 17:01:00 +0900
Subject: Revert "tailcall optimization again (#2528)"

This reverts commit f62f90367fc3bce6714e7c34cbd040e14e43fe07.

diff --git a/test/ruby/test_optimization.rb b/test/ruby/test_optimization.rb
index 0d4f692..6a9e960 100644
--- a/test/ruby/test_optimization.rb
+++ b/test/ruby/test_optimization.rb
@@ -451,19 +451,6 @@ class TestRubyOptimization < Test::Unit::TestCase https://github.com/ruby/ruby/blob/trunk/test/ruby/test_optimization.rb#L451
     }
   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?
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 8e4e02e..f9c825c 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -1872,30 +1872,6 @@ vm_call_iseq_setup_normal_opt_start(rb_execution_context_t *ec, rb_control_frame https://github.com/ruby/ruby/blob/trunk/vm_insnhelper.c#L1872
     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,
@@ -1982,16 +1958,9 @@ vm_callee_setup_arg(rb_execution_context_t *ec, struct rb_calling_info *calling, https://github.com/ruby/ruby/blob/trunk/vm_insnhelper.c#L1958
                 argument_arity_error(ec, iseq, argc, lead_num, lead_num + opt_num);
             }
 
-            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));
-            }
+            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));
 
             /* initialize opt vars for self-references */
             VM_ASSERT((int)iseq->body->param.size == lead_num + opt_num);
-- 
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