From 8efa7b00df2842eaff31cdabc95651a856e40549 Mon Sep 17 00:00:00 2001
From: "Yukihiro \"Matz\" Matsumoto" <matz@ruby-lang.org>
Date: Wed, 15 Feb 2017 11:59:47 +0900
Subject: Preallocate SystemStackError; ref #3421

---
 include/mruby.h | 2 +-
 src/backtrace.c | 2 +-
 src/error.c     | 5 +++--
 src/gc.c        | 1 +
 src/vm.c        | 2 +-
 5 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/mruby.h b/include/mruby.h
index 6950cf904..8adce289b 100644
--- a/include/mruby.h
+++ b/include/mruby.h
@@ -221,6 +221,7 @@ typedef struct mrb_state {
   struct RClass *eException_class;
   struct RClass *eStandardError_class;
   struct RObject *nomem_err;              /* pre-allocated NoMemoryError */
+  struct RObject *stack_err;              /* pre-allocated SysStackError */
 #ifdef MRB_GC_FIXED_ARENA
   struct RObject *arena_err;              /* pre-allocated arena overfow error */
 #endif
@@ -1110,7 +1111,6 @@ MRB_API void mrb_print_error(mrb_state *mrb);
 #define E_SYNTAX_ERROR              (mrb_exc_get(mrb, "SyntaxError"))
 #define E_LOCALJUMP_ERROR           (mrb_exc_get(mrb, "LocalJumpError"))
 #define E_REGEXP_ERROR              (mrb_exc_get(mrb, "RegexpError"))
-#define E_SYSSTACK_ERROR            (mrb_exc_get(mrb, "SystemStackError"))
 
 #define E_NOTIMP_ERROR              (mrb_exc_get(mrb, "NotImplementedError"))
 #define E_FLOATDOMAIN_ERROR         (mrb_exc_get(mrb, "FloatDomainError"))
diff --git a/src/backtrace.c b/src/backtrace.c
index 051d5d4e0..529b0b1c9 100644
--- a/src/backtrace.c
+++ b/src/backtrace.c
@@ -263,7 +263,7 @@ mrb_print_backtrace(mrb_state *mrb)
 {
   mrb_value backtrace;
 
-  if (!mrb->exc || mrb_obj_is_kind_of(mrb, mrb_obj_value(mrb->exc), E_SYSSTACK_ERROR)) {
+  if (!mrb->exc) {
     return;
   }
 
diff --git a/src/error.c b/src/error.c
index a71ee548f..7916ca65b 100644
--- a/src/error.c
+++ b/src/error.c
@@ -532,7 +532,7 @@ mrb_no_method_error(mrb_state *mrb, mrb_sym id, mrb_value args, char const* fmt,
 void
 mrb_init_exception(mrb_state *mrb)
 {
-  struct RClass *exception, *runtime_error, *script_error;
+  struct RClass *exception, *runtime_error, *script_error, *stack_error;
 
   mrb->eException_class = exception = mrb_define_class(mrb, "Exception", mrb->object_class); /* 15.2.22 */
   MRB_SET_INSTANCE_TT(exception, MRB_TT_EXCEPTION);
@@ -553,5 +553,6 @@ mrb_init_exception(mrb_state *mrb)
 #endif
   script_error = mrb_define_class(mrb, "ScriptError", mrb->eException_class);                /* 15.2.37 */
   mrb_define_class(mrb, "SyntaxError", script_error);                                        /* 15.2.38 */
-  mrb_define_class(mrb, "SystemStackError", exception);
+  stack_error = mrb_define_class(mrb, "SystemStackError", exception);
+  mrb->stack_err = mrb_obj_ptr(mrb_exc_new_str_lit(mrb, stack_error, "stack level too deep"));
 }
diff --git a/src/gc.c b/src/gc.c
index 1608dbcb2..63eab8e00 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -865,6 +865,7 @@ root_scan_phase(mrb_state *mrb, mrb_gc *gc)
   }
   /* mark pre-allocated exception */
   mrb_gc_mark(mrb, (struct RBasic*)mrb->nomem_err);
+  mrb_gc_mark(mrb, (struct RBasic*)mrb->stack_err);
 #ifdef MRB_GC_FIXED_ARENA
   mrb_gc_mark(mrb, (struct RBasic*)mrb->arena_err);
 #endif
diff --git a/src/vm.c b/src/vm.c
index 8bb446fa9..91612a0da 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -162,7 +162,7 @@ stack_extend_alloc(mrb_state *mrb, int room, int keep)
      to prevent infinite recursion. However, do this only after resizing the stack, so mrb_raise has stack space to work with. */
   if (size > MRB_STACK_MAX) {
     init_new_stack_space(mrb, room, keep);
-    mrb_raise(mrb, E_SYSSTACK_ERROR, "stack level too deep. (limit=" MRB_STRINGIZE(MRB_STACK_MAX) ")");
+    mrb_exc_raise(mrb, mrb_obj_value(mrb->stack_err));
   }
 }
 
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