diff --git a/Include/internal/pycore_compile.h b/Include/internal/pycore_compile.h
index e8859bb..06a6082 100644
--- a/Include/internal/pycore_compile.h
+++ b/Include/internal/pycore_compile.h
@@ -28,6 +28,9 @@ extern PyObject* _Py_Mangle(PyObject *p, PyObject *name);
 typedef struct {
     int optimize;
     int ff_features;
+
+    int recursion_depth;            /* current recursion depth */
+    int recursion_limit;            /* recursion limit */
 } _PyASTOptimizeState;
 
 extern int _PyAST_Optimize(
diff --git a/Lib/test/test_compile.py b/Lib/test/test_compile.py
index fd1ef61..d40347c 100644
--- a/Lib/test/test_compile.py
+++ b/Lib/test/test_compile.py
@@ -543,21 +543,26 @@ def test_compiler_recursion_limit(self):
         # XXX (ncoghlan): duplicating the scaling factor here is a little
         # ugly. Perhaps it should be exposed somewhere...
         fail_depth = sys.getrecursionlimit() * 3
+        crash_depth = sys.getrecursionlimit() * 300
         success_depth = int(fail_depth * 0.75)
 
-        def check_limit(prefix, repeated):
+        def check_limit(prefix, repeated, mode="single"):
             expect_ok = prefix + repeated * success_depth
-            self.compile_single(expect_ok)
-            broken = prefix + repeated * fail_depth
-            details = "Compiling ({!r} + {!r} * {})".format(
-                         prefix, repeated, fail_depth)
-            with self.assertRaises(RecursionError, msg=details):
-                self.compile_single(broken)
+            compile(expect_ok, '<test>', mode)
+            for depth in (fail_depth, crash_depth):
+                broken = prefix + repeated * depth
+                details = "Compiling ({!r} + {!r} * {})".format(
+                            prefix, repeated, depth)
+                with self.assertRaises(RecursionError, msg=details):
+                    compile(broken, '<test>', mode)
 
         check_limit("a", "()")
         check_limit("a", ".b")
         check_limit("a", "[0]")
         check_limit("a", "*a")
+        # XXX Crashes in the parser.
+        # check_limit("a", " if a else a")
+        # check_limit("if a: pass", "\nelif a: pass", mode="exec")
 
     def test_null_terminated(self):
         # The source code is null-terminated internally, but bytes-like
diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-12-12-14-28-31.bpo-42609.Qcd54b.rst b/Misc/NEWS.d/next/Core and Builtins/2020-12-12-14-28-31.bpo-42609.Qcd54b.rst
new file mode 100644
index 0000000..d2b0c64
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2020-12-12-14-28-31.bpo-42609.Qcd54b.rst
@@ -0,0 +1,3 @@
+Prevented crashes in the AST validator and optimizer when compiling some
+absurdly long expressions like ``"+0"*1000000``. :exc:`RecursionError` is
+now raised instead.
diff --git a/Python/ast.c b/Python/ast.c
index c877953..2b96543 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -4,14 +4,20 @@
  */
 #include "Python.h"
 #include "pycore_ast.h"           // asdl_stmt_seq
+#include "pycore_pystate.h"       // _PyThreadState_GET()
 
 #include <assert.h>
 
-static int validate_stmts(asdl_stmt_seq *);
-static int validate_exprs(asdl_expr_seq*, expr_context_ty, int);
+struct validator {
+    int recursion_depth;            /* current recursion depth */
+    int recursion_limit;            /* recursion limit */
+};
+
+static int validate_stmts(struct validator *, asdl_stmt_seq *);
+static int validate_exprs(struct validator *, asdl_expr_seq*, expr_context_ty, int);
 static int _validate_nonempty_seq(asdl_seq *, const char *, const char *);
-static int validate_stmt(stmt_ty);
-static int validate_expr(expr_ty, expr_context_ty);
+static int validate_stmt(struct validator *, stmt_ty);
+static int validate_expr(struct validator *, expr_ty, expr_context_ty);
 
 static int
 validate_name(PyObject *name)
@@ -33,7 +39,7 @@ validate_name(PyObject *name)
 }
 
 static int
-validate_comprehension(asdl_comprehension_seq *gens)
+validate_comprehension(struct validator *state, asdl_comprehension_seq *gens)
 {
     Py_ssize_t i;
     if (!asdl_seq_LEN(gens)) {
@@ -42,31 +48,31 @@ validate_comprehension(asdl_comprehension_seq *gens)
     }
     for (i = 0; i < asdl_seq_LEN(gens); i++) {
         comprehension_ty comp = asdl_seq_GET(gens, i);
-        if (!validate_expr(comp->target, Store) ||
-            !validate_expr(comp->iter, Load) ||
-            !validate_exprs(comp->ifs, Load, 0))
+        if (!validate_expr(state, comp->target, Store) ||
+            !validate_expr(state, comp->iter, Load) ||
+            !validate_exprs(state, comp->ifs, Load, 0))
             return 0;
     }
     return 1;
 }
 
 static int
-validate_keywords(asdl_keyword_seq *keywords)
+validate_keywords(struct validator *state, asdl_keyword_seq *keywords)
 {
     Py_ssize_t i;
     for (i = 0; i < asdl_seq_LEN(keywords); i++)
-        if (!validate_expr((asdl_seq_GET(keywords, i))->value, Load))
+        if (!validate_expr(state, (asdl_seq_GET(keywords, i))->value, Load))
             return 0;
     return 1;
 }
 
 static int
-validate_args(asdl_arg_seq *args)
+validate_args(struct validator *state, asdl_arg_seq *args)
 {
     Py_ssize_t i;
     for (i = 0; i < asdl_seq_LEN(args); i++) {
         arg_ty arg = asdl_seq_GET(args, i);
-        if (arg->annotation && !validate_expr(arg->annotation, Load))
+        if (arg->annotation && !validate_expr(state, arg->annotation, Load))
             return 0;
     }
     return 1;
@@ -88,19 +94,19 @@ expr_context_name(expr_context_ty ctx)
 }
 
 static int
-validate_arguments(arguments_ty args)
+validate_arguments(struct validator *state, arguments_ty args)
 {
-    if (!validate_args(args->posonlyargs) || !validate_args(args->args)) {
+    if (!validate_args(state, args->posonlyargs) || !validate_args(state, args->args)) {
         return 0;
     }
     if (args->vararg && args->vararg->annotation
-        && !validate_expr(args->vararg->annotation, Load)) {
+        && !validate_expr(state, args->vararg->annotation, Load)) {
             return 0;
     }
-    if (!validate_args(args->kwonlyargs))
+    if (!validate_args(state, args->kwonlyargs))
         return 0;
     if (args->kwarg && args->kwarg->annotation
-        && !validate_expr(args->kwarg->annotation, Load)) {
+        && !validate_expr(state, args->kwarg->annotation, Load)) {
             return 0;
     }
     if (asdl_seq_LEN(args->defaults) > asdl_seq_LEN(args->posonlyargs) + asdl_seq_LEN(args->args)) {
@@ -112,11 +118,11 @@ validate_arguments(arguments_ty args)
                         "kw_defaults on arguments");
         return 0;
     }
-    return validate_exprs(args->defaults, Load, 0) && validate_exprs(args->kw_defaults, Load, 1);
+    return validate_exprs(state, args->defaults, Load, 0) && validate_exprs(state, args->kw_defaults, Load, 1);
 }
 
 static int
-validate_constant(PyObject *value)
+validate_constant(struct validator *state, PyObject *value)
 {
     if (value == Py_None || value == Py_Ellipsis)
         return 1;
@@ -130,9 +136,13 @@ validate_constant(PyObject *value)
         return 1;
 
     if (PyTuple_CheckExact(value) || PyFrozenSet_CheckExact(value)) {
-        PyObject *it;
+        if (++state->recursion_depth > state->recursion_limit) {
+            PyErr_SetString(PyExc_RecursionError,
+                            "maximum recursion depth exceeded during compilation");
+            return 0;
+        }
 
-        it = PyObject_GetIter(value);
+        PyObject *it = PyObject_GetIter(value);
         if (it == NULL)
             return 0;
 
@@ -146,7 +156,7 @@ validate_constant(PyObject *value)
                 break;
             }
 
-            if (!validate_constant(item)) {
+            if (!validate_constant(state, item)) {
                 Py_DECREF(it);
                 Py_DECREF(item);
                 return 0;
@@ -155,6 +165,7 @@ validate_constant(PyObject *value)
         }
 
         Py_DECREF(it);
+        --state->recursion_depth;
         return 1;
     }
 
@@ -167,8 +178,14 @@ validate_constant(PyObject *value)
 }
 
 static int
-validate_expr(expr_ty exp, expr_context_ty ctx)
+validate_expr(struct validator *state, expr_ty exp, expr_context_ty ctx)
 {
+    int ret;
+    if (++state->recursion_depth > state->recursion_limit) {
+        PyErr_SetString(PyExc_RecursionError,
+                        "maximum recursion depth exceeded during compilation");
+        return 0;
+    }
     int check_ctx = 1;
     expr_context_ty actual_ctx;
 
@@ -218,19 +235,24 @@ validate_expr(expr_ty exp, expr_context_ty ctx)
             PyErr_SetString(PyExc_ValueError, "BoolOp with less than 2 values");
             return 0;
         }
-        return validate_exprs(exp->v.BoolOp.values, Load, 0);
+        ret = validate_exprs(state, exp->v.BoolOp.values, Load, 0);
+        break;
     case BinOp_kind:
-        return validate_expr(exp->v.BinOp.left, Load) &&
-            validate_expr(exp->v.BinOp.right, Load);
+        ret = validate_expr(state, exp->v.BinOp.left, Load) &&
+            validate_expr(state, exp->v.BinOp.right, Load);
+        break;
     case UnaryOp_kind:
-        return validate_expr(exp->v.UnaryOp.operand, Load);
+        ret = validate_expr(state, exp->v.UnaryOp.operand, Load);
+        break;
     case Lambda_kind:
-        return validate_arguments(exp->v.Lambda.args) &&
-            validate_expr(exp->v.Lambda.body, Load);
+        ret = validate_arguments(state, exp->v.Lambda.args) &&
+            validate_expr(state, exp->v.Lambda.body, Load);
+        break;
     case IfExp_kind:
-        return validate_expr(exp->v.IfExp.test, Load) &&
-            validate_expr(exp->v.IfExp.body, Load) &&
-            validate_expr(exp->v.IfExp.orelse, Load);
+        ret = validate_expr(state, exp->v.IfExp.test, Load) &&
+            validate_expr(state, exp->v.IfExp.body, Load) &&
+            validate_expr(state, exp->v.IfExp.orelse, Load);
+        break;
     case Dict_kind:
         if (asdl_seq_LEN(exp->v.Dict.keys) != asdl_seq_LEN(exp->v.Dict.values)) {
             PyErr_SetString(PyExc_ValueError,
@@ -239,28 +261,35 @@ validate_expr(expr_ty exp, expr_context_ty ctx)
         }
         /* null_ok=1 for keys expressions to allow dict unpacking to work in
            dict literals, i.e. ``{**{a:b}}`` */
-        return validate_exprs(exp->v.Dict.keys, Load, /*null_ok=*/ 1) &&
-            validate_exprs(exp->v.Dict.values, Load, /*null_ok=*/ 0);
+        ret = validate_exprs(state, exp->v.Dict.keys, Load, /*null_ok=*/ 1) &&
+            validate_exprs(state, exp->v.Dict.values, Load, /*null_ok=*/ 0);
+        break;
     case Set_kind:
-        return validate_exprs(exp->v.Set.elts, Load, 0);
+        ret = validate_exprs(state, exp->v.Set.elts, Load, 0);
+        break;
 #define COMP(NAME) \
         case NAME ## _kind: \
-            return validate_comprehension(exp->v.NAME.generators) && \
-                validate_expr(exp->v.NAME.elt, Load);
+            ret = validate_comprehension(state, exp->v.NAME.generators) && \
+                validate_expr(state, exp->v.NAME.elt, Load); \
+            break;
     COMP(ListComp)
     COMP(SetComp)
     COMP(GeneratorExp)
 #undef COMP
     case DictComp_kind:
-        return validate_comprehension(exp->v.DictComp.generators) &&
-            validate_expr(exp->v.DictComp.key, Load) &&
-            validate_expr(exp->v.DictComp.value, Load);
+        ret = validate_comprehension(state, exp->v.DictComp.generators) &&
+            validate_expr(state, exp->v.DictComp.key, Load) &&
+            validate_expr(state, exp->v.DictComp.value, Load);
+        break;
     case Yield_kind:
-        return !exp->v.Yield.value || validate_expr(exp->v.Yield.value, Load);
+        ret = !exp->v.Yield.value || validate_expr(state, exp->v.Yield.value, Load);
+        break;
     case YieldFrom_kind:
-        return validate_expr(exp->v.YieldFrom.value, Load);
+        ret = validate_expr(state, exp->v.YieldFrom.value, Load);
+        break;
     case Await_kind:
-        return validate_expr(exp->v.Await.value, Load);
+        ret = validate_expr(state, exp->v.Await.value, Load);
+        break;
     case Compare_kind:
         if (!asdl_seq_LEN(exp->v.Compare.comparators)) {
             PyErr_SetString(PyExc_ValueError, "Compare with no comparators");
@@ -272,42 +301,56 @@ validate_expr(expr_ty exp, expr_context_ty ctx)
                             "of comparators and operands");
             return 0;
         }
-        return validate_exprs(exp->v.Compare.comparators, Load, 0) &&
-            validate_expr(exp->v.Compare.left, Load);
+        ret = validate_exprs(state, exp->v.Compare.comparators, Load, 0) &&
+            validate_expr(state, exp->v.Compare.left, Load);
+        break;
     case Call_kind:
-        return validate_expr(exp->v.Call.func, Load) &&
-            validate_exprs(exp->v.Call.args, Load, 0) &&
-            validate_keywords(exp->v.Call.keywords);
+        ret = validate_expr(state, exp->v.Call.func, Load) &&
+            validate_exprs(state, exp->v.Call.args, Load, 0) &&
+            validate_keywords(state, exp->v.Call.keywords);
+        break;
     case Constant_kind:
-        if (!validate_constant(exp->v.Constant.value)) {
+        if (!validate_constant(state, exp->v.Constant.value)) {
             return 0;
         }
-        return 1;
+        ret = 1;
+        break;
     case JoinedStr_kind:
-        return validate_exprs(exp->v.JoinedStr.values, Load, 0);
+        ret = validate_exprs(state, exp->v.JoinedStr.values, Load, 0);
+        break;
     case FormattedValue_kind:
-        if (validate_expr(exp->v.FormattedValue.value, Load) == 0)
+        if (validate_expr(state, exp->v.FormattedValue.value, Load) == 0)
             return 0;
-        if (exp->v.FormattedValue.format_spec)
-            return validate_expr(exp->v.FormattedValue.format_spec, Load);
-        return 1;
+        if (exp->v.FormattedValue.format_spec) {
+            ret = validate_expr(state, exp->v.FormattedValue.format_spec, Load);
+            break;
+        }
+        ret = 1;
+        break;
     case Attribute_kind:
-        return validate_expr(exp->v.Attribute.value, Load);
+        ret = validate_expr(state, exp->v.Attribute.value, Load);
+        break;
     case Subscript_kind:
-        return validate_expr(exp->v.Subscript.slice, Load) &&
-            validate_expr(exp->v.Subscript.value, Load);
+        ret = validate_expr(state, exp->v.Subscript.slice, Load) &&
+            validate_expr(state, exp->v.Subscript.value, Load);
+        break;
     case Starred_kind:
-        return validate_expr(exp->v.Starred.value, ctx);
+        ret = validate_expr(state, exp->v.Starred.value, ctx);
+        break;
     case Slice_kind:
-        return (!exp->v.Slice.lower || validate_expr(exp->v.Slice.lower, Load)) &&
-            (!exp->v.Slice.upper || validate_expr(exp->v.Slice.upper, Load)) &&
-            (!exp->v.Slice.step || validate_expr(exp->v.Slice.step, Load));
+        ret = (!exp->v.Slice.lower || validate_expr(state, exp->v.Slice.lower, Load)) &&
+            (!exp->v.Slice.upper || validate_expr(state, exp->v.Slice.upper, Load)) &&
+            (!exp->v.Slice.step || validate_expr(state, exp->v.Slice.step, Load));
+        break;
     case List_kind:
-        return validate_exprs(exp->v.List.elts, ctx, 0);
+        ret = validate_exprs(state, exp->v.List.elts, ctx, 0);
+        break;
     case Tuple_kind:
-        return validate_exprs(exp->v.Tuple.elts, ctx, 0);
+        ret = validate_exprs(state, exp->v.Tuple.elts, ctx, 0);
+        break;
     case NamedExpr_kind:
-        return validate_expr(exp->v.NamedExpr.value, Load);
+        ret = validate_expr(state, exp->v.NamedExpr.value, Load);
+        break;
     case MatchAs_kind:
         PyErr_SetString(PyExc_ValueError,
                         "MatchAs is only valid in match_case patterns");
@@ -318,10 +361,14 @@ validate_expr(expr_ty exp, expr_context_ty ctx)
         return 0;
     /* This last case doesn't have any checking. */
     case Name_kind:
-        return 1;
+        ret = 1;
+        break;
+    default:
+        PyErr_SetString(PyExc_SystemError, "unexpected expression");
+        return 0;
     }
-    PyErr_SetString(PyExc_SystemError, "unexpected expression");
-    return 0;
+    state->recursion_depth--;
+    return ret;
 }
 
 static int
@@ -342,44 +389,56 @@ _validate_nonempty_seq(asdl_seq *seq, const char *what, const char *owner)
 #define validate_nonempty_seq(seq, what, owner) _validate_nonempty_seq((asdl_seq*)seq, what, owner)
 
 static int
-validate_assignlist(asdl_expr_seq *targets, expr_context_ty ctx)
+validate_assignlist(struct validator *state, asdl_expr_seq *targets, expr_context_ty ctx)
 {
     return validate_nonempty_seq(targets, "targets", ctx == Del ? "Delete" : "Assign") &&
-        validate_exprs(targets, ctx, 0);
+        validate_exprs(state, targets, ctx, 0);
 }
 
 static int
-validate_body(asdl_stmt_seq *body, const char *owner)
+validate_body(struct validator *state, asdl_stmt_seq *body, const char *owner)
 {
-    return validate_nonempty_seq(body, "body", owner) && validate_stmts(body);
+    return validate_nonempty_seq(body, "body", owner) && validate_stmts(state, body);
 }
 
 static int
-validate_stmt(stmt_ty stmt)
+validate_stmt(struct validator *state, stmt_ty stmt)
 {
+    int ret;
     Py_ssize_t i;
+    if (++state->recursion_depth > state->recursion_limit) {
+        PyErr_SetString(PyExc_RecursionError,
+                        "maximum recursion depth exceeded during compilation");
+        return 0;
+    }
     switch (stmt->kind) {
     case FunctionDef_kind:
-        return validate_body(stmt->v.FunctionDef.body, "FunctionDef") &&
-            validate_arguments(stmt->v.FunctionDef.args) &&
-            validate_exprs(stmt->v.FunctionDef.decorator_list, Load, 0) &&
+        ret = validate_body(state, stmt->v.FunctionDef.body, "FunctionDef") &&
+            validate_arguments(state, stmt->v.FunctionDef.args) &&
+            validate_exprs(state, stmt->v.FunctionDef.decorator_list, Load, 0) &&
             (!stmt->v.FunctionDef.returns ||
-             validate_expr(stmt->v.FunctionDef.returns, Load));
+             validate_expr(state, stmt->v.FunctionDef.returns, Load));
+        break;
     case ClassDef_kind:
-        return validate_body(stmt->v.ClassDef.body, "ClassDef") &&
-            validate_exprs(stmt->v.ClassDef.bases, Load, 0) &&
-            validate_keywords(stmt->v.ClassDef.keywords) &&
-            validate_exprs(stmt->v.ClassDef.decorator_list, Load, 0);
+        ret = validate_body(state, stmt->v.ClassDef.body, "ClassDef") &&
+            validate_exprs(state, stmt->v.ClassDef.bases, Load, 0) &&
+            validate_keywords(state, stmt->v.ClassDef.keywords) &&
+            validate_exprs(state, stmt->v.ClassDef.decorator_list, Load, 0);
+        break;
     case Return_kind:
-        return !stmt->v.Return.value || validate_expr(stmt->v.Return.value, Load);
+        ret = !stmt->v.Return.value || validate_expr(state, stmt->v.Return.value, Load);
+        break;
     case Delete_kind:
-        return validate_assignlist(stmt->v.Delete.targets, Del);
+        ret = validate_assignlist(state, stmt->v.Delete.targets, Del);
+        break;
     case Assign_kind:
-        return validate_assignlist(stmt->v.Assign.targets, Store) &&
-            validate_expr(stmt->v.Assign.value, Load);
+        ret = validate_assignlist(state, stmt->v.Assign.targets, Store) &&
+            validate_expr(state, stmt->v.Assign.value, Load);
+        break;
     case AugAssign_kind:
-        return validate_expr(stmt->v.AugAssign.target, Store) &&
-            validate_expr(stmt->v.AugAssign.value, Load);
+        ret = validate_expr(state, stmt->v.AugAssign.target, Store) &&
+            validate_expr(state, stmt->v.AugAssign.value, Load);
+        break;
     case AnnAssign_kind:
         if (stmt->v.AnnAssign.target->kind != Name_kind &&
             stmt->v.AnnAssign.simple) {
@@ -387,74 +446,84 @@ validate_stmt(stmt_ty stmt)
                             "AnnAssign with simple non-Name target");
             return 0;
         }
-        return validate_expr(stmt->v.AnnAssign.target, Store) &&
+        ret = validate_expr(state, stmt->v.AnnAssign.target, Store) &&
                (!stmt->v.AnnAssign.value ||
-                validate_expr(stmt->v.AnnAssign.value, Load)) &&
-               validate_expr(stmt->v.AnnAssign.annotation, Load);
+                validate_expr(state, stmt->v.AnnAssign.value, Load)) &&
+               validate_expr(state, stmt->v.AnnAssign.annotation, Load);
+        break;
     case For_kind:
-        return validate_expr(stmt->v.For.target, Store) &&
-            validate_expr(stmt->v.For.iter, Load) &&
-            validate_body(stmt->v.For.body, "For") &&
-            validate_stmts(stmt->v.For.orelse);
+        ret = validate_expr(state, stmt->v.For.target, Store) &&
+            validate_expr(state, stmt->v.For.iter, Load) &&
+            validate_body(state, stmt->v.For.body, "For") &&
+            validate_stmts(state, stmt->v.For.orelse);
+        break;
     case AsyncFor_kind:
-        return validate_expr(stmt->v.AsyncFor.target, Store) &&
-            validate_expr(stmt->v.AsyncFor.iter, Load) &&
-            validate_body(stmt->v.AsyncFor.body, "AsyncFor") &&
-            validate_stmts(stmt->v.AsyncFor.orelse);
+        ret = validate_expr(state, stmt->v.AsyncFor.target, Store) &&
+            validate_expr(state, stmt->v.AsyncFor.iter, Load) &&
+            validate_body(state, stmt->v.AsyncFor.body, "AsyncFor") &&
+            validate_stmts(state, stmt->v.AsyncFor.orelse);
+        break;
     case While_kind:
-        return validate_expr(stmt->v.While.test, Load) &&
-            validate_body(stmt->v.While.body, "While") &&
-            validate_stmts(stmt->v.While.orelse);
+        ret = validate_expr(state, stmt->v.While.test, Load) &&
+            validate_body(state, stmt->v.While.body, "While") &&
+            validate_stmts(state, stmt->v.While.orelse);
+        break;
     case If_kind:
-        return validate_expr(stmt->v.If.test, Load) &&
-            validate_body(stmt->v.If.body, "If") &&
-            validate_stmts(stmt->v.If.orelse);
+        ret = validate_expr(state, stmt->v.If.test, Load) &&
+            validate_body(state, stmt->v.If.body, "If") &&
+            validate_stmts(state, stmt->v.If.orelse);
+        break;
     case With_kind:
         if (!validate_nonempty_seq(stmt->v.With.items, "items", "With"))
             return 0;
         for (i = 0; i < asdl_seq_LEN(stmt->v.With.items); i++) {
             withitem_ty item = asdl_seq_GET(stmt->v.With.items, i);
-            if (!validate_expr(item->context_expr, Load) ||
-                (item->optional_vars && !validate_expr(item->optional_vars, Store)))
+            if (!validate_expr(state, item->context_expr, Load) ||
+                (item->optional_vars && !validate_expr(state, item->optional_vars, Store)))
                 return 0;
         }
-        return validate_body(stmt->v.With.body, "With");
+        ret = validate_body(state, stmt->v.With.body, "With");
+        break;
     case AsyncWith_kind:
         if (!validate_nonempty_seq(stmt->v.AsyncWith.items, "items", "AsyncWith"))
             return 0;
         for (i = 0; i < asdl_seq_LEN(stmt->v.AsyncWith.items); i++) {
             withitem_ty item = asdl_seq_GET(stmt->v.AsyncWith.items, i);
-            if (!validate_expr(item->context_expr, Load) ||
-                (item->optional_vars && !validate_expr(item->optional_vars, Store)))
+            if (!validate_expr(state, item->context_expr, Load) ||
+                (item->optional_vars && !validate_expr(state, item->optional_vars, Store)))
                 return 0;
         }
-        return validate_body(stmt->v.AsyncWith.body, "AsyncWith");
+        ret = validate_body(state, stmt->v.AsyncWith.body, "AsyncWith");
+        break;
     case Match_kind:
-        if (!validate_expr(stmt->v.Match.subject, Load)
+        if (!validate_expr(state, stmt->v.Match.subject, Load)
             || !validate_nonempty_seq(stmt->v.Match.cases, "cases", "Match")) {
             return 0;
         }
         for (i = 0; i < asdl_seq_LEN(stmt->v.Match.cases); i++) {
             match_case_ty m = asdl_seq_GET(stmt->v.Match.cases, i);
             if (!validate_pattern(m->pattern)
-                || (m->guard && !validate_expr(m->guard, Load))
-                || !validate_body(m->body, "match_case")) {
+                || (m->guard && !validate_expr(state, m->guard, Load))
+                || !validate_body(state, m->body, "match_case")) {
                 return 0;
             }
         }
-        return 1;
+        ret = 1;
+        break;
     case Raise_kind:
         if (stmt->v.Raise.exc) {
-            return validate_expr(stmt->v.Raise.exc, Load) &&
-                (!stmt->v.Raise.cause || validate_expr(stmt->v.Raise.cause, Load));
+            ret = validate_expr(state, stmt->v.Raise.exc, Load) &&
+                (!stmt->v.Raise.cause || validate_expr(state, stmt->v.Raise.cause, Load));
+            break;
         }
         if (stmt->v.Raise.cause) {
             PyErr_SetString(PyExc_ValueError, "Raise with cause but no exception");
             return 0;
         }
-        return 1;
+        ret = 1;
+        break;
     case Try_kind:
-        if (!validate_body(stmt->v.Try.body, "Try"))
+        if (!validate_body(state, stmt->v.Try.body, "Try"))
             return 0;
         if (!asdl_seq_LEN(stmt->v.Try.handlers) &&
             !asdl_seq_LEN(stmt->v.Try.finalbody)) {
@@ -469,55 +538,66 @@ validate_stmt(stmt_ty stmt)
         for (i = 0; i < asdl_seq_LEN(stmt->v.Try.handlers); i++) {
             excepthandler_ty handler = asdl_seq_GET(stmt->v.Try.handlers, i);
             if ((handler->v.ExceptHandler.type &&
-                 !validate_expr(handler->v.ExceptHandler.type, Load)) ||
-                !validate_body(handler->v.ExceptHandler.body, "ExceptHandler"))
+                 !validate_expr(state, handler->v.ExceptHandler.type, Load)) ||
+                !validate_body(state, handler->v.ExceptHandler.body, "ExceptHandler"))
                 return 0;
         }
-        return (!asdl_seq_LEN(stmt->v.Try.finalbody) ||
-                validate_stmts(stmt->v.Try.finalbody)) &&
+        ret = (!asdl_seq_LEN(stmt->v.Try.finalbody) ||
+                validate_stmts(state, stmt->v.Try.finalbody)) &&
             (!asdl_seq_LEN(stmt->v.Try.orelse) ||
-             validate_stmts(stmt->v.Try.orelse));
+             validate_stmts(state, stmt->v.Try.orelse));
+        break;
     case Assert_kind:
-        return validate_expr(stmt->v.Assert.test, Load) &&
-            (!stmt->v.Assert.msg || validate_expr(stmt->v.Assert.msg, Load));
+        ret = validate_expr(state, stmt->v.Assert.test, Load) &&
+            (!stmt->v.Assert.msg || validate_expr(state, stmt->v.Assert.msg, Load));
+        break;
     case Import_kind:
-        return validate_nonempty_seq(stmt->v.Import.names, "names", "Import");
+        ret = validate_nonempty_seq(stmt->v.Import.names, "names", "Import");
+        break;
     case ImportFrom_kind:
         if (stmt->v.ImportFrom.level < 0) {
             PyErr_SetString(PyExc_ValueError, "Negative ImportFrom level");
             return 0;
         }
-        return validate_nonempty_seq(stmt->v.ImportFrom.names, "names", "ImportFrom");
+        ret = validate_nonempty_seq(stmt->v.ImportFrom.names, "names", "ImportFrom");
+        break;
     case Global_kind:
-        return validate_nonempty_seq(stmt->v.Global.names, "names", "Global");
+        ret = validate_nonempty_seq(stmt->v.Global.names, "names", "Global");
+        break;
     case Nonlocal_kind:
-        return validate_nonempty_seq(stmt->v.Nonlocal.names, "names", "Nonlocal");
+        ret = validate_nonempty_seq(stmt->v.Nonlocal.names, "names", "Nonlocal");
+        break;
     case Expr_kind:
-        return validate_expr(stmt->v.Expr.value, Load);
+        ret = validate_expr(state, stmt->v.Expr.value, Load);
+        break;
     case AsyncFunctionDef_kind:
-        return validate_body(stmt->v.AsyncFunctionDef.body, "AsyncFunctionDef") &&
-            validate_arguments(stmt->v.AsyncFunctionDef.args) &&
-            validate_exprs(stmt->v.AsyncFunctionDef.decorator_list, Load, 0) &&
+        ret = validate_body(state, stmt->v.AsyncFunctionDef.body, "AsyncFunctionDef") &&
+            validate_arguments(state, stmt->v.AsyncFunctionDef.args) &&
+            validate_exprs(state, stmt->v.AsyncFunctionDef.decorator_list, Load, 0) &&
             (!stmt->v.AsyncFunctionDef.returns ||
-             validate_expr(stmt->v.AsyncFunctionDef.returns, Load));
+             validate_expr(state, stmt->v.AsyncFunctionDef.returns, Load));
+        break;
     case Pass_kind:
     case Break_kind:
     case Continue_kind:
-        return 1;
+        ret = 1;
+        break;
     default:
         PyErr_SetString(PyExc_SystemError, "unexpected statement");
         return 0;
     }
+    state->recursion_depth--;
+    return ret;
 }
 
 static int
-validate_stmts(asdl_stmt_seq *seq)
+validate_stmts(struct validator *state, asdl_stmt_seq *seq)
 {
     Py_ssize_t i;
     for (i = 0; i < asdl_seq_LEN(seq); i++) {
         stmt_ty stmt = asdl_seq_GET(seq, i);
         if (stmt) {
-            if (!validate_stmt(stmt))
+            if (!validate_stmt(state, stmt))
                 return 0;
         }
         else {
@@ -530,13 +610,13 @@ validate_stmts(asdl_stmt_seq *seq)
 }
 
 static int
-validate_exprs(asdl_expr_seq *exprs, expr_context_ty ctx, int null_ok)
+validate_exprs(struct validator *state, asdl_expr_seq *exprs, expr_context_ty ctx, int null_ok)
 {
     Py_ssize_t i;
     for (i = 0; i < asdl_seq_LEN(exprs); i++) {
         expr_ty expr = asdl_seq_GET(exprs, i);
         if (expr) {
-            if (!validate_expr(expr, ctx))
+            if (!validate_expr(state, expr, ctx))
                 return 0;
         }
         else if (!null_ok) {
@@ -549,26 +629,53 @@ validate_exprs(asdl_expr_seq *exprs, expr_context_ty ctx, int null_ok)
     return 1;
 }
 
+/* See comments in symtable.c. */
+#define COMPILER_STACK_FRAME_SCALE 3
+
 int
 _PyAST_Validate(mod_ty mod)
 {
     int res = 0;
+    struct validator state;
+    PyThreadState *tstate;
+    int recursion_limit = Py_GetRecursionLimit();
+    int starting_recursion_depth;
+
+    /* Setup recursion depth check counters */
+    tstate = _PyThreadState_GET();
+    if (!tstate) {
+        return 0;
+    }
+    /* Be careful here to prevent overflow. */
+    starting_recursion_depth = (tstate->recursion_depth < INT_MAX / COMPILER_STACK_FRAME_SCALE) ?
+        tstate->recursion_depth * COMPILER_STACK_FRAME_SCALE : tstate->recursion_depth;
+    state.recursion_depth = starting_recursion_depth;
+    state.recursion_limit = (recursion_limit < INT_MAX / COMPILER_STACK_FRAME_SCALE) ?
+        recursion_limit * COMPILER_STACK_FRAME_SCALE : recursion_limit;
 
     switch (mod->kind) {
     case Module_kind:
-        res = validate_stmts(mod->v.Module.body);
+        res = validate_stmts(&state, mod->v.Module.body);
         break;
     case Interactive_kind:
-        res = validate_stmts(mod->v.Interactive.body);
+        res = validate_stmts(&state, mod->v.Interactive.body);
         break;
     case Expression_kind:
-        res = validate_expr(mod->v.Expression.body, Load);
+        res = validate_expr(&state, mod->v.Expression.body, Load);
         break;
     default:
         PyErr_SetString(PyExc_SystemError, "impossible module node");
         res = 0;
         break;
     }
+
+    /* Check that the recursion depth counting balanced correctly */
+    if (res && state.recursion_depth != starting_recursion_depth) {
+        PyErr_Format(PyExc_SystemError,
+            "AST validator recursion depth mismatch (before=%d, after=%d)",
+            starting_recursion_depth, state.recursion_depth);
+        return 0;
+    }
     return res;
 }
 
diff --git a/Python/ast_opt.c b/Python/ast_opt.c
index dea20da..6eb514e 100644
--- a/Python/ast_opt.c
+++ b/Python/ast_opt.c
@@ -2,6 +2,7 @@
 #include "Python.h"
 #include "pycore_ast.h"           // _PyAST_GetDocString()
 #include "pycore_compile.h"       // _PyASTOptimizeState
+#include "pycore_pystate.h"       // _PyThreadState_GET()
 
 
 static int
@@ -488,6 +489,11 @@ astfold_mod(mod_ty node_, PyArena *ctx_, _PyASTOptimizeState *state)
 static int
 astfold_expr(expr_ty node_, PyArena *ctx_, _PyASTOptimizeState *state)
 {
+    if (++state->recursion_depth > state->recursion_limit) {
+        PyErr_SetString(PyExc_RecursionError,
+                        "maximum recursion depth exceeded during compilation");
+        return 0;
+    }
     switch (node_->kind) {
     case BoolOp_kind:
         CALL_SEQ(astfold_expr, expr, node_->v.BoolOp.values);
@@ -586,6 +592,7 @@ astfold_expr(expr_ty node_, PyArena *ctx_, _PyASTOptimizeState *state)
     case Name_kind:
         if (node_->v.Name.ctx == Load &&
                 _PyUnicode_EqualToASCIIString(node_->v.Name.id, "__debug__")) {
+            state->recursion_depth--;
             return make_const(node_, PyBool_FromLong(!state->optimize), ctx_);
         }
         break;
@@ -602,6 +609,7 @@ astfold_expr(expr_ty node_, PyArena *ctx_, _PyASTOptimizeState *state)
     // No default case, so the compiler will emit a warning if new expression
     // kinds are added without being handled here
     }
+    state->recursion_depth--;
     return 1;
 }
 
@@ -648,6 +656,11 @@ astfold_arg(arg_ty node_, PyArena *ctx_, _PyASTOptimizeState *state)
 static int
 astfold_stmt(stmt_ty node_, PyArena *ctx_, _PyASTOptimizeState *state)
 {
+    if (++state->recursion_depth > state->recursion_limit) {
+        PyErr_SetString(PyExc_RecursionError,
+                        "maximum recursion depth exceeded during compilation");
+        return 0;
+    }
     switch (node_->kind) {
     case FunctionDef_kind:
         CALL(astfold_arguments, arguments_ty, node_->v.FunctionDef.args);
@@ -757,6 +770,7 @@ astfold_stmt(stmt_ty node_, PyArena *ctx_, _PyASTOptimizeState *state)
     // No default case, so the compiler will emit a warning if new statement
     // kinds are added without being handled here
     }
+    state->recursion_depth--;
     return 1;
 }
 
@@ -906,10 +920,38 @@ astfold_match_case(match_case_ty node_, PyArena *ctx_, _PyASTOptimizeState *stat
 #undef CALL_SEQ
 #undef CALL_INT_SEQ
 
+/* See comments in symtable.c. */
+#define COMPILER_STACK_FRAME_SCALE 3
+
 int
 _PyAST_Optimize(mod_ty mod, PyArena *arena, _PyASTOptimizeState *state)
 {
+    PyThreadState *tstate;
+    int recursion_limit = Py_GetRecursionLimit();
+    int starting_recursion_depth;
+
+    /* Setup recursion depth check counters */
+    tstate = _PyThreadState_GET();
+    if (!tstate) {
+        return 0;
+    }
+    /* Be careful here to prevent overflow. */
+    starting_recursion_depth = (tstate->recursion_depth < INT_MAX / COMPILER_STACK_FRAME_SCALE) ?
+        tstate->recursion_depth * COMPILER_STACK_FRAME_SCALE : tstate->recursion_depth;
+    state->recursion_depth = starting_recursion_depth;
+    state->recursion_limit = (recursion_limit < INT_MAX / COMPILER_STACK_FRAME_SCALE) ?
+        recursion_limit * COMPILER_STACK_FRAME_SCALE : recursion_limit;
+
     int ret = astfold_mod(mod, arena, state);
     assert(ret || PyErr_Occurred());
+
+    /* Check that the recursion depth counting balanced correctly */
+    if (ret && state->recursion_depth != starting_recursion_depth) {
+        PyErr_Format(PyExc_SystemError,
+            "AST optimizer recursion depth mismatch (before=%d, after=%d)",
+            starting_recursion_depth, state->recursion_depth);
+        return 0;
+    }
+
     return ret;
 }