Currently, when typeck discovers that a return statement will refer to a
local variable it rewrites to return a null pointer. This causes the
error messages for using the return value in a constant expression to be
unhelpful, especially for reference return values, and is also a visible
change to otherwise valid code (as in the linked PR).
The transformation is nonetheless important, however, both as a safety
guard against attackers being able to gain a handle to other data on the
stack, and to prevent duplicate warnings from later null-dereference
warning passes.
As such, this patch just delays the transformation until cp_genericize,
after constexpr function definitions have been generated.
PR c++/110619
gcc/cp/ChangeLog:
* cp-gimplify.cc (cp_genericize_r): Transform RETURN_EXPRs to
not return dangling pointers.
* cp-tree.h (RETURN_EXPR_LOCAL_ADDR_P): New flag.
(check_return_expr): Add a new parameter.
* semantics.cc (finish_return_stmt): Set flag on RETURN_EXPR
when referring to dangling pointer.
* typeck.cc (check_return_expr): Disable transformation of
dangling pointers, instead pass this information to caller.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1y/constexpr-110619.C: New test.
Signed-off-by: Nathaniel Shead <nathanielosh...@gmail.com>
---
gcc/cp/cp-gimplify.cc | 23 ++++++++++++++++---
gcc/cp/cp-tree.h | 8 ++++++-
gcc/cp/semantics.cc | 4 +++-
gcc/cp/typeck.cc | 9 ++++----
gcc/testsuite/g++.dg/cpp1y/constexpr-110619.C | 10 ++++++++
5 files changed, 45 insertions(+), 9 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-110619.C
diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index f5734197774..0a5d6300aca 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -1336,9 +1336,26 @@ cp_genericize_r (tree *stmt_p, int *walk_subtrees, void
*data)
break;
case RETURN_EXPR:
- if (TREE_OPERAND (stmt, 0) && is_invisiref_parm (TREE_OPERAND (stmt, 0)))
- /* Don't dereference an invisiref RESULT_DECL inside a RETURN_EXPR. */
- *walk_subtrees = 0;
+ if (TREE_OPERAND (stmt, 0))
+ {
+ if (is_invisiref_parm (TREE_OPERAND (stmt, 0)))
+ /* Don't dereference an invisiref RESULT_DECL inside a RETURN_EXPR.
*/
+ *walk_subtrees = 0;
+ if (RETURN_EXPR_LOCAL_ADDR_P (stmt))
+ {
+ /* Don't return the address of a local variable. */
+ tree *p = &TREE_OPERAND (stmt, 0);
+ while (TREE_CODE (*p) == COMPOUND_EXPR)
+ p = &TREE_OPERAND (*p, 0);
+ if (TREE_CODE (*p) == INIT_EXPR)
+ {
+ tree op = TREE_OPERAND (*p, 1);
+ tree new_op = build2 (COMPOUND_EXPR, TREE_TYPE (op), op,
+ build_zero_cst (TREE_TYPE (op)));
+ TREE_OPERAND (*p, 1) = new_op;
+ }
+ }
+ }
break;
case OMP_CLAUSE:
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 3de0e154c12..e0c181d9aef 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -447,6 +447,7 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
INIT_EXPR_NRV_P (in INIT_EXPR)
ATOMIC_CONSTR_MAP_INSTANTIATED_P (in ATOMIC_CONSTR)
contract_semantic (in ASSERTION_, PRECONDITION_, POSTCONDITION_STMT)
+ RETURN_EXPR_LOCAL_ADDR_P (in RETURN_EXPR)
1: IDENTIFIER_KIND_BIT_1 (in IDENTIFIER_NODE)
TI_PENDING_TEMPLATE_FLAG.
TEMPLATE_PARMS_FOR_INLINE.
@@ -4071,6 +4072,11 @@ struct GTY(()) lang_decl {
(LANG_DECL_FN_CHECK (FUNCTION_DECL_CHECK (NODE)) \
->u.saved_auto_return_type)
+/* In a RETURN_EXPR, whether the expression refers to the address
+ of a local variable. */
+#define RETURN_EXPR_LOCAL_ADDR_P(NODE) \
+ TREE_LANG_FLAG_0 (RETURN_EXPR_CHECK (NODE))
+
/* True if NODE is an implicit INDIRECT_REF from convert_from_reference. */
#define REFERENCE_REF_P(NODE) \
(INDIRECT_REF_P (NODE) \
@@ -8139,7 +8145,7 @@ extern tree composite_pointer_type (const
op_location_t &,
tsubst_flags_t);
extern tree merge_types (tree, tree);
extern tree strip_array_domain (tree);
-extern tree check_return_expr (tree, bool *);
+extern tree check_return_expr (tree, bool *, bool *);
extern tree spaceship_type (tree, tsubst_flags_t =
tf_warning_or_error);
extern tree genericize_spaceship (location_t, tree, tree, tree);
extern tree cp_build_binary_op (const op_location_t &,
diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 8fb47fd179e..720521b7f1a 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -1240,8 +1240,9 @@ finish_return_stmt (tree expr)
{
tree r;
bool no_warning;
+ bool dangling;
- expr = check_return_expr (expr, &no_warning);
+ expr = check_return_expr (expr, &no_warning, &dangling);
if (error_operand_p (expr)
|| (flag_openmp && !check_omp_return ()))
@@ -1259,6 +1260,7 @@ finish_return_stmt (tree expr)
}
r = build_stmt (input_location, RETURN_EXPR, expr);
+ RETURN_EXPR_LOCAL_ADDR_P (r) = dangling;
if (no_warning)
suppress_warning (r, OPT_Wreturn_type);
r = maybe_cleanup_point_expr_void (r);
diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 859b133a18d..d5c0c85ed51 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -10920,10 +10920,11 @@ maybe_warn_pessimizing_move (tree expr, tree type,
bool return_p)
change RETVAL into the function return type, and to assign it to
the DECL_RESULT for the function. Set *NO_WARNING to true if
code reaches end of non-void function warning shouldn't be issued
- on this RETURN_EXPR. */
+ on this RETURN_EXPR. Set *DANGLING to true if code returns the
+ address of a local variable. */
tree
-check_return_expr (tree retval, bool *no_warning)
+check_return_expr (tree retval, bool *no_warning, bool *dangling)
{
tree result;
/* The type actually returned by the function. */
@@ -10935,6 +10936,7 @@ check_return_expr (tree retval, bool *no_warning)
location_t loc = cp_expr_loc_or_input_loc (retval);
*no_warning = false;
+ *dangling = false;
/* A `volatile' function is one that isn't supposed to return, ever.
(This is a G++ extension, used to get better code for functions
@@ -11273,8 +11275,7 @@ check_return_expr (tree retval, bool *no_warning)
else if (!processing_template_decl
&& maybe_warn_about_returning_address_of_local (retval, loc)
&& INDIRECT_TYPE_P (valtype))
- retval = build2 (COMPOUND_EXPR, TREE_TYPE (retval), retval,
- build_zero_cst (TREE_TYPE (retval)));
+ *dangling = true;
}
/* A naive attempt to reduce the number of -Wdangling-reference false
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-110619.C
b/gcc/testsuite/g++.dg/cpp1y/constexpr-110619.C
new file mode 100644
index 00000000000..cca13302238
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-110619.C
@@ -0,0 +1,10 @@
+// { dg-do compile { target c++14 } }
+// { dg-options "-Wno-return-local-addr" }
+// PR c++/110619
+
+constexpr auto f() {
+ int i = 0;
+ return &i;
+};
+
+static_assert( f() != nullptr );
--
2.41.0
