My recent patch for -Winit-list-lifetime added a warning for returning
a temporary initializer_list, but the bug report is correct that we
*could* promote the constant array to .rodata as an optimization, did
in GCC 7, and don't in GCC 8. That we did in GCC 7 was for the wrong
reason, because we were treating it as a C compound literal, but this
patch allows arbitrary constant aggregate temporaries to be allocated
to .rodata if appropriate.
Tested x86_64-pc-linux-gnu, applying to trunk.
commit 7f55bc360af52bd19996f030ad4c506dd153ff66
Author: Jason Merrill <ja...@redhat.com>
Date: Fri May 25 12:43:29 2018 -0400
PR c++/85873 - constant initializer_list array not in .rodata.
* tree.c (build_target_expr): Set TREE_READONLY.
* call.c (set_up_extended_ref_temp): Set TREE_READONLY.
diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 2bbf9837487..67e404d1cb2 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -10923,13 +10923,11 @@ set_up_extended_ref_temp (tree decl, tree expr, vec<tree, va_gc> **cleanups,
lvalue-rvalue conversion applied to "a glvalue of literal type
that refers to a non-volatile temporary object initialized
with a constant expression". Rather than try to communicate
- that this VAR_DECL is a temporary, just mark it constexpr.
-
- Currently this is only useful for initializer_list temporaries,
- since reference vars can't appear in constant expressions. */
+ that this VAR_DECL is a temporary, just mark it constexpr. */
DECL_DECLARED_CONSTEXPR_P (var) = true;
DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (var) = true;
TREE_CONSTANT (var) = true;
+ TREE_READONLY (var) = true;
}
DECL_INITIAL (var) = init;
init = NULL_TREE;
diff --git a/gcc/cp/tree.c b/gcc/cp/tree.c
index f21daaca1d0..53bc9c7176f 100644
--- a/gcc/cp/tree.c
+++ b/gcc/cp/tree.c
@@ -461,6 +461,14 @@ build_target_expr (tree decl, tree value, tsubst_flags_t complain)
|| useless_type_conversion_p (TREE_TYPE (decl),
TREE_TYPE (value)));
+ /* Set TREE_READONLY for optimization, such as gimplify_init_constructor
+ moving a constant aggregate into .rodata. */
+ if (CP_TYPE_CONST_NON_VOLATILE_P (type)
+ && !TYPE_HAS_NONTRIVIAL_DESTRUCTOR (type)
+ && !VOID_TYPE_P (TREE_TYPE (value))
+ && reduced_constant_expression_p (value))
+ TREE_READONLY (decl) = true;
+
if (complain & tf_no_cleanup)
/* The caller is building a new-expr and does not need a cleanup. */
t = NULL_TREE;
diff --git a/gcc/testsuite/g++.dg/tree-ssa/array-temp1.C b/gcc/testsuite/g++.dg/tree-ssa/array-temp1.C
new file mode 100644
index 00000000000..97c2e0521c9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/array-temp1.C
@@ -0,0 +1,21 @@
+// PR c++/85873
+// Test that these array temporaries are promoted to static variables as an
+// optimization.
+
+// { dg-do compile { target c++11 } }
+// { dg-additional-options -fdump-tree-gimple }
+// { dg-final { scan-tree-dump-not "= 42" "gimple" } }
+
+#include <initializer_list>
+
+int f()
+{
+ using AR = const int[];
+ return AR{ 1,42,3,4,5,6,7,8,9,0 }[5];
+}
+
+int g()
+{
+ std::initializer_list<int> a = {1,42,3};
+ return a.begin()[0];
+}
