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C++ PATCH for c++/54021 (__builtin_constant_p and constexpr)
- From: Jason Merrill <jason at redhat dot com>
- To: gcc-patches List <gcc-patches at gcc dot gnu dot org>
- Date: Thu, 19 Jul 2012 16:01:44 -0400
- Subject: C++ PATCH for c++/54021 (__builtin_constant_p and constexpr)
When not optimizing, fold_builtin_1 assumes that __builtin_constant_p
isn't going to give any better answer than we can see at first folding.
But this isn't true in a constexpr function, for which the parameters
can later be replaced by constants. So I've taken Jakub's suggestion of
setting optimize across the call to fold in this case.
Tested x86_64-pc-linux-gnu, applying to trunk.
commit 0fe036307f9101c5157a680f816fc816895fe3ea
Author: Jason Merrill <jason@redhat.com>
Date: Thu Jul 19 09:47:04 2012 -0400
PR c++/54021
* call.c (build_cxx_call): Set optimize when folding
__builtin_constant_p in a constexpr function.
diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 5b3245f..cf50e88 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -6900,6 +6900,7 @@ tree
build_cxx_call (tree fn, int nargs, tree *argarray)
{
tree fndecl;
+ int optimize_sav;
/* Remember roughly where this call is. */
location_t loc = EXPR_LOC_OR_HERE (fn);
@@ -6916,8 +6917,15 @@ build_cxx_call (tree fn, int nargs, tree *argarray)
return error_mark_node;
/* Some built-in function calls will be evaluated at compile-time in
- fold (). */
+ fold (). Set optimize to 1 when folding __builtin_constant_p inside
+ a constexpr function so that fold_builtin_1 doesn't fold it to 0. */
+ optimize_sav = optimize;
+ if (!optimize && fndecl && DECL_IS_BUILTIN_CONSTANT_P (fndecl)
+ && current_function_decl
+ && DECL_DECLARED_CONSTEXPR_P (current_function_decl))
+ optimize = 1;
fn = fold_if_not_in_template (fn);
+ optimize = optimize_sav;
if (VOID_TYPE_P (TREE_TYPE (fn)))
return fn;
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-builtin2.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-builtin2.C
new file mode 100644
index 0000000..dde38f0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-builtin2.C
@@ -0,0 +1,16 @@
+// PR c++/54021
+// { dg-do compile { target c++11 } }
+
+extern int nonconst_func(int);
+constexpr int identity(int x) { return x; }
+constexpr int zero() { return identity(0); }
+constexpr int one() { return identity(1); }
+
+// These are the same. Only the latter is accepted, though.
+constexpr int rejected_const_4(int x)
+{ return __builtin_constant_p(x) ? 4 : nonconst_func(x); }
+constexpr int accepted_const_4(int x)
+{ return identity(__builtin_constant_p(x)) ? 4 : nonconst_func(x); }
+
+// This is rejected. I would like it to work.
+constexpr int four = accepted_const_4(1);