PR c++/55513: do not fold away builtins in COMPOUND_EXPRs

[Aldy Hernandez]

The problem here is that we fold away calls to built-ins in 
COMPOUND_EXPRs such as:

const int t = (__builtin_memcpy (s, "Hello", 6), 777);
Since we return true for any built-in in 
potential_constant_expression_1, this means that 
check_initializer->store_init_value->maybe_constant_init will chop off 
the built-in side of a COMPOUND_EXPR when setting a DECL_INITIAL, 
transforming:

(COMPOUND_EXPR (CALL_EXPR built-in) INTEGER_CST)

into

INTEGER_CST

Fixed by setting `non_constant_p' if folding a built-in does not yield a 
constant.

Tested on x86-64 Linux.

OK for trunk?
commit 9775b837cf9ece71cbf9560c35c638b3f8c0a778
Author: Aldy Hernandez <al...@redhat.com>
Date:   Fri Dec 7 10:40:25 2012 -0600

        PR c++/55513
        * semantics.c (cxx_eval_builtin_function_call): Set non_constant_p
        after folding.

diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 491d97c..f487a61 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -6437,7 +6437,10 @@ cxx_eval_builtin_function_call (const constexpr_call 
*call, tree t,
     return t;
   new_call = build_call_array_loc (EXPR_LOCATION (t), TREE_TYPE (t),
                                    CALL_EXPR_FN (t), nargs, args);
-  return fold (new_call);
+  new_call = fold (new_call);
+  if (!TREE_CONSTANT (new_call))
+    *non_constant_p = true;
+  return new_call;
 }
 
 /* TEMP is the constant value of a temporary object of type TYPE.  Adjust
diff --git a/gcc/testsuite/g++.dg/pr55513.C b/gcc/testsuite/g++.dg/pr55513.C
new file mode 100644
index 0000000..06eedee
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr55513.C
@@ -0,0 +1,12 @@
+// { dg-do compile }
+// { dg-options "-O0 -fdump-tree-gimple" }
+
+main ()
+{
+  char s[10];
+  const int t = (__builtin_memcpy (s, "Hello", 6), 777);
+  __builtin_printf ("%d %s\n", t, s);
+}
+
+// { dg-final { scan-tree-dump-times "memcpy" 1 "gimple" } }
+// { dg-final { cleanup-tree-dump "gimple" } }