C++ PATCH for c++/68006 (broken value-initialization with -flifetime-dse)

[Jason Merrill]

My change to clobber the object contents on entry to a constructor was 
forgetting that for value-initialization of a class with an 
implicitly-defined default constructor, we do zero-initialization before 
calling the copy constructor, and that zero-initialization should not be 
clobbered.

Tested x86_64-pc-linux-gnu, applied to trunk.
commit 8f05ea76d518ab25685e8c33da630aaef39725b1
Author: Jason Merrill <ja...@redhat.com>
Date:   Sat Oct 17 23:48:12 2015 -1000

    	PR c++/68006
    
    	* decl.c (implicit_default_ctor_p): New.
    	(start_preparsed_function): Don't clobber on entry to one.

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 8eb9cc2..8036fb7 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -13630,6 +13630,16 @@ check_function_type (tree decl, tree current_function_parms)
     abstract_virtuals_error (decl, TREE_TYPE (fntype));
 }
 
+/* True iff FN is an implicitly-defined default constructor.  */
+
+static bool
+implicit_default_ctor_p (tree fn)
+{
+  return (DECL_CONSTRUCTOR_P (fn)
+	  && !user_provided_p (fn)
+	  && sufficient_parms_p (FUNCTION_FIRST_USER_PARMTYPE (fn)));
+}
+
 /* Create the FUNCTION_DECL for a function definition.
    DECLSPECS and DECLARATOR are the parts of the declaration;
    they describe the function's name and the type it returns,
@@ -14036,7 +14046,11 @@ start_preparsed_function (tree decl1, tree attrs, int flags)
   store_parm_decls (current_function_parms);
 
   if (!processing_template_decl
-      && flag_lifetime_dse && DECL_CONSTRUCTOR_P (decl1))
+      && flag_lifetime_dse && DECL_CONSTRUCTOR_P (decl1)
+      /* We can't clobber safely for an implicitly-defined default constructor
+	 because part of the initialization might happen before we enter the
+	 consructor, via AGGR_INIT_ZERO_FIRST (c++/68006).  */
+      && !implicit_default_ctor_p (decl1))
     {
       /* Insert a clobber to let the back end know that the object storage
 	 is dead when we enter the constructor.  */
diff --git a/gcc/testsuite/g++.dg/opt/flifetime-dse3.C b/gcc/testsuite/g++.dg/opt/flifetime-dse3.C
new file mode 100644
index 0000000..7a03acc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/flifetime-dse3.C
@@ -0,0 +1,22 @@
+// PR c++/68006
+// { dg-do run { target c++11 } }
+// { dg-options -O2 }
+
+inline void* operator new(__SIZE_TYPE__, void* ptr)
+{
+  return ptr;
+}
+
+struct X { int x; int y; int z = 42; };
+
+void test_bar(void* p)
+{
+  new(p) X{};   // Bad.
+}
+
+int main()
+{
+  int ar[3] = { 1,2,3 };
+  test_bar (ar);
+  return (ar[0] != 0 || ar[1] != 0 || ar[2] != 42);
+}