Add -flifetime-dse=1.

gcc/
	* common.opt (flifetime-dse): Add -flifetime-dse=1.
gcc/cp/
	* decl.c (start_preparsed_function): Condition ctor clobber on
	flag_lifetime_dse > 1.

From-SVN: r233672

gcc/ChangeLog

+2016-02-24  Jason Merrill  <jason@redhat.com>
+
+	* common.opt (flifetime-dse): Add -flifetime-dse=1.
+
 2016-02-24  Richard Biener  <rguenther@suse.de>
 	Jakub Jelinek  <jakub@redhat.com>
 

gcc/common.opt

@@ -1946,10 +1946,13 @@ Common Ignore
 Does nothing. Preserved for backward compatibility.
 
 flifetime-dse
-Common Report Var(flag_lifetime_dse) Init(1) Optimization
+Common Report Var(flag_lifetime_dse,2) Init(2) Optimization
 Tell DSE that the storage for a C++ object is dead when the constructor
 starts and when the destructor finishes.
 
+flifetime-dse=
+Common Joined RejectNegative UInteger Var(flag_lifetime_dse) Optimization
+
 flive-range-shrinkage
 Common Report Var(flag_live_range_shrinkage) Init(0) Optimization
 Relief of register pressure through live range shrinkage.

gcc/cp/ChangeLog

 2016-02-24  Jason Merrill  <jason@redhat.com>
 
+	* decl.c (start_preparsed_function): Condition ctor clobber on
+	flag_lifetime_dse > 1.
+
 	* cp-gimplify.c (cp_fold): Don't fold constexpr calls if -fno-inline.
 
 2016-02-19  Jason Merrill  <jason@redhat.com>

gcc/cp/decl.c

@@ -14104,7 +14104,8 @@ 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 > 1)
+      && 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
 	 constructor, via AGGR_INIT_ZERO_FIRST (c++/68006).  */

gcc/doc/invoke.texi

@@ -6809,7 +6809,10 @@ value, and any changes during the lifetime of the object are dead when
 the object is destroyed.  Normally dead store elimination will take
 advantage of this; if your code relies on the value of the object
 storage persisting beyond the lifetime of the object, you can use this
-flag to disable this optimization.
+flag to disable this optimization.  To preserve stores before the
+constructor starts (e.g. because your operator new clears the object
+storage) but still treat the object as dead after the destructor you,
+can use -flifetime-dse=1.
 
 @item -flive-range-shrinkage
 @opindex flive-range-shrinkage

gcc/testsuite/g++.dg/opt/flifetime-dse4.C

+// { dg-options "-O3 -flifetime-dse=1" }
+// { dg-do run }
+
+typedef __SIZE_TYPE__ size_t;
+inline void * operator new (size_t, void *p) { return p; }
+
+struct A
+{
+  int i;
+  A() {}
+  ~A() {}
+};
+
+int main()
+{
+  int ar[1] = { 42 };
+  A* ap = new(ar) A;
+
+  // With -flifetime-dse=1 we retain the old value.
+  if (ap->i != 42) __builtin_abort();
+
+  ap->i = 42;
+  ap->~A();
+
+  // When the destructor ends the object no longer exists.
+  if (ar[0] == 42) __builtin_abort();
+}