common.opt (-flifetime-dse): New.

gcc/
	* common.opt (-flifetime-dse): New.
gcc/cp/
	* decl.c (begin_destructor_body): Condition clobber on
	-flifetime-dse.

From-SVN: r220657

6 files changed, 60 insertions, 9 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 60af263..767e880 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,7 @@
+2015-02-12  Jason Merrill  <jason@redhat.com>
+
+	* common.opt (-flifetime-dse): New.
+
 2015-02-12  Jakub Jelinek  <jakub@redhat.com>
 
 	PR sanitizer/65019
diff --git a/gcc/common.opt b/gcc/common.opt
index cf4e503..6e65757 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1856,6 +1856,11 @@ fregmove
 Common Ignore
 Does nothing. Preserved for backward compatibility.
 
+flifetime-dse
+Common Report Var(flag_lifetime_dse) Init(1) Optimization
+Tell DSE that the storage for a C++ object is dead when the constructor
+starts and when the destructor finishes.
+
 flive-range-shrinkage
 Common Report Var(flag_live_range_shrinkage) Init(0) Optimization
 Relief of register pressure through live range shrinkage
diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index e460c55..4f75646 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,8 @@
+2015-02-12  Jason Merrill  <jason@redhat.com>
+
+	* decl.c (begin_destructor_body): Condition clobber on
+	-flifetime-dse.
+
 2015-02-12 Andrea Azzarone <azzaronea@gmail.com>
 
 	PR c++/64959
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 50b0624..810acd5 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -13936,15 +13936,19 @@ begin_destructor_body (void)
       initialize_vtbl_ptrs (current_class_ptr);
       finish_compound_stmt (compound_stmt);
 
-      /* Insert a cleanup to let the back end know that the object is dead
-	 when we exit the destructor, either normally or via exception.  */
-      tree btype = CLASSTYPE_AS_BASE (current_class_type);
-      tree clobber = build_constructor (btype, NULL);
-      TREE_THIS_VOLATILE (clobber) = true;
-      tree bref = build_nop (build_reference_type (btype), current_class_ptr);
-      bref = convert_from_reference (bref);
-      tree exprstmt = build2 (MODIFY_EXPR, btype, bref, clobber);
-      finish_decl_cleanup (NULL_TREE, exprstmt);
+      if (flag_lifetime_dse)
+	{
+	  /* Insert a cleanup to let the back end know that the object is dead
+	     when we exit the destructor, either normally or via exception.  */
+	  tree btype = CLASSTYPE_AS_BASE (current_class_type);
+	  tree clobber = build_constructor (btype, NULL);
+	  TREE_THIS_VOLATILE (clobber) = true;
+	  tree bref = build_nop (build_reference_type (btype),
+				 current_class_ptr);
+	  bref = convert_from_reference (bref);
+	  tree exprstmt = build2 (MODIFY_EXPR, btype, bref, clobber);
+	  finish_decl_cleanup (NULL_TREE, exprstmt);
+	}
 
       /* And insert cleanups for our bases and members so that they
 	 will be properly destroyed if we throw.  */
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 8a04790..5cce4f7 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -7888,6 +7888,16 @@ registers after writing to their lower 32-bit half.
 Enabled for Alpha, AArch64 and x86 at levels @option{-O2},
 @option{-O3}, @option{-Os}.
 
+@item -fno-lifetime-dse
+@opindex fno-lifetime-dse
+In C++ the value of an object is only affected by changes within its
+lifetime: when the constructor begins, the object has an indeterminate
+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.
+
 @item -flive-range-shrinkage
 @opindex flive-range-shrinkage
 Attempt to decrease register pressure through register live range
diff --git a/gcc/testsuite/g++.dg/opt/flifetime-dse1.C b/gcc/testsuite/g++.dg/opt/flifetime-dse1.C
new file mode 100644
index 0000000..733d28a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/flifetime-dse1.C
@@ -0,0 +1,23 @@
+// { dg-options "-O3 -fno-lifetime-dse" }
+// { 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];
+
+  A* ap = new(ar) A;
+  ap->i = 42;
+  ap->~A();
+
+  if (ar[0] != 42) __builtin_abort();
+}