From: Jason Merrill <jason@redhat.com>
Date: Thu, 26 Feb 2015 02:43:52 +0000 (-0500)
Subject: common.opt (-flifetime-dse): New.
X-Git-Tag: releases/gcc-4.9.3~322
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=f85347cdaee5fb3ce8b77266e7929e29d60ed5cf;p=thirdparty%2Fgcc.git

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: r220996
---

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 326a98ddfb44..0d8f66fff7c7 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,7 @@
+2015-02-25  Jason Merrill  <jason@redhat.com>
+
+	* common.opt (-flifetime-dse): New.
+
 2015-02-25  Kai Tietz  <ktietz@redhat.com>
 
 	PR tree-optimization/61917
diff --git a/gcc/common.opt b/gcc/common.opt
index 2259f29d19e4..51ddd77c9a9c 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1745,6 +1745,11 @@ 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 90f2923f99e1..30b1c3f24fd1 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,8 @@
+2015-02-25  Jason Merrill  <jason@redhat.com>
+
+	* decl.c (begin_destructor_body): Condition clobber on
+	-flifetime-dse.
+
 2015-02-13  Jason Merrill  <jason@redhat.com>
 
 	PR c++/62017
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 631ce9191325..b0bbf9e4fb25 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -13667,15 +13667,19 @@ 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 870e637f93df..1aea050d0cdd 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -7401,6 +7401,16 @@ 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 000000000000..733d28a794f4
--- /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();
+}