PATCH to add -flifetime-dse=1

[Jason Merrill <jason at redhat dot com>]

Various projects have been having trouble with the new -flifetime-dse 
clobber at the beginning of the constructor, so this patch allows them 
to pass -flifetime-dse=1 to disable that clobber while still keeping the 
one at the end of the destructor.

Tested x86_64-pc-linux-gnu, applying to trunk.

commit 4795b095eba78ee05f46548fbc9aa9343ff34a7e
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Feb 22 15:18:13 2016 -0500

    	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.

diff --git a/gcc/common.opt b/gcc/common.opt
index bc5b4c4..e91f225 100644
--- a/gcc/common.opt
+++ b/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.
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 30eef5c..2df3398 100644
--- a/gcc/cp/decl.c
+++ b/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).  */
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 9ca3793..b8b2e70 100644
--- a/gcc/doc/invoke.texi
+++ b/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
diff --git a/gcc/testsuite/g++.dg/opt/flifetime-dse4.C b/gcc/testsuite/g++.dg/opt/flifetime-dse4.C
new file mode 100644
index 0000000..c72444a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/flifetime-dse4.C
@@ -0,0 +1,27 @@
+// { 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();
+}