commit ca9986d97112f804e4cc416b15f9baf74d713aab
Author: Jason Merrill <jason@redhat.com>
Date: Tue Feb 10 23:45:19 2015 -0500
gcc/
* common.opt (-flifetime-dse): New.
gcc/cp/
* decl.c (begin_destructor_body): Condition clobber on
-flifetime-dse.
@@ -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
@@ -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. */
@@ -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
new file mode 100644
@@ -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();
+}