[CopyPropagation] Delete dead copy_values.

When canonicalizing an owned value's lifetime, also check whether the
value is dead.  If it is, track it for deletion.  In particular, this
eliminates dead copy_values.

Author: nate-chandler

lib/SILOptimizer/Transforms/CopyPropagation.cpp

@@ -559,6 +559,9 @@ void CopyPropagation::run() {
   while (!defWorklist.ownedValues.empty()) {
     SILValue def = defWorklist.ownedValues.pop_back_val();
     canonicalizer.canonicalizeValueLifetime(def);
+    // Copies of borrowed values may be dead.
+    if (auto *inst = def->getDefiningInstruction())
+      deleter.trackIfDead(inst);
   }
   // Recursively cleanup dead defs after removing uses.
   deleter.cleanupDeadInstructions();

test/SILOptimizer/copy_propagation.sil

@@ -701,6 +701,10 @@ bb1:
   destroy_value %copy : $AnyObject
   %obj = load [copy] %access : $*AnyObject
   end_access %access : $*AnyObject
+  // Use %obj so it doesn't get deleted because it's unused a trigger a cascade
+  // of deletions.
+  %f2 = function_ref @takeGuaranteedAnyObject : $@convention(thin) (@guaranteed AnyObject) -> ()
+  apply %f2(%obj) : $@convention(thin) (@guaranteed AnyObject) -> ()
   destroy_value %obj : $AnyObject
   destroy_value %box : ${ var AnyObject }
   %v = tuple ()
@@ -876,3 +880,24 @@ entry(%instance : @owned $C):
   %retval = tuple ()
   return %retval : $()
 }
+
+// Verify that a dead copy_value is deleted.
+// CHECK-LABEL: sil [ossa] @delete_dead_reborrow_copy : {{.*}} {
+// CHECK-NOT:     copy_value
+// CHECK-LABEL: } // end sil function 'delete_dead_reborrow_copy'
+sil [ossa] @delete_dead_reborrow_copy : $@convention(thin) (@owned X) -> () {
+bb0(%instance : @owned $X):
+    %lifetime = begin_borrow %instance : $X
+    br bb1(%lifetime : $X)
+
+bb1(%reborrow : @guaranteed $X):
+    %dead_copy = copy_value %reborrow : $X
+    end_borrow %reborrow : $X
+    destroy_value %dead_copy : $X
+    br exit
+
+exit:
+    destroy_value %instance : $X
+    %retval = tuple ()
+    return %retval : $()
+}