[arc] guaranteed parameters are always known safe BU.

The reason why this is true is that we know that a guaranteed parameter must out
live the current function. That means that no releases on that guaranteed
parameter can be "last" releases.

rdar://25091228

Author: gottesmm

lib/SILOptimizer/ARC/RCStateTransitionVisitors.cpp

@@ -80,6 +80,15 @@ BottomUpDataflowRCStateVisitor<ARCState>::visitStrongDecrement(ValueBase *V) {
     }
   }
 
+  // A guaranteed function argument is guaranteed to outlive the function we are
+  // processing. So bottom up for such a parameter, we are always known safe.
+  if (auto *Arg = dyn_cast<SILArgument>(Op)) {
+    if (Arg->isFunctionArg() &&
+        Arg->hasConvention(SILArgumentConvention::Direct_Guaranteed)) {
+      State.updateKnownSafe(true);
+    }
+  }
+
   DEBUG(llvm::dbgs() << "    REF COUNT DECREMENT! Known Safe: "
                      << (State.isKnownSafe() ? "yes" : "no") << "\n");
 

test/SILOptimizer/arcsequenceopts.sil

@@ -2116,3 +2116,18 @@ bb2:
   %5 = tuple()
   return %5 : $()
 }
+
+// CHECK-LABEL: sil @guaranteed_always_known_safe_bu : $@convention(thin) (@guaranteed Builtin.NativeObject) -> () {
+// CHECK: strong_retain
+// CHECK-NOT: strong_retain
+// CHECK-NOT: strong_release
+sil @guaranteed_always_known_safe_bu : $@convention(thin) (@guaranteed Builtin.NativeObject) -> () {
+bb0(%0 : $Builtin.NativeObject):
+  strong_retain %0 : $Builtin.NativeObject
+  strong_retain %0 : $Builtin.NativeObject
+  %1 = function_ref @use : $@convention(thin) (Builtin.NativeObject) -> ()
+  apply %1(%0) : $@convention(thin) (Builtin.NativeObject) -> ()
+  apply %1(%0) : $@convention(thin) (Builtin.NativeObject) -> ()
+  strong_release %0 : $Builtin.NativeObject
+  return undef : $()
+}