[a64] Omit redundant smi checks in write barriers.

R=rmcilroy@chromium.org

Review URL: https://codereview.chromium.org/156603007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19324 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/a64/lithium-codegen-a64.cc

@@ -5051,11 +5051,17 @@ void LCodeGen::DoStoreNamedField(LStoreNamedField* instr) {
   }
 
   Handle<Map> transition = instr->transition();
+  SmiCheck check_needed =
+      instr->hydrogen()->value()->IsHeapObject()
+          ? OMIT_SMI_CHECK : INLINE_SMI_CHECK;
 
   if (FLAG_track_heap_object_fields && representation.IsHeapObject()) {
     Register value = ToRegister(instr->value());
     if (!instr->hydrogen()->value()->type().IsHeapObject()) {
       DeoptimizeIfSmi(value, instr->environment());
+
+      // We know that value is a smi now, so we can omit the check below.
+      check_needed = OMIT_SMI_CHECK;
     }
   } else if (representation.IsDouble()) {
     ASSERT(transition.is_null());
@@ -5087,9 +5093,6 @@ void LCodeGen::DoStoreNamedField(LStoreNamedField* instr) {
   // Do the store.
   Register value = ToRegister(instr->value());
   Register destination;
-  SmiCheck check_needed =
-      instr->hydrogen()->value()->IsHeapObject()
-          ? OMIT_SMI_CHECK : INLINE_SMI_CHECK;
   if (access.IsInobject()) {
     destination = object;
   } else {