[turbofan] Utilize the fact that empty string is canonicalized.

src/compiler/js-typed-lowering.cc

Index: src/compiler/js-typed-lowering.cc
diff --git a/src/compiler/js-typed-lowering.cc b/src/compiler/js-typed-lowering.cc
index de471c9e9e1a9c6ea75ef5cff2832d7c8ab369be..fdd3da7743a6aaf9224761466a67ecadaef217bf 100644
--- a/src/compiler/js-typed-lowering.cc
+++ b/src/compiler/js-typed-lowering.cc
@@ -528,6 +528,18 @@ JSTypedLowering::JSTypedLowering(Editor* editor,
       dependencies_(dependencies),
       flags_(flags),
       jsgraph_(jsgraph),
+      pointer_comparable_type_(Type::Union(
+          Type::BooleanOrNullOrUndefined(),
+          Type::Union(Type::Symbol(),
+                      Type::Union(Type::Receiver(),
+                                  Type::Union(Type::Hole(),
+                                              Type::HeapConstant(
+                                                  factory()->empty_string(),
+                                                  graph()->zone()),
+                                              graph()->zone()),
+                                  graph()->zone()),
+                      graph()->zone()),
+          graph()->zone())),

[Yang, 2017/02/09 06:41:41]

can we have a helper function that returns this monstrosity?

[Benedikt Meurer, 2017/02/09 06:58:24]

Done.

       type_cache_(TypeCache::Get()) {
   for (size_t k = 0; k < arraysize(shifted_int32_ranges_); ++k) {
     double min = kMinInt / (1 << k);
@@ -1010,7 +1022,7 @@ Reduction JSTypedLowering::ReduceJSStrictEqual(Node* node, bool invert) {
   if (r.BothInputsAre(Type::Unique())) {
     return r.ChangeToPureOperator(simplified()->ReferenceEqual(), invert);
   }
-  if (r.OneInputIs(Type::NonStringUniqueOrHole())) {
+  if (r.OneInputIs(pointer_comparable_type_)) {
     return r.ChangeToPureOperator(simplified()->ReferenceEqual(), invert);
   }
   if (r.IsInternalizedStringCompareOperation()) {
@@ -1074,6 +1086,14 @@ Reduction JSTypedLowering::ReduceJSToBoolean(Node* node) {
     node->TrimInputCount(1);
     NodeProperties::ChangeOp(node, simplified()->BooleanNot());
     return Changed(node);
+  } else if (input_type->Is(Type::String())) {
+    // JSToBoolean(x:string) => BooleanNot(ReferenceEqual(x,""))
+    node->ReplaceInput(0,
+                       graph()->NewNode(simplified()->ReferenceEqual(), input,
+                                        jsgraph()->EmptyStringConstant()));
+    node->TrimInputCount(1);
+    NodeProperties::ChangeOp(node, simplified()->BooleanNot());
+    return Changed(node);
   }
   return NoChange();
 }