[turbofan] Improve typed lowering for JSToBoolean.

src/compiler/js-typed-lowering.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/access-builder.h"
 #include "src/compiler/graph-inl.h"
 #include "src/compiler/js-builtin-reducer.h"
 #include "src/compiler/js-typed-lowering.h"
 #include "src/compiler/node-aux-data-inl.h"
 #include "src/compiler/node-properties-inl.h"
@@ skipping 528 matching lines @@
     // JSToBoolean(x:undetectable) => #false
     return ReplaceWith(jsgraph()->FalseConstant());
   }
   if (input_type->Is(Type::OrderedNumber())) {
     // JSToBoolean(x:ordered-number) => BooleanNot(NumberEqual(x, #0))
     Node* cmp = graph()->NewNode(simplified()->NumberEqual(), input,
                                  jsgraph()->ZeroConstant());
     Node* inv = graph()->NewNode(simplified()->BooleanNot(), cmp);
     return ReplaceWith(inv);
   }
-  // TODO(turbofan): js-typed-lowering of ToBoolean(string)
+  if (input_type->Is(Type::String())) {
+    // JSToBoolean(x:string) => BooleanNot(NumberEqual(x.length, #0))
+    FieldAccess access = AccessBuilder::ForStringLength();
+    Node* length = graph()->NewNode(simplified()->LoadField(access), input,
+                                    graph()->start(), graph()->start());
+    Node* cmp = graph()->NewNode(simplified()->NumberEqual(), length,
+                                 jsgraph()->ZeroConstant());
+    Node* inv = graph()->NewNode(simplified()->BooleanNot(), cmp);
+    return ReplaceWith(inv);
+  }
+  if (input->opcode() == IrOpcode::kPhi && input_type->Is(Type::Primitive())) {
+    // JSToBoolean(phi(x1,...,xn):primitive)
+    //   => phi(JSToBoolean(x1),...,JSToBoolean(xn))
+    int input_count = input->InputCount() - 1;
+    Node** inputs = zone()->NewArray<Node*>(input_count + 1);
+    for (int i = 0; i < input_count; ++i) {
+      Node* value = input->InputAt(i);
+      // Recursively try to reduce the value first.
+      Reduction result = ReduceJSToBooleanInput(value);
+      if (result.Changed()) {
+        inputs[i] = result.replacement();
+      } else {
+        inputs[i] = graph()->NewNode(javascript()->ToBoolean(), value,
+                                     jsgraph()->ZeroConstant(),
+                                     graph()->start(), graph()->start());
+      }
+    }
+    inputs[input_count] = input->InputAt(input_count);
+    Node* phi = graph()->NewNode(common()->Phi(kMachAnyTagged, input_count),
+                                 input_count + 1, inputs);
+    return ReplaceWith(phi);
+  }
   return NoChange();
 }
 
 
 Reduction JSTypedLowering::ReduceJSLoadProperty(Node* node) {
   Node* key = NodeProperties::GetValueInput(node, 1);
   Node* base = NodeProperties::GetValueInput(node, 0);
   Type* key_type = NodeProperties::GetBounds(key).upper;
   Type* base_type = NodeProperties::GetBounds(base).upper;
   // TODO(mstarzinger): This lowering is not correct if:
@@ skipping 158 matching lines @@
       return JSBuiltinReducer(jsgraph()).Reduce(node);
     default:
       break;
   }
   return NoChange();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8