[turbofan] Specify better type for FixedArray::length field.

src/compiler/access-builder.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/types-inl.h"
 
 namespace v8 {
 namespace internal {
 namespace compiler {
@@ skipping 28 matching lines @@
 
 // static
 FieldAccess AccessBuilder::ForJSArrayBufferBackingStore() {
   return {kTaggedBase, JSArrayBuffer::kBackingStoreOffset, MaybeHandle<Name>(),
           Type::UntaggedPointer(), kMachPtr};
 }
 
 
 // static
 FieldAccess AccessBuilder::ForFixedArrayLength() {
+  // TODO(turbofan): 2^30 is a valid upper limit for the FixedArray::length
+  // field, although it's not the best. If we had a Zone we could create an
+  // appropriate range type instead.
+  STATIC_ASSERT(FixedArray::kMaxLength <= 1 << 30);
   return {kTaggedBase, FixedArray::kLengthOffset, MaybeHandle<Name>(),
-          Type::TaggedSigned(), kMachAnyTagged};
+          Type::Intersect(Type::Unsigned30(), Type::TaggedSigned()),
+          kMachAnyTagged};
 }
 
 
 // static
 FieldAccess AccessBuilder::ForExternalArrayPointer() {
   return {kTaggedBase, ExternalArray::kExternalPointerOffset,
           MaybeHandle<Name>(), Type::UntaggedPointer(), kMachPtr};
 }
 
 
@@ skipping 127 matching lines @@
 
 // static
 FieldAccess AccessBuilder::ForSharedFunctionInfoTypeFeedbackVector() {
   return {kTaggedBase, SharedFunctionInfo::kFeedbackVectorOffset,
           Handle<Name>(), Type::Any(), kMachAnyTagged};
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8