[turbofan] Improve typing for CheckBounds.

src/compiler/typer.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/typer.h"
 
 #include <iomanip>
 
 #include "src/base/flags.h"
 #include "src/bootstrapper.h"
@@ skipping 1519 matching lines @@
 Type* Typer::Visitor::TypeStringCharCodeAt(Node* node) {
   // TODO(bmeurer): We could do better here based on inputs.
   return Type::Range(0, kMaxUInt16, zone());
 }
 
 Type* Typer::Visitor::TypeStringFromCharCode(Node* node) {
   return TypeUnaryOp(node, StringFromCharCodeTyper);
 }
 
 Type* Typer::Visitor::TypeCheckBounds(Node* node) {
-  // TODO(bmeurer): We could do better here based on the limit.
-  return Type::Unsigned31();
+  Type* index = Operand(node, 0);
+  Type* length = Operand(node, 1);
+  index = Type::Intersect(index, Type::Integral32(), zone());
+  if (!index->IsInhabited() || !length->IsInhabited()) return Type::None();
+  double min = std::max(index->Min(), 0.0);
+  double max = std::min(index->Max(), length->Min() - 1);
+  if (max < min) return Type::None();
+  return Type::Range(min, max, zone());
 }
 
 Type* Typer::Visitor::TypeCheckMaps(Node* node) {
   UNREACHABLE();
   return nullptr;
 }
 
 Type* Typer::Visitor::TypeCheckNumber(Node* node) {
   Type* arg = Operand(node, 0);
   return Type::Intersect(arg, Type::Number(), zone());
@@ skipping 772 matching lines @@
   }
   if (Type::IsInteger(*value)) {
     return Type::Range(value->Number(), value->Number(), zone());
   }
   return Type::Constant(value, zone());
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8