[turbofan] Ensure monotonicity for induction variable typing.

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 617 matching lines @@
   DCHECK_EQ(IrOpcode::kLoop, NodeProperties::GetControlInput(node)->opcode());
   DCHECK_EQ(2, NodeProperties::GetControlInput(node)->InputCount());
 
   Type* initial_type = Operand(node, 0);
   Type* increment_type = Operand(node, 2);
 
   // We only handle integer induction variables (otherwise ranges
   // do not apply and we cannot do anything).
   if (!initial_type->Is(typer_->cache_.kInteger) ||
       !increment_type->Is(typer_->cache_.kInteger)) {
-    // Fallback to normal phi typing.
-    Type* type = Operand(node, 0);
-    for (int i = 1; i < arity; ++i) {
+    // Fallback to normal phi typing, but ensure monotonicity.
+    // (Unfortunately, without baking in the previous type, monotonicity might
+    // be violated because we might not yet have retyped the incrementing
+    // operation even though the increment's type might been already reflected
+    // in the induction variable phi.)
+    Type* type = NodeProperties::IsTyped(node) ? NodeProperties::GetType(node)
+                                               : Type::None();
+    for (int i = 0; i < arity; ++i) {
       type = Type::Union(type, Operand(node, i), zone());
     }
     return type;
   }
   // If we do not have enough type information for the initial value or
   // the increment, just return the initial value's type.
   if (!initial_type->IsInhabited() || !increment_type->IsInhabited()) {
     return initial_type;
   }
 
@@ skipping 1052 matching lines @@
 Type* Typer::Visitor::TypeConstant(Handle<Object> value) {
   if (Type::IsInteger(*value)) {
     return Type::Range(value->Number(), value->Number(), zone());
   }
   return Type::Constant(value, zone());
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8