[turbofan] Mark fast StringAdd as not causing side effects.

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/js-typed-lowering.h"
 
 #include "src/ast/modules.h"
 #include "src/builtins/builtins-utils.h"
 #include "src/code-factory.h"
 #include "src/compilation-dependencies.h"
@@ skipping 554 matching lines @@
   if (r.OneInputIs(Type::String())) {
     if (r.ShouldCreateConsString()) {
       return ReduceCreateConsString(node);
     }
     StringAddFlags flags = STRING_ADD_CHECK_NONE;
     if (!r.LeftInputIs(Type::String())) {
       flags = STRING_ADD_CONVERT_LEFT;
     } else if (!r.RightInputIs(Type::String())) {
       flags = STRING_ADD_CONVERT_RIGHT;
     }
+    Operator::Properties properties = node->op()->properties();
+    if (r.NeitherInputCanBe(Type::Receiver())) {
+      // Both sides are already strings, so we know that the
+      // string addition will not cause any observable side
+      // effects; it can still throw obviously.
+      properties = Operator::kNoWrite | Operator::kNoDeopt;
+    }
     // JSAdd(x:string, y) => CallStub[StringAdd](x, y)
     // JSAdd(x, y:string) => CallStub[StringAdd](x, y)
     Callable const callable =
         CodeFactory::StringAdd(isolate(), flags, NOT_TENURED);
     CallDescriptor const* const desc = Linkage::GetStubCallDescriptor(
         isolate(), graph()->zone(), callable.descriptor(), 0,
-        CallDescriptor::kNeedsFrameState, node->op()->properties());
+        CallDescriptor::kNeedsFrameState, properties);
     DCHECK_EQ(1, OperatorProperties::GetFrameStateInputCount(node->op()));
     node->InsertInput(graph()->zone(), 0,
                       jsgraph()->HeapConstant(callable.code()));
     NodeProperties::ChangeOp(node, common()->Call(desc));
     return Changed(node);
   }
   return NoChange();
 }
 
 Reduction JSTypedLowering::ReduceNumberBinop(Node* node) {
@@ skipping 1794 matching lines @@
 }
 
 
 CompilationDependencies* JSTypedLowering::dependencies() const {
   return dependencies_;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8