Fast-path in SlowFlatten for ConsStrings with empty first part

test/cctest/test-strings.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
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 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
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 //     * Redistributions in binary form must reproduce the above
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@@ skipping 582 matching lines @@
   Traverse(flat, right_asymmetric);
   printf("14\n");
   String::Flatten(symmetric);
   printf("15\n");
   Traverse(flat, symmetric);
   printf("16\n");
   String::Flatten(left_deep_asymmetric);
   printf("18\n");
 }
 
+TEST(ConsStringWithEmptyFirstFlatten) {
+  printf("ConsStringWithEmptyFirstFlatten\n");
+  CcTest::InitializeVM();
+  v8::HandleScope scope(CcTest::isolate());
+  Isolate* isolate = CcTest::i_isolate();
+
+  i::Handle<i::String> initial_fst =
+      isolate->factory()->NewStringFromAsciiChecked("fst012345");
+  i::Handle<i::String> initial_snd =
+      isolate->factory()->NewStringFromAsciiChecked("snd012345");
+  i::Handle<i::String> str = isolate->factory()
+                                 ->NewConsString(initial_fst, initial_snd)
+                                 .ToHandleChecked();
+  CHECK(str->IsConsString());
+  auto cons = i::Handle<i::ConsString>::cast(str);
+
+  const int initial_length = cons->length();
+
+  // set_first / set_second does not update the length (which the heap verifier
+  // checks), so we need to ensure the length stays the same.
+
+  i::Handle<i::String> new_fst = isolate->factory()->empty_string();
+  i::Handle<i::String> new_snd =
+      isolate->factory()->NewStringFromAsciiChecked("snd012345012345678");
+  cons->set_first(*new_fst);
+  cons->set_second(*new_snd);
+  CHECK(!cons->IsFlat());
+  CHECK_EQ(initial_length, new_fst->length() + new_snd->length());
+  CHECK_EQ(initial_length, cons->length());
+
+  // Make sure Flatten doesn't alloc a new string.
+  DisallowHeapAllocation no_alloc;
+  i::Handle<i::String> flat = i::String::Flatten(cons);
+  CHECK(flat->IsFlat());
+  CHECK_EQ(initial_length, flat->length());
+}
 
 static void VerifyCharacterStream(
     String* flat_string, String* cons_string) {
   // Do not want to test ConString traversal on flat string.
   CHECK(flat_string->IsFlat() && !flat_string->IsConsString());
   CHECK(cons_string->IsConsString());
   // TODO(dcarney) Test stream reset as well.
   int length = flat_string->length();
   // Iterate start search in multiple places in the string.
   int outer_iterations = length > 20 ? 20 : length;
@@ skipping 952 matching lines @@
   }
   {
     HandleScope scope(isolate);
     v8::Local<v8::Value> result = CompileRun(
         "String.fromCharCode(432, 432, 432, 432, 432, "
         "432, 432, 432, 432, 432, 432, 432, 432, 432, "
         "432, 432, 432, 432, 432, 432, 432, 432, 432)");
     CHECK(v8::Utils::OpenHandle(*result)->IsSeqTwoByteString());
   }
 }