[ast] Add a Flatten method for AstConsString

This allows off-thread flattening.

Bug: chromium:1011762
Change-Id: If83f7bbcbf74165987a4c157184f5b92dc554971
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1924437
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65075}

src/ast/ast-value-factory.cc

@@ -246,6 +246,50 @@ AstConsString* AstValueFactory::NewConsString(const AstRawString* str1,
   return NewConsString()->AddString(zone_, str1)->AddString(zone_, str2);
 }
 
+const AstRawString* AstValueFactory::Flatten(const AstConsString* str) {
+  if (str->IsEmpty()) return empty_string();
+  AstConsString::Segment segment = str->segment_;
+  if (!segment.next) return segment.string;
+
+  int length = segment.string->length();
+  bool is_one_byte = segment.string->is_one_byte();
+  while (segment.next) {
+    segment = *segment.next;
+    length += segment.string->length();
+    is_one_byte &= segment.string->is_one_byte();
+  }
+
+  if (is_one_byte) {
+    Vector<byte> data(zone()->NewArray<byte>(length), length);
+    segment = str->segment_;
+    byte* p = data.begin();
+    while (true) {
+      CopyChars(p, segment.string->raw_data(), segment.string->length());
+      p += segment.string->length();
+      if (!segment.next) break;
+      segment = *segment.next;
+    }
+    return GetOneByteString(data);
+  }
+
+  Vector<uint16_t> data(zone()->NewArray<uint16_t>(length), length);
+  segment = str->segment_;
+  uint16_t* p = data.begin();
+  while (true) {
+    if (segment.string->is_one_byte()) {
+      CopyChars(p, segment.string->raw_data(), segment.string->length());
+    } else {
+      CopyChars(p,
+                reinterpret_cast<const uint16_t*>(segment.string->raw_data()),
+                segment.string->length());
+    }
+    p += segment.string->length();
+    if (!segment.next) break;
+    segment = *segment.next;
+  }
+  return GetTwoByteString(data);
+}
+
 void AstValueFactory::Internalize(Isolate* isolate) {
   // Strings need to be internalized before values, because values refer to
   // strings.

src/ast/ast-value-factory.h

@@ -312,6 +312,7 @@ class AstValueFactory {
   V8_EXPORT_PRIVATE AstConsString* NewConsString(const AstRawString* str);
   V8_EXPORT_PRIVATE AstConsString* NewConsString(const AstRawString* str1,
                                                  const AstRawString* str2);
+  V8_EXPORT_PRIVATE const AstRawString* Flatten(const AstConsString* str);
 
   V8_EXPORT_PRIVATE void Internalize(Isolate* isolate);