Automatically grow ReplacementStringBuilder if needed

The ReplacementStringBuilder had a tricky API that required manually
growing the backing store if needed. This saves a few int additions
and comparisons, but is probably not worth the potential for errors.
This CL changes to automatic growth that is invisible to the user.

Bug: chromium:933521
Change-Id: I7d2404a48fe47ccce9af919910c06a9eca598120
Reviewed-on: https://chromium-review.googlesource.com/c/1477748Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59726}

src/runtime/runtime-regexp.cc

@@ -652,16 +652,9 @@ V8_WARN_UNUSED_RESULT static Object StringReplaceGlobalRegExpWithString(
   int expected_parts = (compiled_replacement.parts() + 1) * 4 + 1;
   ReplacementStringBuilder builder(isolate->heap(), subject, expected_parts);
 
-  // Number of parts added by compiled replacement plus preceding
-  // string and possibly suffix after last match.  It is possible for
-  // all components to use two elements when encoded as two smis.
-  const int parts_added_per_loop = 2 * (compiled_replacement.parts() + 2);
-
   int prev = 0;
 
   do {
-    builder.EnsureCapacity(parts_added_per_loop);
-
     int start = current_match[0];
     int end = current_match[1];
 
@@ -682,7 +675,6 @@ V8_WARN_UNUSED_RESULT static Object StringReplaceGlobalRegExpWithString(
   if (global_cache.HasException()) return ReadOnlyRoots(isolate).exception();
 
   if (prev < subject_length) {
-    builder.EnsureCapacity(2);
     builder.AddSubjectSlice(prev, subject_length);
   }
 

src/string-builder-inl.h

@@ -65,6 +65,7 @@ class ReplacementStringBuilder {
   ReplacementStringBuilder(Heap* heap, Handle<String> subject,
                            int estimated_part_count);
 
+  // Caution: Callers must ensure the builder has enough capacity.
   static inline void AddSubjectSlice(FixedArrayBuilder* builder, int from,
                                      int to) {
     DCHECK_GE(from, 0);
@@ -82,9 +83,8 @@ class ReplacementStringBuilder {
     }
   }
 
-  void EnsureCapacity(int elements);
-
   void AddSubjectSlice(int from, int to) {
+    EnsureCapacity(2);  // Subject slices are encoded with up to two smis.
     AddSubjectSlice(&array_builder_, from, to);
     IncrementCharacterCount(to - from);
   }
@@ -104,6 +104,7 @@ class ReplacementStringBuilder {
 
  private:
   void AddElement(Object element);
+  void EnsureCapacity(int elements);
 
   Heap* heap_;
   FixedArrayBuilder array_builder_;

src/string-builder.cc

@@ -226,6 +226,7 @@ MaybeHandle<String> ReplacementStringBuilder::ToString() {
 void ReplacementStringBuilder::AddElement(Object element) {
   DCHECK(element->IsSmi() || element->IsString());
   DCHECK(array_builder_.capacity() > array_builder_.length());
+  EnsureCapacity(1);
   array_builder_.Add(element);
 }