[zone] Keep one page when we Zone::Reset for reuse

Change-Id: I50c6124d3da5b35d4156c066f38d10d2dc966567
Reviewed-on: https://chromium-review.googlesource.com/c/1349246Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57793}

src/ast/scopes.cc

@@ -1488,7 +1488,7 @@ void DeclarationScope::ResetAfterPreparsing(AstValueFactory* ast_value_factory,
   has_rest_ = false;
 
   DCHECK_NE(zone_, ast_value_factory->zone());
-  zone_->ReleaseMemory();
+  zone_->Reset();
 
   if (aborted) {
     // Prepare scope for use in the outer zone.

src/zone/zone.cc

@@ -73,10 +73,16 @@ void* Zone::AsanNew(size_t size) {
   return reinterpret_cast<void*>(result);
 }
 
-void Zone::ReleaseMemory() {
+void Zone::Reset() {
+  if (!segment_head_) return;
   allocator_->ZoneDestruction(this);
+  Segment* keep = segment_head_;
+  segment_head_ = segment_head_->next();
+  keep->set_next(nullptr);
   DeleteAll();
   allocator_->ZoneCreation(this);
+  keep->ZapContents();
+  segment_head_ = keep;
 }
 
 void Zone::DeleteAll() {

src/zone/zone.h

@@ -69,9 +69,10 @@ class V8_EXPORT_PRIVATE Zone final {
   // Seals the zone to prevent any further allocation.
   void Seal() { sealed_ = true; }
 
-  // Allows the zone to be safely reused. Releases the memory and fires zone
-  // destruction and creation events for the accounting allocator.
-  void ReleaseMemory();
+  // Allows the zone to be safely reused. Releases the memory except for the
+  // last page, and fires zone destruction and creation events for the
+  // accounting allocator.
+  void Reset();
 
   // Returns true if more memory has been allocated in zones than
   // the limit allows.