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

This is a reland of 2418d22a

Reland fixes:
  * Rebase this 2+ year old change 😱
  * Unpoison the kept segment before zapping it to make ASAN happy.
  * Carefully adjust allocation size tracking fields to compensate for
    kept segment.

Original change's description:
> [zone] Keep one page when we Zone::Reset for reuse
>
> Change-Id: I50c6124d3da5b35d4156c066f38d10d2dc966567
> Reviewed-on: https://chromium-review.googlesource.com/c/1349246
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Commit-Queue: Toon Verwaest <verwaest@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#57793}

Change-Id: Iaffde5b38b3d683af081b1878464dd4c66be5af8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3322833Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78348}

src/ast/scopes.cc

@@ -1631,7 +1631,7 @@ void DeclarationScope::ResetAfterPreparsing(AstValueFactory* ast_value_factory,
     // Get the zone, while variables_ is still valid
     Zone* zone = this->zone();
     variables_.Invalidate();
-    zone->ReleaseMemory();
+    zone->Reset();
   }
 
   if (aborted) {

src/parsing/parse-info.h

@@ -197,7 +197,7 @@ class V8_EXPORT_PRIVATE ReusableUnoptimizedCompileState {
   // Zone pointer in the AstValueFactory.
   Zone* single_parse_zone() { return &single_parse_zone_; }
 
-  void NotifySingleParseCompleted() { single_parse_zone_.ReleaseMemory(); }
+  void NotifySingleParseCompleted() { single_parse_zone_.Reset(); }
 
   AstValueFactory* ast_value_factory() const {
     return ast_value_factory_.get();

src/zone/zone.cc

@@ -70,9 +70,30 @@ void* Zone::AsanNew(size_t size) {
   return reinterpret_cast<void*>(result);
 }
 
-void Zone::ReleaseMemory() {
+void Zone::Reset() {
+  if (!segment_head_) return;
+  Segment* keep = segment_head_;
+  segment_head_ = segment_head_->next();
+  if (segment_head_ != nullptr) {
+    // Reset the position to the end of the new head, and uncommit its
+    // allocation size (which will be re-committed in DeleteAll).
+    position_ = segment_head_->end();
+    allocation_size_ -= segment_head_->end() - segment_head_->start();
+  }
+  keep->set_next(nullptr);
   DeleteAll();
   allocator_->TraceZoneCreation(this);
+
+  // Un-poison the kept segment content so we can zap and re-use it.
+  ASAN_UNPOISON_MEMORY_REGION(reinterpret_cast<void*>(keep->start()),
+                              keep->capacity());
+  keep->ZapContents();
+
+  segment_head_ = keep;
+  position_ = keep->start();
+  limit_ = keep->end();
+  DCHECK_EQ(allocation_size(), 0);
+  DCHECK_EQ(segment_bytes_allocated_, keep->total_size());
 }
 
 void Zone::DeleteAll() {

src/zone/zone.h

@@ -134,9 +134,10 @@ 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.