[base] Apply empty base class optimization to base::TemplateHashMapImpl

... this reduces the size of VariableMap object by one word which in
turn reduces zone memory pressure. The Scope class which contains
VariableMap as a field is usually in top 5 of all allocated objects
in zone memory.

Bug: v8:9923
Change-Id: I79c6bd9ae97db72f24b831fd5e3733d8d7e4c0fd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2300486
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68896}

src/ast/scopes.cc

@@ -21,6 +21,7 @@
 #include "src/parsing/parser.h"
 #include "src/parsing/preparse-data.h"
 #include "src/zone/zone-list-inl.h"
+#include "src/zone/zone.h"
 
 namespace v8 {
 namespace internal {
@@ -34,6 +35,10 @@ namespace internal {
 //       use. Because a Variable holding a handle with the same location exists
 //       this is ensured.
 
+static_assert(sizeof(VariableMap) == (sizeof(void*) + 2 * sizeof(uint32_t) +
+                                      sizeof(ZoneAllocationPolicy)),
+              "Empty base optimization didn't kick in for VariableMap");
+
 VariableMap::VariableMap(Zone* zone)
     : ZoneHashMap(8, ZoneAllocationPolicy(zone)) {}
 

src/zone/zone.h

@@ -192,7 +192,7 @@ class ZoneObject {
 
 // The ZoneAllocationPolicy is used to specialize generic data
 // structures to allocate themselves and their elements in the Zone.
-class ZoneAllocationPolicy final {
+class ZoneAllocationPolicy {
  public:
   // Creates unusable allocation policy.
   ZoneAllocationPolicy() : zone_(nullptr) {}