[compiler] Use SmallVector for Parser->BG thread use counts

Change the off-thread parse to fill a SmallVector<UseCounterFeature, 8>
on the BG compile task, rather than an int[kUseCounterFeatureCount]
array. This allows us to keep the loop over use counts in the compile
task finalization short by avoiding looping over unused counters.

The value 8 was chosen as a "reasonable small number"; experimenting on
our benchmarks shows a max of 3 use counts collected per compile (and
at a vanishingly low percentage of all compiles).

Passing around an explicit SmallVector<UseCounterFeature, 8> pointer,
complete with size, is a bit ugly, but since it's used only in this one
place (Parser -> BackgroundCompileTask) I can live with it to avoid
further indirections. Typedeffing it is possible, but it's not clear
where, since it's needed in both src/codegen/compiler.h and
src/parsing/parser.h, and neither includes the other.

Change-Id: Idb73e2f56fa9e8911ea29fb810d7562246f19d46
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3318662Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78305}

src/codegen/compiler.cc

@@ -1547,7 +1547,7 @@ void BackgroundCompileTask::Run(
 
   parser.ParseOnBackground(isolate, &info, start_position_, end_position_,
                            function_literal_id_);
-  parser.UpdateStatistics(script_, use_counts_, &total_preparse_skipped_);
+  parser.UpdateStatistics(script_, &use_counts_, &total_preparse_skipped_);
 
   // Save the language mode.
   language_mode_ = info.language_mode();
@@ -1691,14 +1691,13 @@ void BackgroundCompileTask::AbortFunction() {
 
 void BackgroundCompileTask::ReportStatistics(Isolate* isolate) {
   // Update use-counts.
-  for (int i = 0; i < static_cast<int>(v8::Isolate::kUseCounterFeatureCount);
-       ++i) {
-    v8::Isolate::UseCounterFeature feature =
-        static_cast<v8::Isolate::UseCounterFeature>(i);
-    isolate->CountUsage(feature, use_counts_[i]);
+  for (auto feature : use_counts_) {
+    isolate->CountUsage(feature);
   }
+  if (total_preparse_skipped_ > 0) {
     isolate->counters()->total_preparse_skipped()->Increment(
         total_preparse_skipped_);
+  }
 }
 
 BackgroundDeserializeTask::BackgroundDeserializeTask(

src/codegen/compiler.h

@@ -10,6 +10,7 @@
 
 #include "src/ast/ast-value-factory.h"
 #include "src/base/platform/elapsed-timer.h"
+#include "src/base/small-vector.h"
 #include "src/codegen/bailout-reason.h"
 #include "src/common/globals.h"
 #include "src/execution/isolate.h"
@@ -552,7 +553,7 @@ class V8_EXPORT_PRIVATE BackgroundCompileTask {
   IsCompiledScope is_compiled_scope_;
   FinalizeUnoptimizedCompilationDataList finalize_unoptimized_compilation_data_;
   DeferredFinalizationJobDataList jobs_to_retry_finalization_on_main_thread_;
-  int use_counts_[v8::Isolate::kUseCounterFeatureCount] = {0};
+  base::SmallVector<v8::Isolate::UseCounterFeature, 8> use_counts_;
   int total_preparse_skipped_ = 0;
 
   // Single function data for top-level function compilation.

src/parsing/parser.cc

@@ -3370,19 +3370,21 @@ void Parser::UpdateStatistics(Isolate* isolate, Handle<Script> script) {
       total_preparse_skipped_);
 }
 
-void Parser::UpdateStatistics(Handle<Script> script, int* use_counts,
+void Parser::UpdateStatistics(
+    Handle<Script> script,
+    base::SmallVector<v8::Isolate::UseCounterFeature, 8>* use_counts,
     int* preparse_skipped) {
   // Move statistics to Isolate.
   for (int feature = 0; feature < v8::Isolate::kUseCounterFeatureCount;
        ++feature) {
     if (use_counts_[feature] > 0) {
-      use_counts[feature]++;
+      use_counts->emplace_back(v8::Isolate::UseCounterFeature(feature));
     }
   }
   if (scanner_.FoundHtmlComment()) {
-    use_counts[v8::Isolate::kHtmlComment]++;
+    use_counts->emplace_back(v8::Isolate::kHtmlComment);
     if (script->line_offset() == 0 && script->column_offset() == 0) {
-      use_counts[v8::Isolate::kHtmlCommentInExternalScript]++;
+      use_counts->emplace_back(v8::Isolate::kHtmlCommentInExternalScript);
     }
   }
   *preparse_skipped = total_preparse_skipped_;

src/parsing/parser.h

@@ -12,6 +12,7 @@
 #include "src/ast/ast.h"
 #include "src/ast/scopes.h"
 #include "src/base/compiler-specific.h"
+#include "src/base/small-vector.h"
 #include "src/base/threaded-list.h"
 #include "src/common/globals.h"
 #include "src/parsing/import-assertions.h"
@@ -163,7 +164,9 @@ class V8_EXPORT_PRIVATE Parser : public NON_EXPORTED_BASE(ParserBase<Parser>) {
 
   // Move statistics to Isolate
   void UpdateStatistics(Isolate* isolate, Handle<Script> script);
-  void UpdateStatistics(Handle<Script> script, int* use_counters,
+  void UpdateStatistics(
+      Handle<Script> script,
+      base::SmallVector<v8::Isolate::UseCounterFeature, 8>* use_counters,
       int* preparse_skipped);
   template <typename IsolateT>
   void HandleSourceURLComments(IsolateT* isolate, Handle<Script> script);