[nci] Add flags to configure NCI codegen and ageing

Codegen timing and cache ageing are two important parameters for NCI
performance. This adds runtime flags for them:

 --turbo-nci-cache-ageing (default: false)
 --turbo-nci-delayed-codegen (default: true)

Note the behavioral change of disabling ageing by default for now.

Bug: v8:8888
Change-Id: Id9611185566f5c4828ad48e58c42424833d3323b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2438456
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70303}

src/codegen/compilation-cache.cc

@@ -76,8 +76,10 @@ void CompilationCacheScript::Age() { AgeCustom(this); }
 void CompilationCacheEval::Age() { AgeCustom(this); }
 void CompilationCacheRegExp::Age() { AgeByGeneration(this); }
 void CompilationCacheCode::Age() {
-  if (FLAG_trace_turbo_nci) CompilationCacheCode::TraceAgeing();
-  AgeByGeneration(this);
+  if (FLAG_turbo_nci_cache_ageing) {
+    if (FLAG_trace_turbo_nci) CompilationCacheCode::TraceAgeing();
+    AgeByGeneration(this);
+  }
 }
 
 void CompilationSubCache::Iterate(RootVisitor* v) {

src/flags/flag-definitions.h

@@ -740,6 +740,14 @@ DEFINE_BOOL(print_nci_code, false, "print native context independent code.")
 DEFINE_BOOL(trace_turbo_nci, false, "trace native context independent code.")
 DEFINE_BOOL(turbo_collect_feedback_in_generic_lowering, true,
             "enable experimental feedback collection in generic lowering.")
+// TODO(jgruber,v8:8888): Remove this flag once we've settled on a codegen
+// strategy.
+DEFINE_BOOL(turbo_nci_delayed_codegen, true,
+            "delay NCI codegen to reduce useless compilation work.")
+// TODO(jgruber,v8:8888): Remove this flag once we've settled on an ageing
+// strategy.
+DEFINE_BOOL(turbo_nci_cache_ageing, false,
+            "enable ageing of the NCI code cache.")
 
 // Favor memory over execution speed.
 DEFINE_BOOL(optimize_for_size, false,

src/runtime/runtime-compiler.cc

@@ -78,12 +78,13 @@ Object CompileOptimized(Isolate* isolate, Handle<JSFunction> function,
 
   // Possibly compile for NCI caching.
   if (MaybeSpawnNativeContextIndependentCompilationJob()) {
-    // The first optimization request does not trigger NCI compilation,
-    // since we try to avoid compiling Code that remains unused in the future.
-    // Repeated optimization (possibly in different native contexts) is taken
-    // as a signal that this SFI will continue to be used in the future, thus
-    // we trigger NCI compilation.
-    if (function->shared().has_optimized_at_least_once()) {
+    // If delayed codegen is enabled, the first optimization request does not
+    // trigger NCI compilation, since we try to avoid compiling Code that
+    // remains unused in the future.  Repeated optimization (possibly in
+    // different native contexts) is taken as a signal that this SFI will
+    // continue to be used in the future, thus we trigger NCI compilation.
+    if (!FLAG_turbo_nci_delayed_codegen ||
+        function->shared().has_optimized_at_least_once()) {
       if (!Compiler::CompileOptimized(function, mode,
                                       CodeKind::NATIVE_CONTEXT_INDEPENDENT)) {
         return ReadOnlyRoots(isolate).exception();