[nci] Add a --isolate-script-cache-ageing flag

The NCI cache depends on the script cache (which is essentially a
SharedFunctionInfo cache, and the SFIs contain bytecode) to
deduplicate SFIs across native contexts. NCI caching does not work
without script caching. Thus the lifetimes of both caches should
match; in particular, disabling cache ageing for NCI is only effective
if script cache ageing is also disabled.

This CL adds an --isolate-script-cache-ageing flag. It's separate from
the --turbo-nci-cache-ageing flag s.t. we can disable script cache
ageing *without NCI* for benchmarking purposes.

Future work: Good ageing heuristics for both script and nci caches.

Bug: v8:8888
Change-Id: Ia5546feeced5821a538d97db1bb8f5bb92528114
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2452471
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70339}

src/codegen/compilation-cache.cc

@@ -72,7 +72,9 @@ void CompilationSubCache::AgeCustom(CompilationSubCache* c) {
   CompilationCacheTable::cast(c->tables_[0]).Age();
 }
 
-void CompilationCacheScript::Age() { AgeCustom(this); }
+void CompilationCacheScript::Age() {
+  if (FLAG_isolate_script_cache_ageing) AgeCustom(this);
+}
 void CompilationCacheEval::Age() { AgeCustom(this); }
 void CompilationCacheRegExp::Age() { AgeByGeneration(this); }
 void CompilationCacheCode::Age() {

src/flags/flag-definitions.h

@@ -748,6 +748,10 @@ DEFINE_BOOL(turbo_nci_delayed_codegen, true,
 // strategy.
 DEFINE_BOOL(turbo_nci_cache_ageing, false,
             "enable ageing of the NCI code cache.")
+// TODO(jgruber,v8:8888): Remove this flag once we've settled on an ageing
+// strategy.
+DEFINE_BOOL(isolate_script_cache_ageing, true,
+            "enable ageing of the isolate script cache.")
 
 // Favor memory over execution speed.
 DEFINE_BOOL(optimize_for_size, false,