Bug 1715759 - Add a pref to control allocation site based pretrening r=jandem

This doesn't change the state of this feature which currently enabled, but will
allow us to disable it easily if necessary.

Differential Revision: https://phabricator.services.mozilla.com/D117441

js/public/GCAPI.h

@@ -913,6 +913,12 @@ class JS_PUBLIC_API AutoDisableGenerationalGC {
  */
 extern JS_PUBLIC_API bool IsGenerationalGCEnabled(JSRuntime* rt);
 
+/**
+ * Enable or disable support for pretenuring allocations based on their
+ * allocation site.
+ */
+extern JS_PUBLIC_API void SetSiteBasedPretenuringEnabled(bool enable);
+
 /**
  * Pass a subclass of this "abstract" class to callees to require that they
  * never GC. Subclasses can use assertions or the hazard analysis to ensure no

js/src/gc/Pretenuring.cpp

@@ -55,9 +55,16 @@ static constexpr size_t HighNurserySurvivalCountBeforeRecovery = 2;
 
 AllocSite* const AllocSite::EndSentinel = reinterpret_cast<AllocSite*>(1);
 
+static bool SiteBasedPretenuringEnabled = true;
+
+JS_PUBLIC_API void JS::SetSiteBasedPretenuringEnabled(bool enable) {
+  SiteBasedPretenuringEnabled = enable;
+}
+
 bool PretenuringNursery::canCreateAllocSite() {
   MOZ_ASSERT(allocSitesCreated <= MaxAllocSitesPerMinorGC);
-  return allocSitesCreated < MaxAllocSitesPerMinorGC;
+  return SiteBasedPretenuringEnabled &&
+         allocSitesCreated < MaxAllocSitesPerMinorGC;
 }
 
 size_t PretenuringNursery::doPretenuring(GCRuntime* gc, bool validPromotionRate,

js/xpconnect/src/XPCJSContext.cpp

@@ -910,6 +910,10 @@ static void LoadStartupJSPrefs(XPCJSContext* xpccx) {
 
   JS::SetLargeArrayBuffersEnabled(
       StaticPrefs::javascript_options_large_arraybuffers_DoNotUseDirectly());
+
+  JS::SetSiteBasedPretenuringEnabled(
+      StaticPrefs::
+          javascript_options_site_based_pretenuring_DoNotUseDirectly());
 }
 
 static void ReloadPrefsCallback(const char* pref, void* aXpccx) {

modules/libpref/init/StaticPrefList.yaml

@@ -5969,6 +5969,13 @@
   mirror: always  # LoadStartupJSPrefs
   do_not_use_directly: true
 
+# Support for pretenuring allocations based on their allocation site.
+- name: javascript.options.site_based_pretenuring
+  type: bool
+  value: true
+  mirror: always  # LoadStartupJSPrefs
+  do_not_use_directly: true
+
 #if !defined(JS_CODEGEN_MIPS32) && !defined(JS_CODEGEN_MIPS64)
   # Spectre security vulnerability mitigations for the JS JITs.
   #