Try to use the CompileOptions::usePinnedBytecode option for ScriptPreloader since that mmap file is alive for as long as the runtime. This should reduce Base Content JS overhead.

https://hg.mozilla.org/mozilla-central/rev/a75f9be5cd6e

Backed out for causing talos crashes @ js::InternalCallOrConstruct

Backout link

Push with failures

Failure log

This accidentally enabled bytecode pinning also for StartupCache, because the same option is used for both StartupCache and ScriptPreloader when reading.
StartupCache has cache invalidation, and we cannot use bytecode pinning.
we should enable bytecode pinning only on ScriptPreloader cache.

also there's lifetime issue around shutdown, that ccov hit
we need to make sure the buffer is freed only after the JS is fully shutdown

Backout merged: https://hg.mozilla.org/mozilla-central/rev/9e567d4a8c18

ScriptPreloader singleton is freed on ShutdownPhase::XPCOMShutdownFinal phase,
and the mmap-ed file is munmap-ed there.

https://searchfox.org/mozilla-central/rev/a2118a17fe5e4f63d9f011db3111beb995b96a91/js/xpconnect/loader/ScriptPreloader.cpp#108

ScriptPreloader& ScriptPreloader::GetSingleton() {
  static RefPtr<ScriptPreloader> singleton;

  if (!singleton) {
    if (XRE_IsParentProcess()) {
      singleton = new ScriptPreloader();
      singleton->mChildCache = &GetChildSingleton();
      Unused << singleton->InitCache();
    } else {
      singleton = &GetChildSingleton();
    }

    ClearOnShutdown(&singleton);
  }

  return *singleton;
}

ScriptPreloader& ScriptPreloader::GetChildSingleton() {
  static RefPtr<ScriptPreloader> singleton;

  if (!singleton) {
    singleton = new ScriptPreloader();
    if (XRE_IsParentProcess()) {
      Unused << singleton->InitCache(u"scriptCache-child"_ns);
    }
    ClearOnShutdown(&singleton);
  }

  return *singleton;
}

https://searchfox.org/mozilla-central/rev/a2118a17fe5e4f63d9f011db3111beb995b96a91/xpcom/base/ClearOnShutdown.h#110-119

template <class SmartPtr>
inline void ClearOnShutdown(
    SmartPtr* aPtr, ShutdownPhase aPhase = ShutdownPhase::XPCOMShutdownFinal) {
  using namespace ClearOnShutdown_Internal;

  MOZ_ASSERT(NS_IsMainThread());
  MOZ_ASSERT(aPhase != ShutdownPhase::ShutdownPhase_Length);

  InsertIntoShutdownList(new PointerClearer<SmartPtr>(aPtr), aPhase);
}

that happens before JS_ShutDown.

https://searchfox.org/mozilla-central/rev/a2118a17fe5e4f63d9f011db3111beb995b96a91/xpcom/build/XPCOMInit.cpp#650-734

nsresult ShutdownXPCOM(nsIServiceManager* aServMgr) {
...
  // Notify observers of xpcom shutting down
  {
...
    mozilla::KillClearOnShutdown(ShutdownPhase::XPCOMShutdownFinal);
...
  }
...
  nsCycleCollector_shutdown(shutdownCollect);
...
  if (sInitializedJS) {
    // Shut down the JS engine.
    JS_ShutDown();
    sInitializedJS = false;
  }

ccov's failure happens in GC in nsCycleCollector_shutdown.

ScriptPreloader must be live longer than JS_ShutDown, that means we need to have ShutdownPhase after that.

Just moving the ScriptPreloader shutdown after JS_ShutDown causes another crash around SharedImmutableStringsCache
https://treeherder.mozilla.org/jobs?repo=try&group_state=expanded&revision=78d72c7a97bc8ceb661b7c65e1cc403de6847958&selectedTaskRun=DIiwUXxpReG5GkprStm8lg.0

there seems to be another dependency

Depends on D126632

mmap-ed file needs to be live longer than JS runtime, and it's different than
ScriptPreloader's lifetime.

Depends on D126633

https://hg.mozilla.org/mozilla-central/rev/12ed363393af
https://hg.mozilla.org/mozilla-central/rev/1fcebc384393
https://hg.mozilla.org/mozilla-central/rev/8b2fb3f5ed01
https://hg.mozilla.org/mozilla-central/rev/8bbe0638c3b7