Bug 1559659 - 3. Get rid of per-thread jscontext instantiation and use the global pool instead r=jandem

Got rid of the per-thread JSContext created at the start of each thread. Tasks that require JSContext (ParseTasks, IonBuilder, Wasm tier 2 generators, GCParallel) now request an unused context to set to their thread. Tasks which do not use JSContext will not request one.

Differential Revision: https://phabricator.services.mozilla.com/D37495

--HG--
extra : moz-landing-system : lando

js/src/jit/IonBuilder.cpp

@@ -1127,6 +1127,7 @@ AbortReasonOr<Ok> IonBuilder::buildInline(IonBuilder* callerBuilder,
 void IonBuilder::runTask() {
   // This is the entry point when ion compiles are run offthread.
   JSRuntime* rt = script()->runtimeFromAnyThread();
+  AutoSetHelperThreadContext usesContext;
 
   TraceLoggerThread* logger = TraceLoggerForCurrentThread();
   TraceLoggerEvent event(TraceLogger_AnnotateScripts, script());

js/src/vm/HelperThreads.cpp

@@ -542,11 +542,14 @@ size_t ParseTask::sizeOfExcludingThis(
 }
 
 void ParseTask::runTask() {
+  AutoSetHelperThreadContext usesContext;
+
   JSContext* cx = TlsContext.get();
   JSRuntime* runtime = parseGlobal->runtimeFromAnyThread();
 
   AutoSetContextRuntime ascr(runtime);
   AutoSetContextParse parsetask(this);
+  gc::AutoSuppressNurseryCellAlloc noNurseryAlloc(cx);
 
   Zone* zone = parseGlobal->zoneFromAnyThread();
   zone->setHelperThreadOwnerContext(cx);
@@ -1764,11 +1767,12 @@ void js::GCParallelTask::runFromMainThread(JSRuntime* rt) {
 void js::GCParallelTask::runFromHelperThread(AutoLockHelperThreadState& lock) {
   MOZ_ASSERT(isDispatched(lock));
 
-  AutoSetContextRuntime ascr(runtime());
-  gc::AutoSetThreadIsPerformingGC performingGC;
-
   {
     AutoUnlockHelperThreadState parallelSection(lock);
+    AutoSetHelperThreadContext usesContext;
+    AutoSetContextRuntime ascr(runtime());
+    gc::AutoSetThreadIsPerformingGC performingGC;
+
     TimeStamp timeStart = ReallyNow();
     runTask();
     duration_ = TimeSince(timeStart);
@@ -2172,7 +2176,6 @@ void HelperThread::handleIonWorkload(AutoLockHelperThreadState& locked) {
 
   {
     AutoUnlockHelperThreadState unlock(locked);
-    AutoSetContextRuntime ascr(rt);
 
     builder->runTask();
   }
@@ -2540,22 +2543,9 @@ void HelperThread::threadLoop() {
 
   ensureRegisteredWithProfiler();
 
-  JSContext cx(nullptr, JS::ContextOptions());
-  {
-    AutoEnterOOMUnsafeRegion oomUnsafe;
-    cx.setThread();
-    if (!cx.init(ContextKind::HelperThread)) {
-      oomUnsafe.crash("HelperThread cx.init()");
-    }
-  }
-  gc::AutoSuppressNurseryCellAlloc noNurseryAlloc(&cx);
-  JS_SetNativeStackQuota(&cx, HELPER_STACK_QUOTA);
-
   while (!terminate) {
     MOZ_ASSERT(idle());
 
-    maybeFreeUnusedMemory(&cx);
-
     // The selectors may depend on the HelperThreadState not changing
     // between task selection and task execution, in particular, on new
     // tasks not being added (because of the lifo structure of the work

js/src/wasm/WasmModule.cpp

@@ -62,6 +62,7 @@ class Module::Tier2GeneratorTaskImpl : public Tier2GeneratorTask {
   void cancel() override { cancelled_ = true; }
 
   void runTask() override {
+    AutoSetHelperThreadContext usesContext;
     CompileTier2(*compileArgs_, bytecode_->bytes, *module_, &cancelled_);
   }
   ThreadType threadType() override {