Support JIT on a separate thread

src/Reactor/LLVMReactor.cpp

@@ -632,6 +632,9 @@ Config Nucleus::getDefaultConfig()
 
 std::shared_ptr<Routine> Nucleus::acquireRoutine(const char *name, const Config::Edit &cfgEdit /* = Config::Edit::None */)
 {
+	std::shared_ptr<Routine> routine;
+
+	auto acquire = [&]() {
 		auto cfg = cfgEdit.apply(jit->config);
 
 		if(jit->builder->GetInsertBlock()->empty() || !jit->builder->GetInsertBlock()->back().isTerminator())
@@ -679,8 +682,20 @@ std::shared_ptr<Routine> Nucleus::acquireRoutine(const char *name, const Config:
 			jit->module->print(file, 0);
 		}
 
-	auto routine = jit->acquireRoutine(&jit->function, 1, cfg);
+		routine = jit->acquireRoutine(&jit->function, 1, cfg);
 		jit.reset();
+	};
+
+#ifdef JIT_IN_SEPARATE_THREAD
+	// Perform optimizations and codegen in a separate thread to avoid stack overflow.
+	// FIXME(b/149829034): This is not a long-term solution. Reactor has no control
+	// over the threading and stack sizes of its users, so this should be addressed
+	// at a higher level instead.
+	std::thread thread(acquire);
+	thread.join();
+#else
+	acquire();
+#endif
 
 	return routine;
 }