issue/461 make current runtime thread local (#503)

Author: PanZezhong1725

src/infinicore/context/context_impl.cc

@@ -4,6 +4,8 @@
 
 namespace infinicore {
 
+thread_local Runtime *ContextImpl::current_runtime_ = nullptr;
+
 Runtime *ContextImpl::getCurrentRuntime() {
     return current_runtime_;
 }

src/infinicore/context/context_impl.hpp

@@ -8,8 +8,10 @@
 namespace infinicore {
 class ContextImpl {
 private:
+    // Table of runtimes for every device (type and index)
     std::array<std::vector<std::unique_ptr<Runtime>>, size_t(Device::Type::COUNT)> runtime_table_;
-    Runtime *current_runtime_ = nullptr;
+    // Active runtime for current thread. Can use "static thread local" because context is a process singleton.
+    static thread_local Runtime *current_runtime_;
 
 protected:
     ContextImpl();