FlowChecker: Use StackExecutor for cloning generic nodes

Summary: This mitigates the problem of cloning deep in the stack.

Reviewed By: tmikov

Differential Revision: D66383991

fbshipit-source-id: 821724baa8722b8c4283df94645efe1eff11c4b7

lib/Sema/FlowChecker.cpp

@@ -1949,19 +1949,21 @@ sema::Decl *FlowChecker::specializeGenericWithParsedTypes(
   };
 
   // Create specialization if it doesn't exist.
-  // TODO: Determine if the actual clone can be deferred to avoid potential
-  // stack overflow by cloning from deep in the AST.
   if (doClone) {
     LLVM_DEBUG(
         llvh::dbgs() << "Creating specialization for: " << oldDecl->name.str()
                      << "\n");
-    specialization = cloneNode(
-        astContext_,
-        semContext_,
-        declCollectorMap_,
-        generic.originalNode,
-        scope->parentFunction,
-        scope);
+    // Avoid stack overflow in the clone by executing in a separate stack.
+    executeInStack(
+        *stackExecutor_, [this, &generic, scope, &specialization]() -> void {
+          specialization = cloneNode(
+              astContext_,
+              semContext_,
+              declCollectorMap_,
+              generic.originalNode,
+              scope->parentFunction,
+              scope);
+        });
     if (!specialization) {
       sm_.error(
           typeArgsNode->getSourceRange(),

lib/Sema/FlowChecker.h

@@ -16,6 +16,7 @@
 #include "hermes/Sema/FlowContext.h"
 #include "hermes/Sema/Keywords.h"
 #include "hermes/Sema/SemContext.h"
+#include "hermes/Support/StackExecutor.h"
 
 namespace hermes {
 namespace flow {
@@ -73,6 +74,13 @@ class FlowChecker : public ESTree::RecursionDepthTracker<FlowChecker> {
   /// Keywords we will be checking for.
   sema::Keywords &kw_;
 
+  /// Use an 8MB stack, which is the default size on mac and linux.
+  static constexpr size_t kExecutorStackSize = 1 << 23;
+
+  /// The executor used for cloning generics.
+  std::shared_ptr<StackExecutor> stackExecutor_ =
+      newStackExecutor(kExecutorStackSize);
+
   struct TypeDecl {
     /// nullptr is used to indicate a generic declaration.
     Type *type;