Avoid running lowerIR multiple times.

Summary:
When calling with different values of `SegmentRange`, we don't need
to rerun the whole `lowerIR` pipeline.
Store a flag in the `Module` and check it before running `lowerIR`.

Reviewed By: dulinriley

Differential Revision: D16766752

fbshipit-source-id: f9cb358f36a5a16f1dc6355980ce82eb31b9af8b

include/hermes/IR/IR.h

@@ -1743,6 +1743,9 @@ class Module : public Value {
   std::unordered_map<RawStringList, uint32_t, HashRawStrings>
       templateObjectIDMap_;
 
+  /// Set to true when lowerIR has been called on this Module.
+  bool isLowered_{false};
+
  public:
   explicit Module(std::shared_ptr<Context> ctx)
       : Value(ValueKind::ModuleKind), Ctx(std::move(ctx)) {}
@@ -1894,6 +1897,14 @@ class Module : public Value {
   /// Given a list of raw strings from a template literal, get its unique id.
   uint32_t getTemplateObjectID(RawStringList &&rawStrings);
 
+  bool isLowered() const {
+    return isLowered_;
+  }
+
+  void setLowered(bool isLowered) {
+    isLowered_ = isLowered;
+  }
+
   inline iterator begin() {
     return FunctionList.begin();
   }

lib/BCGen/HBC/HBC.cpp

@@ -47,6 +47,9 @@ const unsigned kFastRegisterAllocationThreshold = 250;
 const uint64_t kRegisterAllocationMemoryLimit = 10L * 1024 * 1024;
 
 void lowerIR(Module *M, const BytecodeGenerationOptions &options) {
+  if (M->isLowered())
+    return;
+
   PassManager PM;
   PM.addPass(new LowerLoadStoreFrameInst());
   if (options.optimizationEnabled) {
@@ -89,6 +92,7 @@ void lowerIR(Module *M, const BytecodeGenerationOptions &options) {
   PM.addHoistStartGenerator();
 
   PM.run(M);
+  M->setLowered(true);
 
   if (options.verifyIR &&
       verifyModule(*M, &llvm::errs(), VerificationMode::IR_VALID)) {