Index: include/llvm/Transforms/Instrumentation.h
===================================================================
--- include/llvm/Transforms/Instrumentation.h
+++ include/llvm/Transforms/Instrumentation.h
@@ -88,6 +88,10 @@
 createPGOInstrumentationUseLegacyPass(StringRef Filename = StringRef(""));
 ModulePass *createPGOIndirectCallPromotionLegacyPass(bool InLTO = false);
 
+// Helper function to check if it is legal to promote indirect call \p Inst
+// to a direct call of function \p F. Stores the reason in \p Reason.
+bool isLegalToPromote(Instruction *Inst, Function *F, const char **Reason);
+
 // Helper function that transforms Inst (either an indirect-call instruction, or
 // an invoke instruction , to a conditional call to F. This is like:
 //     if (Inst.CalledValue == F)
Index: lib/Transforms/Instrumentation/IndirectCallPromotion.cpp
===================================================================
--- lib/Transforms/Instrumentation/IndirectCallPromotion.cpp
+++ lib/Transforms/Instrumentation/IndirectCallPromotion.cpp
@@ -144,17 +144,9 @@
   // defines.
   InstrProfSymtab *Symtab;
 
-  enum TargetStatus {
-    OK,                   // Should be able to promote.
-    NotAvailableInModule, // Cannot find the target in current module.
-    ReturnTypeMismatch,   // Return type mismatch b/w target and indirect-call.
-    NumArgsMismatch,      // Number of arguments does not match.
-    ArgTypeMismatch       // Type mismatch in the arguments (cannot bitcast).
-  };
-
   // Test if we can legally promote this direct-call of Target.
-  TargetStatus isPromotionLegal(Instruction *Inst, uint64_t Target,
-                                Function *&F);
+  bool isPromotionLegal(Instruction *Inst, uint64_t Target, Function *&F,
+                        const char **Reason = nullptr);
 
   // A struct that records the direct target and it's call count.
   struct PromotionCandidate {
@@ -178,22 +170,6 @@
                         const std::vector<PromotionCandidate> &Candidates,
                         uint64_t &TotalCount);
 
-  static const char *StatusToString(const TargetStatus S) {
-    switch (S) {
-    case OK:
-      return "OK to promote";
-    case NotAvailableInModule:
-      return "Cannot find the target";
-    case ReturnTypeMismatch:
-      return "Return type mismatch";
-    case NumArgsMismatch:
-      return "The number of arguments mismatch";
-    case ArgTypeMismatch:
-      return "Argument Type mismatch";
-    }
-    llvm_unreachable("Should not reach here");
-  }
-
   // Noncopyable
   ICallPromotionFunc(const ICallPromotionFunc &other) = delete;
   ICallPromotionFunc &operator=(const ICallPromotionFunc &other) = delete;
@@ -207,43 +183,58 @@
 };
 } // end anonymous namespace
 
-ICallPromotionFunc::TargetStatus
-ICallPromotionFunc::isPromotionLegal(Instruction *Inst, uint64_t Target,
-                                     Function *&TargetFunction) {
-  Function *DirectCallee = Symtab->getFunction(Target);
-  if (DirectCallee == nullptr)
-    return NotAvailableInModule;
+bool llvm::isLegalToPromote(Instruction *Inst, Function *F,
+                            const char **Reason) {
   // Check the return type.
   Type *CallRetType = Inst->getType();
   if (!CallRetType->isVoidTy()) {
-    Type *FuncRetType = DirectCallee->getReturnType();
+    Type *FuncRetType = F->getReturnType();
     if (FuncRetType != CallRetType &&
-        !CastInst::isBitCastable(FuncRetType, CallRetType))
-      return ReturnTypeMismatch;
+        !CastInst::isBitCastable(FuncRetType, CallRetType)) {
+      if (Reason)
+        *Reason = "Return type mismatch";
+      return false;
+    }
   }
 
   // Check if the arguments are compatible with the parameters
-  FunctionType *DirectCalleeType = DirectCallee->getFunctionType();
+  FunctionType *DirectCalleeType = F->getFunctionType();
   unsigned ParamNum = DirectCalleeType->getFunctionNumParams();
   CallSite CS(Inst);
   unsigned ArgNum = CS.arg_size();
 
-  if (ParamNum != ArgNum && !DirectCalleeType->isVarArg())
-    return NumArgsMismatch;
+  if (ParamNum != ArgNum && !DirectCalleeType->isVarArg()) {
+    if (Reason)
+      *Reason = "The number of arguments mismatch";
+    return false;
+  }
 
   for (unsigned I = 0; I < ParamNum; ++I) {
     Type *PTy = DirectCalleeType->getFunctionParamType(I);
     Type *ATy = CS.getArgument(I)->getType();
     if (PTy == ATy)
       continue;
-    if (!CastInst::castIsValid(Instruction::BitCast, CS.getArgument(I), PTy))
-      return ArgTypeMismatch;
+    if (!CastInst::castIsValid(Instruction::BitCast, CS.getArgument(I), PTy)) {
+      if (Reason)
+        return "Argument Type mismatch";
+      return false;
+    }
   }
 
   DEBUG(dbgs() << " #" << NumOfPGOICallPromotion << " Promote the icall to "
-               << Symtab->getFuncName(Target) << "\n");
-  TargetFunction = DirectCallee;
-  return OK;
+               << F->getName() << "\n");
+  return true;
+}
+
+bool ICallPromotionFunc::isPromotionLegal(Instruction *Inst, uint64_t Target,
+                                          Function *&TargetFunction,
+                                          const char **Reason) {
+  TargetFunction = Symtab->getFunction(Target);
+  if (TargetFunction == nullptr) {
+    *Reason = "Cannot find the target";
+    return false;
+  }
+  return isLegalToPromote(Inst, TargetFunction, Reason);
 }
 
 // Indirect-call promotion heuristic. The direct targets are sorted based on
@@ -283,10 +274,9 @@
       break;
     }
     Function *TargetFunction = nullptr;
-    TargetStatus Status = isPromotionLegal(Inst, Target, TargetFunction);
-    if (Status != OK) {
+    const char *Reason = nullptr;
+    if (!isPromotionLegal(Inst, Target, TargetFunction, &Reason)) {
       StringRef TargetFuncName = Symtab->getFuncName(Target);
-      const char *Reason = StatusToString(Status);
       DEBUG(dbgs() << " Not promote: " << Reason << "\n");
       emitOptimizationRemarkMissed(
           F.getContext(), "pgo-icall-prom", F, Inst->getDebugLoc(),