Add return instruction to functions without it

LLVM does not legally require a return instruction in a function.
However, return is needed by IGC BE to properly generate return code for control flow.
This change will add a return (if it does not exist) as the last instruction of the function.

Author: dlei6g

IGC/AdaptorCommon/LegalizeFunctionSignatures.cpp

@@ -80,6 +80,8 @@ bool LegalizeFunctionSignatures::runOnModule(Module& M)
 {
     auto pMdUtils = getAnalysis<MetaDataUtilsWrapper>().getMetaDataUtils();
 
+    // Add an unreachable return for functions without the return instruction
+    FixNoReturn(M);
     // Creates a new function declaration with modified signature
     FixFunctionSignatures(M);
     // Transforms all callers of the old function to the new function
@@ -665,3 +667,34 @@ void LegalizeFunctionSignatures::FixCallInstruction(Module& M, CallInst* callIns
         callInst->eraseFromParent();
     }
 }
+
+void LegalizeFunctionSignatures::FixNoReturn(Module& M)
+{
+    for (auto& FI : M)
+    {
+        if (!FI.empty())
+        {
+            // Search each BB for the ReturnInst
+            BasicBlock* lastBB = nullptr;
+            bool hasRetInst = false;
+            for (auto& BB : FI)
+            {
+                if (isa<ReturnInst>(BB.getTerminator()))
+                {
+                    hasRetInst = true;
+                    break;
+                }
+                lastBB = &BB;
+            }
+            // Insert an unreachable ReturnInst in the last BB
+            if (!hasRetInst)
+            {
+                IGCLLVM::IRBuilder<> builder(lastBB);
+                ReturnInst* dummyRet = FI.getReturnType()->isVoidTy() ?
+                    builder.CreateRetVoid() :
+                    builder.CreateRet(UndefValue::get(FI.getReturnType()));
+                dummyRet->moveAfter(lastBB->getTerminator());
+            }
+        }
+    }
+}

IGC/AdaptorCommon/LegalizeFunctionSignatures.h

@@ -40,6 +40,7 @@ class LegalizeFunctionSignatures : public llvm::ModulePass
     void FixFunctionBody(llvm::Module& M);
     void FixFunctionUsers(llvm::Module& M);
     void FixCallInstruction(llvm::Module& M, llvm::CallInst* callInst);
+    void FixNoReturn(llvm::Module& M);
 
     virtual llvm::StringRef getPassName() const
     {