diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -1016,8 +1016,8 @@
   }
 }
 
-void CodeGenFunction::ExpandTypeFromArgs(
-    QualType Ty, LValue LV, SmallVectorImpl<llvm::Value *>::iterator &AI) {
+void CodeGenFunction::ExpandTypeFromArgs(QualType Ty, LValue LV,
+                                         llvm::Function::arg_iterator &AI) {
   assert(LV.isSimple() &&
          "Unexpected non-simple lvalue during struct expansion.");
 
@@ -1046,17 +1046,17 @@
       ExpandTypeFromArgs(FD->getType(), SubLV, AI);
     }
   } else if (isa<ComplexExpansion>(Exp.get())) {
-    auto realValue = *AI++;
-    auto imagValue = *AI++;
+    auto realValue = &*AI++;
+    auto imagValue = &*AI++;
     EmitStoreOfComplex(ComplexPairTy(realValue, imagValue), LV, /*init*/ true);
   } else {
     // Call EmitStoreOfScalar except when the lvalue is a bitfield to emit a
     // primitive store.
     assert(isa<NoExpansion>(Exp.get()));
     if (LV.isBitField())
-      EmitStoreThroughLValue(RValue::get(*AI++), LV);
+      EmitStoreThroughLValue(RValue::get(&*AI++), LV);
     else
-      EmitStoreOfScalar(*AI++, LV);
+      EmitStoreOfScalar(&*AI++, LV);
   }
 }
 
@@ -2323,19 +2323,13 @@
   // simplify.
 
   ClangToLLVMArgMapping IRFunctionArgs(CGM.getContext(), FI);
-  // Flattened function arguments.
-  SmallVector<llvm::Value *, 16> FnArgs;
-  FnArgs.reserve(IRFunctionArgs.totalIRArgs());
-  for (auto &Arg : Fn->args()) {
-    FnArgs.push_back(&Arg);
-  }
-  assert(FnArgs.size() == IRFunctionArgs.totalIRArgs());
+  assert(Fn->arg_size() == IRFunctionArgs.totalIRArgs());
 
   // If we're using inalloca, all the memory arguments are GEPs off of the last
   // parameter, which is a pointer to the complete memory area.
   Address ArgStruct = Address::invalid();
   if (IRFunctionArgs.hasInallocaArg()) {
-    ArgStruct = Address(FnArgs[IRFunctionArgs.getInallocaArgNo()],
+    ArgStruct = Address(Fn->getArg(IRFunctionArgs.getInallocaArgNo()),
                         FI.getArgStructAlignment());
 
     assert(ArgStruct.getType() == FI.getArgStruct()->getPointerTo());
@@ -2343,7 +2337,7 @@
 
   // Name the struct return parameter.
   if (IRFunctionArgs.hasSRetArg()) {
-    auto AI = cast<llvm::Argument>(FnArgs[IRFunctionArgs.getSRetArgNo()]);
+    auto AI = Fn->getArg(IRFunctionArgs.getSRetArgNo());
     AI->setName("agg.result");
     AI->addAttr(llvm::Attribute::NoAlias);
   }
@@ -2394,7 +2388,8 @@
 
     case ABIArgInfo::Indirect: {
       assert(NumIRArgs == 1);
-      Address ParamAddr = Address(FnArgs[FirstIRArg], ArgI.getIndirectAlign());
+      Address ParamAddr =
+          Address(Fn->getArg(FirstIRArg), ArgI.getIndirectAlign());
 
       if (!hasScalarEvaluationKind(Ty)) {
         // Aggregates and complex variables are accessed by reference.  All we
@@ -2436,8 +2431,7 @@
           ArgI.getCoerceToType() == ConvertType(Ty) &&
           ArgI.getDirectOffset() == 0) {
         assert(NumIRArgs == 1);
-        llvm::Value *V = FnArgs[FirstIRArg];
-        auto AI = cast<llvm::Argument>(V);
+        auto AI = Fn->getArg(FirstIRArg);
 
         if (const ParmVarDecl *PVD = dyn_cast<ParmVarDecl>(Arg)) {
           if (getNonNullAttr(CurCodeDecl, PVD, PVD->getType(),
@@ -2499,6 +2493,7 @@
 
         // LLVM expects swifterror parameters to be used in very restricted
         // ways.  Copy the value into a less-restricted temporary.
+        llvm::Value *V = AI;
         if (FI.getExtParameterInfo(ArgNo).getABI()
               == ParameterABI::SwiftErrorResult) {
           QualType pointeeTy = Ty->getPointeeType();
@@ -2560,7 +2555,7 @@
 
         assert(STy->getNumElements() == NumIRArgs);
         for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
-          auto AI = FnArgs[FirstIRArg + i];
+          auto AI = Fn->getArg(FirstIRArg + i);
           AI->setName(Arg->getName() + ".coerce" + Twine(i));
           Address EltPtr = Builder.CreateStructGEP(AddrToStoreInto, i);
           Builder.CreateStore(AI, EltPtr);
@@ -2573,7 +2568,7 @@
       } else {
         // Simple case, just do a coerced store of the argument into the alloca.
         assert(NumIRArgs == 1);
-        auto AI = FnArgs[FirstIRArg];
+        auto AI = Fn->getArg(FirstIRArg);
         AI->setName(Arg->getName() + ".coerce");
         CreateCoercedStore(AI, Ptr, /*DstIsVolatile=*/false, *this);
       }
@@ -2606,7 +2601,7 @@
           continue;
 
         auto eltAddr = Builder.CreateStructGEP(alloca, i);
-        auto elt = FnArgs[argIndex++];
+        auto elt = Fn->getArg(argIndex++);
         Builder.CreateStore(elt, eltAddr);
       }
       assert(argIndex == FirstIRArg + NumIRArgs);
@@ -2621,11 +2616,11 @@
       LValue LV = MakeAddrLValue(Alloca, Ty);
       ArgVals.push_back(ParamValue::forIndirect(Alloca));
 
-      auto FnArgIter = FnArgs.begin() + FirstIRArg;
+      auto FnArgIter = Fn->arg_begin() + FirstIRArg;
       ExpandTypeFromArgs(Ty, LV, FnArgIter);
-      assert(FnArgIter == FnArgs.begin() + FirstIRArg + NumIRArgs);
+      assert(FnArgIter == Fn->arg_begin() + FirstIRArg + NumIRArgs);
       for (unsigned i = 0, e = NumIRArgs; i != e; ++i) {
-        auto AI = FnArgs[FirstIRArg + i];
+        auto AI = Fn->getArg(FirstIRArg + i);
         AI->setName(Arg->getName() + "." + Twine(i));
       }
       break;
diff --git a/clang/lib/CodeGen/CodeGenFunction.h b/clang/lib/CodeGen/CodeGenFunction.h
--- a/clang/lib/CodeGen/CodeGenFunction.h
+++ b/clang/lib/CodeGen/CodeGenFunction.h
@@ -4355,7 +4355,7 @@
   ///
   /// \param AI - The first function argument of the expansion.
   void ExpandTypeFromArgs(QualType Ty, LValue Dst,
-                          SmallVectorImpl<llvm::Value *>::iterator &AI);
+                          llvm::Function::arg_iterator &AI);
 
   /// ExpandTypeToArgs - Expand an CallArg \arg Arg, with the LLVM type for \arg
   /// Ty, into individual arguments on the provided vector \arg IRCallArgs,