Index: llvm/include/llvm/IR/InstrTypes.h
===================================================================
--- llvm/include/llvm/IR/InstrTypes.h
+++ llvm/include/llvm/IR/InstrTypes.h
@@ -1185,6 +1185,14 @@
   /// Return true if the callsite is an indirect call.
   bool isIndirectCall() const;
 
+  /// Determine whether the passed iterator points to the callee operand's Use.
+  bool isCallee(Value::const_user_iterator UI) const {
+    return isCallee(&UI.getUse());
+  }
+
+  /// Determine whether this Use is the callee operand's Use.
+  bool isCallee(const Use *U) const { return &getCalledOperandUse() == U; }
+
   /// Helper to get the caller (the parent function).
   Function *getCaller();
   const Function *getCaller() const {
Index: llvm/include/llvm/IR/PatternMatch.h
===================================================================
--- llvm/include/llvm/IR/PatternMatch.h
+++ llvm/include/llvm/IR/PatternMatch.h
@@ -31,7 +31,6 @@
 
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/APInt.h"
-#include "llvm/IR/CallSite.h"
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/InstrTypes.h"
@@ -1486,8 +1485,9 @@
   Argument_match(unsigned OpIdx, const Opnd_t &V) : OpI(OpIdx), Val(V) {}
 
   template <typename OpTy> bool match(OpTy *V) {
-    CallSite CS(V);
-    return CS.isCall() && Val.match(CS.getArgument(OpI));
+    // FIXME: Should likely be switched to use `CallBase`.
+    auto *CI = dyn_cast<CallInst>(V);
+    return CI && Val.match(CI->getArgOperand(OpI));
   }
 };
 
Index: llvm/lib/IR/AsmWriter.cpp
===================================================================
--- llvm/lib/IR/AsmWriter.cpp
+++ llvm/lib/IR/AsmWriter.cpp
@@ -36,7 +36,6 @@
 #include "llvm/IR/Attributes.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/CFG.h"
-#include "llvm/IR/CallSite.h"
 #include "llvm/IR/CallingConv.h"
 #include "llvm/IR/Comdat.h"
 #include "llvm/IR/Constant.h"
@@ -998,9 +997,9 @@
 
       // We allow direct calls to any llvm.foo function here, because the
       // target may not be linked into the optimizer.
-      if (auto CS = ImmutableCallSite(&I)) {
+      if (const auto *Call = dyn_cast<CallBase>(&I)) {
         // Add all the call attributes to the table.
-        AttributeSet Attrs = CS.getAttributes().getFnAttributes();
+        AttributeSet Attrs = Call->getAttributes().getFnAttributes();
         if (Attrs.hasAttributes())
           CreateAttributeSetSlot(Attrs);
       }
@@ -2359,7 +2358,7 @@
 
   void writeOperand(const Value *Op, bool PrintType);
   void writeParamOperand(const Value *Operand, AttributeSet Attrs);
-  void writeOperandBundles(ImmutableCallSite CS);
+  void writeOperandBundles(const CallBase *Call);
   void writeSyncScope(const LLVMContext &Context,
                       SyncScope::ID SSID);
   void writeAtomic(const LLVMContext &Context,
@@ -2510,15 +2509,15 @@
   WriteAsOperandInternal(Out, Operand, &TypePrinter, &Machine, TheModule);
 }
 
-void AssemblyWriter::writeOperandBundles(ImmutableCallSite CS) {
-  if (!CS.hasOperandBundles())
+void AssemblyWriter::writeOperandBundles(const CallBase *Call) {
+  if (!Call->hasOperandBundles())
     return;
 
   Out << " [ ";
 
   bool FirstBundle = true;
-  for (unsigned i = 0, e = CS.getNumOperandBundles(); i != e; ++i) {
-    OperandBundleUse BU = CS.getOperandBundleAt(i);
+  for (unsigned i = 0, e = Call->getNumOperandBundles(); i != e; ++i) {
+    OperandBundleUse BU = Call->getOperandBundleAt(i);
 
     if (!FirstBundle)
       Out << ", ";
Index: llvm/lib/IR/Core.cpp
===================================================================
--- llvm/lib/IR/Core.cpp
+++ llvm/lib/IR/Core.cpp
@@ -15,7 +15,6 @@
 #include "llvm-c/Core.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/IR/Attributes.h"
-#include "llvm/IR/CallSite.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/DerivedTypes.h"
@@ -2656,43 +2655,43 @@
   if (FuncletPadInst *FPI = dyn_cast<FuncletPadInst>(unwrap(Instr))) {
     return FPI->getNumArgOperands();
   }
-  return CallSite(unwrap<Instruction>(Instr)).getNumArgOperands();
+  return unwrap<CallBase>(Instr)->getNumArgOperands();
 }
 
 /*--.. Call and invoke instructions ........................................--*/
 
 unsigned LLVMGetInstructionCallConv(LLVMValueRef Instr) {
-  return CallSite(unwrap<Instruction>(Instr)).getCallingConv();
+  return unwrap<CallBase>(Instr)->getCallingConv();
 }
 
 void LLVMSetInstructionCallConv(LLVMValueRef Instr, unsigned CC) {
-  return CallSite(unwrap<Instruction>(Instr))
-    .setCallingConv(static_cast<CallingConv::ID>(CC));
+  return unwrap<CallBase>(Instr)->setCallingConv(
+      static_cast<CallingConv::ID>(CC));
 }
 
 void LLVMSetInstrParamAlignment(LLVMValueRef Instr, unsigned index,
                                 unsigned align) {
-  CallSite Call = CallSite(unwrap<Instruction>(Instr));
+  auto *Call = unwrap<CallBase>(Instr);
   Attribute AlignAttr = Attribute::getWithAlignment(Call->getContext(), align);
-  Call.addAttribute(index, AlignAttr);
+  Call->addAttribute(index, AlignAttr);
 }
 
 void LLVMAddCallSiteAttribute(LLVMValueRef C, LLVMAttributeIndex Idx,
                               LLVMAttributeRef A) {
-  CallSite(unwrap<Instruction>(C)).addAttribute(Idx, unwrap(A));
+  unwrap<CallBase>(C)->addAttribute(Idx, unwrap(A));
 }
 
 unsigned LLVMGetCallSiteAttributeCount(LLVMValueRef C,
                                        LLVMAttributeIndex Idx) {
-  auto CS = CallSite(unwrap<Instruction>(C));
-  auto AS = CS.getAttributes().getAttributes(Idx);
+  auto *Call = unwrap<CallBase>(C);
+  auto AS = Call->getAttributes().getAttributes(Idx);
   return AS.getNumAttributes();
 }
 
 void LLVMGetCallSiteAttributes(LLVMValueRef C, LLVMAttributeIndex Idx,
                                LLVMAttributeRef *Attrs) {
-  auto CS = CallSite(unwrap<Instruction>(C));
-  auto AS = CS.getAttributes().getAttributes(Idx);
+  auto *Call = unwrap<CallBase>(C);
+  auto AS = Call->getAttributes().getAttributes(Idx);
   for (auto A : AS)
     *Attrs++ = wrap(A);
 }
@@ -2700,30 +2699,28 @@
 LLVMAttributeRef LLVMGetCallSiteEnumAttribute(LLVMValueRef C,
                                               LLVMAttributeIndex Idx,
                                               unsigned KindID) {
-  return wrap(CallSite(unwrap<Instruction>(C))
-    .getAttribute(Idx, (Attribute::AttrKind)KindID));
+  return wrap(
+      unwrap<CallBase>(C)->getAttribute(Idx, (Attribute::AttrKind)KindID));
 }
 
 LLVMAttributeRef LLVMGetCallSiteStringAttribute(LLVMValueRef C,
                                                 LLVMAttributeIndex Idx,
                                                 const char *K, unsigned KLen) {
-  return wrap(CallSite(unwrap<Instruction>(C))
-    .getAttribute(Idx, StringRef(K, KLen)));
+  return wrap(unwrap<CallBase>(C)->getAttribute(Idx, StringRef(K, KLen)));
 }
 
 void LLVMRemoveCallSiteEnumAttribute(LLVMValueRef C, LLVMAttributeIndex Idx,
                                      unsigned KindID) {
-  CallSite(unwrap<Instruction>(C))
-    .removeAttribute(Idx, (Attribute::AttrKind)KindID);
+  unwrap<CallBase>(C)->removeAttribute(Idx, (Attribute::AttrKind)KindID);
 }
 
 void LLVMRemoveCallSiteStringAttribute(LLVMValueRef C, LLVMAttributeIndex Idx,
                                        const char *K, unsigned KLen) {
-  CallSite(unwrap<Instruction>(C)).removeAttribute(Idx, StringRef(K, KLen));
+  unwrap<CallBase>(C)->removeAttribute(Idx, StringRef(K, KLen));
 }
 
 LLVMValueRef LLVMGetCalledValue(LLVMValueRef Instr) {
-  return wrap(CallSite(unwrap<Instruction>(Instr)).getCalledValue());
+  return wrap(unwrap<CallBase>(Instr)->getCalledValue());
 }
 
 /*--.. Operations on call instructions (only) ..............................--*/
Index: llvm/lib/IR/Function.cpp
===================================================================
--- llvm/lib/IR/Function.cpp
+++ llvm/lib/IR/Function.cpp
@@ -24,7 +24,6 @@
 #include "llvm/IR/Argument.h"
 #include "llvm/IR/Attributes.h"
 #include "llvm/IR/BasicBlock.h"
-#include "llvm/IR/CallSite.h"
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DerivedTypes.h"
@@ -1257,13 +1256,13 @@
     const User *FU = U.getUser();
     if (isa<BlockAddress>(FU))
       continue;
-    if (!isa<CallInst>(FU) && !isa<InvokeInst>(FU)) {
+    const auto *Call = dyn_cast<CallBase>(FU);
+    if (!Call) {
       if (PutOffender)
         *PutOffender = FU;
       return true;
     }
-    ImmutableCallSite CS(cast<Instruction>(FU));
-    if (!CS.isCallee(&U)) {
+    if (!Call->isCallee(&U)) {
       if (PutOffender)
         *PutOffender = FU;
       return true;
@@ -1291,8 +1290,8 @@
 bool Function::callsFunctionThatReturnsTwice() const {
   for (const_inst_iterator
          I = inst_begin(this), E = inst_end(this); I != E; ++I) {
-    ImmutableCallSite CS(&*I);
-    if (CS && CS.hasFnAttr(Attribute::ReturnsTwice))
+    const auto *Call = dyn_cast<CallBase>(&*I);
+    if (Call && Call->hasFnAttr(Attribute::ReturnsTwice))
       return true;
   }
 
Index: llvm/lib/IR/Value.cpp
===================================================================
--- llvm/lib/IR/Value.cpp
+++ llvm/lib/IR/Value.cpp
@@ -16,7 +16,6 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SetVector.h"
-#include "llvm/IR/CallSite.h"
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
@@ -503,8 +502,8 @@
         return V;
       V = GA->getAliasee();
     } else {
-      if (auto CS = ImmutableCallSite(V)) {
-        if (const Value *RV = CS.getReturnedArgOperand()) {
+      if (const auto *Call = dyn_cast<CallBase>(V)) {
+        if (const Value *RV = Call->getReturnedArgOperand()) {
           V = RV;
           continue;
         }
@@ -512,9 +511,9 @@
         // but it can't be marked with returned attribute, that's why it needs
         // special case.
         if (StripKind == PSK_ZeroIndicesAndAliasesAndInvariantGroups &&
-            (CS.getIntrinsicID() == Intrinsic::launder_invariant_group ||
-             CS.getIntrinsicID() == Intrinsic::strip_invariant_group)) {
-          V = CS.getArgOperand(0);
+            (Call->getIntrinsicID() == Intrinsic::launder_invariant_group ||
+             Call->getIntrinsicID() == Intrinsic::strip_invariant_group)) {
+          V = Call->getArgOperand(0);
           continue;
         }
       }
@@ -573,8 +572,8 @@
     } else if (auto *GA = dyn_cast<GlobalAlias>(V)) {
       V = GA->getAliasee();
     } else {
-      if (auto CS = ImmutableCallSite(V))
-        if (const Value *RV = CS.getReturnedArgOperand()) {
+      if (const auto *Call = dyn_cast<CallBase>(V))
+        if (const Value *RV = Call->getReturnedArgOperand()) {
           V = RV;
           continue;
         }
@@ -608,10 +607,11 @@
       DerefBytes = A->getDereferenceableOrNullBytes();
       CanBeNull = true;
     }
-  } else if (auto CS = ImmutableCallSite(this)) {
-    DerefBytes = CS.getDereferenceableBytes(AttributeList::ReturnIndex);
+  } else if (const auto *Call = dyn_cast<CallBase>(this)) {
+    DerefBytes = Call->getDereferenceableBytes(AttributeList::ReturnIndex);
     if (DerefBytes == 0) {
-      DerefBytes = CS.getDereferenceableOrNullBytes(AttributeList::ReturnIndex);
+      DerefBytes =
+          Call->getDereferenceableOrNullBytes(AttributeList::ReturnIndex);
       CanBeNull = true;
     }
   } else if (const LoadInst *LI = dyn_cast<LoadInst>(this)) {
@@ -683,8 +683,8 @@
       if (AllocatedType->isSized())
         Align = DL.getPrefTypeAlignment(AllocatedType);
     }
-  } else if (auto CS = ImmutableCallSite(this))
-    Align = CS.getAttributes().getRetAlignment();
+  } else if (const auto *Call = dyn_cast<CallBase>(this))
+    Align = Call->getAttributes().getRetAlignment();
   else if (const LoadInst *LI = dyn_cast<LoadInst>(this))
     if (MDNode *MD = LI->getMetadata(LLVMContext::MD_align)) {
       ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(0));