diff --git a/llvm/include/llvm/Transforms/IPO/Attributor.h b/llvm/include/llvm/Transforms/IPO/Attributor.h
--- a/llvm/include/llvm/Transforms/IPO/Attributor.h
+++ b/llvm/include/llvm/Transforms/IPO/Attributor.h
@@ -217,23 +217,6 @@
     return IRPosition(const_cast<CallBase &>(CB), Kind(ArgNo));
   }
 
-  /// Create a position describing the function scope of \p ICS.
-  static const IRPosition callsite_function(ImmutableCallSite ICS) {
-    return IRPosition::callsite_function(cast<CallBase>(*ICS.getInstruction()));
-  }
-
-  /// Create a position describing the returned value of \p ICS.
-  static const IRPosition callsite_returned(ImmutableCallSite ICS) {
-    return IRPosition::callsite_returned(cast<CallBase>(*ICS.getInstruction()));
-  }
-
-  /// Create a position describing the argument of \p ICS at position \p ArgNo.
-  static const IRPosition callsite_argument(ImmutableCallSite ICS,
-                                            unsigned ArgNo) {
-    return IRPosition::callsite_argument(cast<CallBase>(*ICS.getInstruction()),
-                                         ArgNo);
-  }
-
   /// Create a position describing the argument of \p ACS at position \p ArgNo.
   static const IRPosition callsite_argument(AbstractCallSite ACS,
                                             unsigned ArgNo) {
@@ -418,9 +401,9 @@
       return;
 
     AttributeList AttrList;
-    CallSite CS = CallSite(&getAnchorValue());
-    if (CS)
-      AttrList = CS.getAttributes();
+    auto *CB = dyn_cast<CallBase>(&getAnchorValue());
+    if (CB)
+      AttrList = CB->getAttributes();
     else
       AttrList = getAssociatedFunction()->getAttributes();
 
@@ -428,8 +411,8 @@
     for (Attribute::AttrKind AK : AKs)
       AttrList = AttrList.removeAttribute(Ctx, getAttrIdx(), AK);
 
-    if (CS)
-      CS.setAttributes(AttrList);
+    if (CB)
+      CB->setAttributes(AttrList);
     else
       getAssociatedFunction()->setAttributes(AttrList);
   }
diff --git a/llvm/lib/Transforms/IPO/Attributor.cpp b/llvm/lib/Transforms/IPO/Attributor.cpp
--- a/llvm/lib/Transforms/IPO/Attributor.cpp
+++ b/llvm/lib/Transforms/IPO/Attributor.cpp
@@ -228,7 +228,7 @@
   case IRPosition::IRP_CALL_SITE:
   case IRPosition::IRP_CALL_SITE_RETURNED:
   case IRPosition::IRP_CALL_SITE_ARGUMENT:
-    Attrs = ImmutableCallSite(&IRP.getAnchorValue()).getAttributes();
+    Attrs = cast<CallBase>(IRP.getAnchorValue()).getAttributes();
     break;
   }
 
@@ -253,7 +253,7 @@
   case IRPosition::IRP_CALL_SITE:
   case IRPosition::IRP_CALL_SITE_RETURNED:
   case IRPosition::IRP_CALL_SITE_ARGUMENT:
-    CallSite(&IRP.getAnchorValue()).setAttributes(Attrs);
+    cast<CallBase>(IRP.getAnchorValue()).setAttributes(Attrs);
     break;
   case IRPosition::IRP_INVALID:
   case IRPosition::IRP_FLOAT:
@@ -269,7 +269,7 @@
 SubsumingPositionIterator::SubsumingPositionIterator(const IRPosition &IRP) {
   IRPositions.emplace_back(IRP);
 
-  ImmutableCallSite ICS(&IRP.getAnchorValue());
+  const auto *CB = dyn_cast<CallBase>(&IRP.getAnchorValue());
   switch (IRP.getPositionKind()) {
   case IRPosition::IRP_INVALID:
   case IRPosition::IRP_FLOAT:
@@ -280,41 +280,40 @@
     IRPositions.emplace_back(IRPosition::function(*IRP.getAnchorScope()));
     return;
   case IRPosition::IRP_CALL_SITE:
-    assert(ICS && "Expected call site!");
+    assert(CB && "Expected call site!");
     // TODO: We need to look at the operand bundles similar to the redirection
     //       in CallBase.
-    if (!ICS.hasOperandBundles())
-      if (const Function *Callee = ICS.getCalledFunction())
+    if (!CB->hasOperandBundles())
+      if (const Function *Callee = CB->getCalledFunction())
         IRPositions.emplace_back(IRPosition::function(*Callee));
     return;
   case IRPosition::IRP_CALL_SITE_RETURNED:
-    assert(ICS && "Expected call site!");
+    assert(CB && "Expected call site!");
     // TODO: We need to look at the operand bundles similar to the redirection
     //       in CallBase.
-    if (!ICS.hasOperandBundles()) {
-      if (const Function *Callee = ICS.getCalledFunction()) {
+    if (!CB->hasOperandBundles()) {
+      if (const Function *Callee = CB->getCalledFunction()) {
         IRPositions.emplace_back(IRPosition::returned(*Callee));
         IRPositions.emplace_back(IRPosition::function(*Callee));
         for (const Argument &Arg : Callee->args())
           if (Arg.hasReturnedAttr()) {
             IRPositions.emplace_back(
-                IRPosition::callsite_argument(ICS, Arg.getArgNo()));
+                IRPosition::callsite_argument(*CB, Arg.getArgNo()));
             IRPositions.emplace_back(
-                IRPosition::value(*ICS.getArgOperand(Arg.getArgNo())));
+                IRPosition::value(*CB->getArgOperand(Arg.getArgNo())));
             IRPositions.emplace_back(IRPosition::argument(Arg));
           }
       }
     }
-    IRPositions.emplace_back(
-        IRPosition::callsite_function(cast<CallBase>(*ICS.getInstruction())));
+    IRPositions.emplace_back(IRPosition::callsite_function(*CB));
     return;
   case IRPosition::IRP_CALL_SITE_ARGUMENT: {
     int ArgNo = IRP.getArgNo();
-    assert(ICS && ArgNo >= 0 && "Expected call site!");
+    assert(CB && ArgNo >= 0 && "Expected call site!");
     // TODO: We need to look at the operand bundles similar to the redirection
     //       in CallBase.
-    if (!ICS.hasOperandBundles()) {
-      const Function *Callee = ICS.getCalledFunction();
+    if (!CB->hasOperandBundles()) {
+      const Function *Callee = CB->getCalledFunction();
       if (Callee && Callee->arg_size() > unsigned(ArgNo))
         IRPositions.emplace_back(IRPosition::argument(*Callee->getArg(ArgNo)));
       if (Callee)
@@ -369,8 +368,8 @@
     return false;
 
   AttributeList AttrList;
-  if (ImmutableCallSite ICS = ImmutableCallSite(&getAnchorValue()))
-    AttrList = ICS.getAttributes();
+  if (const auto *CB = dyn_cast<CallBase>(&getAnchorValue()))
+    AttrList = CB->getAttributes();
   else
     AttrList = getAssociatedFunction()->getAttributes();
 
@@ -510,12 +509,12 @@
     return isAssumedDead(IRPosition::value(*U.get()), QueryingAA, FnLivenessAA,
                          CheckBBLivenessOnly, DepClass);
 
-  if (CallSite CS = CallSite(UserI)) {
+  if (auto *CB = dyn_cast<CallBase>(UserI)) {
     // For call site argument uses we can check if the argument is
     // unused/dead.
-    if (CS.isArgOperand(&U)) {
+    if (CB->isArgOperand(&U)) {
       const IRPosition &CSArgPos =
-          IRPosition::callsite_argument(CS, CS.getArgumentNo(&U));
+          IRPosition::callsite_argument(*CB, CB->getArgOperandNo(&U));
       return isAssumedDead(CSArgPos, QueryingAA, FnLivenessAA,
                            CheckBBLivenessOnly, DepClass);
     }
@@ -1617,8 +1616,8 @@
     // Note: There are no concrete attributes now so this is initially empty.
     switch (I.getOpcode()) {
     default:
-      assert((!ImmutableCallSite(&I)) && (!isa<CallBase>(&I)) &&
-             "New call site/base instruction type needs to be known in the "
+      assert(!isa<CallBase>(&I) &&
+             "New call base instruction type needs to be known in the "
              "Attributor.");
       break;
     case Instruction::Call:
@@ -1687,8 +1686,8 @@
   InformationCache::FunctionInfo &FI = InfoCache.getFunctionInfo(F);
   if (!isModulePass() && !FI.CalledViaMustTail) {
     for (const Use &U : F.uses())
-      if (ImmutableCallSite ICS = ImmutableCallSite(U.getUser()))
-        if (ICS.isCallee(&U) && ICS.isMustTailCall())
+      if (const auto *CB = dyn_cast<CallBase>(U.getUser()))
+        if (CB->isCallee(&U) && CB->isMustTailCall())
           FI.CalledViaMustTail = true;
   }
 
@@ -1800,14 +1799,14 @@
   }
 
   auto CallSitePred = [&](Instruction &I) -> bool {
-    CallSite CS(&I);
-    IRPosition CSRetPos = IRPosition::callsite_returned(CS);
+    auto *CB = dyn_cast<CallBase>(&I);
+    IRPosition CBRetPos = IRPosition::callsite_returned(*CB);
 
     // Call sites might be dead if they do not have side effects and no live
     // users. The return value might be dead if there are no live users.
-    getOrCreateAAFor<AAIsDead>(CSRetPos);
+    getOrCreateAAFor<AAIsDead>(CBRetPos);
 
-    Function *Callee = CS.getCalledFunction();
+    Function *Callee = CB->getCalledFunction();
     // TODO: Even if the callee is not known now we might be able to simplify
     //       the call/callee.
     if (!Callee)
@@ -1819,46 +1818,46 @@
         !Callee->hasMetadata(LLVMContext::MD_callback))
       return true;
 
-    if (!Callee->getReturnType()->isVoidTy() && !CS->use_empty()) {
+    if (!Callee->getReturnType()->isVoidTy() && !CB->use_empty()) {
 
-      IRPosition CSRetPos = IRPosition::callsite_returned(CS);
+      IRPosition CBRetPos = IRPosition::callsite_returned(*CB);
 
       // Call site return integer values might be limited by a constant range.
       if (Callee->getReturnType()->isIntegerTy())
-        getOrCreateAAFor<AAValueConstantRange>(CSRetPos);
+        getOrCreateAAFor<AAValueConstantRange>(CBRetPos);
     }
 
-    for (int i = 0, e = CS.getNumArgOperands(); i < e; i++) {
+    for (int I = 0, E = CB->getNumArgOperands(); I < E; ++I) {
 
-      IRPosition CSArgPos = IRPosition::callsite_argument(CS, i);
+      IRPosition CBArgPos = IRPosition::callsite_argument(*CB, I);
 
       // Every call site argument might be dead.
-      getOrCreateAAFor<AAIsDead>(CSArgPos);
+      getOrCreateAAFor<AAIsDead>(CBArgPos);
 
       // Call site argument might be simplified.
-      getOrCreateAAFor<AAValueSimplify>(CSArgPos);
+      getOrCreateAAFor<AAValueSimplify>(CBArgPos);
 
-      if (!CS.getArgument(i)->getType()->isPointerTy())
+      if (!CB->getArgOperand(I)->getType()->isPointerTy())
         continue;
 
       // Call site argument attribute "non-null".
-      getOrCreateAAFor<AANonNull>(CSArgPos);
+      getOrCreateAAFor<AANonNull>(CBArgPos);
 
       // Call site argument attribute "no-alias".
-      getOrCreateAAFor<AANoAlias>(CSArgPos);
+      getOrCreateAAFor<AANoAlias>(CBArgPos);
 
       // Call site argument attribute "dereferenceable".
-      getOrCreateAAFor<AADereferenceable>(CSArgPos);
+      getOrCreateAAFor<AADereferenceable>(CBArgPos);
 
       // Call site argument attribute "align".
-      getOrCreateAAFor<AAAlign>(CSArgPos);
+      getOrCreateAAFor<AAAlign>(CBArgPos);
 
       // Call site argument attribute
       // "readnone/readonly/writeonly/..."
-      getOrCreateAAFor<AAMemoryBehavior>(CSArgPos);
+      getOrCreateAAFor<AAMemoryBehavior>(CBArgPos);
 
       // Call site argument attribute "nofree".
-      getOrCreateAAFor<AANoFree>(CSArgPos);
+      getOrCreateAAFor<AANoFree>(CBArgPos);
     }
     return true;
   };
@@ -1983,9 +1982,9 @@
     // do it eagerly.
     if (F->hasLocalLinkage()) {
       if (llvm::all_of(F->uses(), [&Functions](const Use &U) {
-            ImmutableCallSite ICS(U.getUser());
-            return ICS && ICS.isCallee(&U) &&
-                   Functions.count(const_cast<Function *>(ICS.getCaller()));
+            const auto *CB = dyn_cast<CallBase>(U.getUser());
+            return CB && CB->isCallee(&U) &&
+                   Functions.count(const_cast<Function *>(CB->getCaller()));
           }))
         continue;
     }
diff --git a/llvm/lib/Transforms/IPO/AttributorAttributes.cpp b/llvm/lib/Transforms/IPO/AttributorAttributes.cpp
--- a/llvm/lib/Transforms/IPO/AttributorAttributes.cpp
+++ b/llvm/lib/Transforms/IPO/AttributorAttributes.cpp
@@ -280,11 +280,11 @@
     if (V->getType()->isPointerTy()) {
       NewV = V->stripPointerCasts();
     } else {
-      CallSite CS(V);
-      if (CS && CS.getCalledFunction()) {
-        for (Argument &Arg : CS.getCalledFunction()->args())
+      auto *CB = dyn_cast<CallBase>(V);
+      if (CB && CB->getCalledFunction()) {
+        for (Argument &Arg : CB->getCalledFunction()->args())
           if (Arg.hasReturnedAttr()) {
-            NewV = CS.getArgOperand(Arg.getArgNo());
+            NewV = CB->getArgOperand(Arg.getArgNo());
             break;
           }
       }
@@ -688,9 +688,9 @@
       if (!I.mayThrow())
         return true;
 
-      if (ImmutableCallSite ICS = ImmutableCallSite(&I)) {
+      if (const auto *CB = dyn_cast<CallBase>(&I)) {
         const auto &NoUnwindAA =
-            A.getAAFor<AANoUnwind>(*this, IRPosition::callsite_function(ICS));
+            A.getAAFor<AANoUnwind>(*this, IRPosition::callsite_function(*CB));
         return NoUnwindAA.isAssumedNoUnwind();
       }
       return false;
@@ -1273,8 +1273,7 @@
 }
 
 bool AANoSyncImpl::isVolatile(Instruction *I) {
-  assert(!ImmutableCallSite(I) && !isa<CallBase>(I) &&
-         "Calls should not be checked here");
+  assert(!isa<CallBase>(I) && "Calls should not be checked here");
 
   switch (I->getOpcode()) {
   case Instruction::AtomicRMW:
@@ -1299,12 +1298,12 @@
     if (isa<IntrinsicInst>(&I) && isNoSyncIntrinsic(&I))
       return true;
 
-    if (ImmutableCallSite ICS = ImmutableCallSite(&I)) {
-      if (ICS.hasFnAttr(Attribute::NoSync))
+    if (const auto *CB = dyn_cast<CallBase>(&I)) {
+      if (CB->hasFnAttr(Attribute::NoSync))
         return true;
 
       const auto &NoSyncAA =
-          A.getAAFor<AANoSync>(*this, IRPosition::callsite_function(ICS));
+          A.getAAFor<AANoSync>(*this, IRPosition::callsite_function(*CB));
       if (NoSyncAA.isAssumedNoSync())
         return true;
       return false;
@@ -1323,7 +1322,7 @@
       return true;
 
     // non-convergent and readnone imply nosync.
-    return !ImmutableCallSite(&I).isConvergent();
+    return !cast<CallBase>(I).isConvergent();
   };
 
   if (!A.checkForAllReadWriteInstructions(CheckRWInstForNoSync, *this) ||
@@ -1377,12 +1376,12 @@
   /// See AbstractAttribute::updateImpl(...).
   ChangeStatus updateImpl(Attributor &A) override {
     auto CheckForNoFree = [&](Instruction &I) {
-      ImmutableCallSite ICS(&I);
-      if (ICS.hasFnAttr(Attribute::NoFree))
+      const auto &CB = cast<CallBase>(I);
+      if (CB.hasFnAttr(Attribute::NoFree))
         return true;
 
       const auto &NoFreeAA =
-          A.getAAFor<AANoFree>(*this, IRPosition::callsite_function(ICS));
+          A.getAAFor<AANoFree>(*this, IRPosition::callsite_function(CB));
       return NoFreeAA.isAssumedNoFree();
     };
 
@@ -1559,17 +1558,17 @@
   bool NullPointerIsDefined =
       F ? llvm::NullPointerIsDefined(F, PtrTy->getPointerAddressSpace()) : true;
   const DataLayout &DL = A.getInfoCache().getDL();
-  if (ImmutableCallSite ICS = ImmutableCallSite(I)) {
-    if (ICS.isBundleOperand(U))
+  if (const auto *CB = dyn_cast<CallBase>(I)) {
+    if (CB->isBundleOperand(U))
       return 0;
 
-    if (ICS.isCallee(U)) {
+    if (CB->isCallee(U)) {
       IsNonNull |= !NullPointerIsDefined;
       return 0;
     }
 
-    unsigned ArgNo = ICS.getArgumentNo(U);
-    IRPosition IRP = IRPosition::callsite_argument(ICS, ArgNo);
+    unsigned ArgNo = CB->getArgOperandNo(U);
+    IRPosition IRP = IRPosition::callsite_argument(*CB, ArgNo);
     // As long as we only use known information there is no need to track
     // dependences here.
     auto &DerefAA = A.getAAFor<AADereferenceable>(QueryingAA, IRP,
@@ -1803,17 +1802,17 @@
 
     // If the above check does not hold anymore we look at the calls.
     auto CheckForNoRecurse = [&](Instruction &I) {
-      ImmutableCallSite ICS(&I);
-      if (ICS.hasFnAttr(Attribute::NoRecurse))
+      const auto &CB = cast<CallBase>(I);
+      if (CB.hasFnAttr(Attribute::NoRecurse))
         return true;
 
       const auto &NoRecurseAA =
-          A.getAAFor<AANoRecurse>(*this, IRPosition::callsite_function(ICS));
+          A.getAAFor<AANoRecurse>(*this, IRPosition::callsite_function(CB));
       if (!NoRecurseAA.isAssumedNoRecurse())
         return false;
 
       // Recursion to the same function
-      if (ICS.getCalledFunction() == getAnchorScope())
+      if (CB.getCalledFunction() == getAnchorScope())
         return false;
 
       return true;
@@ -2114,7 +2113,7 @@
   /// See AbstractAttribute::updateImpl(...).
   ChangeStatus updateImpl(Attributor &A) override {
     auto CheckForWillReturn = [&](Instruction &I) {
-      IRPosition IPos = IRPosition::callsite_function(ImmutableCallSite(&I));
+      IRPosition IPos = IRPosition::callsite_function(cast<CallBase>(I));
       const auto &WillReturnAA = A.getAAFor<AAWillReturn>(*this, IPos);
       if (WillReturnAA.isKnownWillReturn())
         return true;
@@ -2321,8 +2320,8 @@
   /// See AbstractAttribute::initialize(...).
   void initialize(Attributor &A) override {
     // See callsite argument attribute and callee argument attribute.
-    ImmutableCallSite ICS(&getAnchorValue());
-    if (ICS.paramHasAttr(getArgNo(), Attribute::NoAlias))
+    const auto &CB = cast<CallBase>(getAnchorValue());
+    if (CB.paramHasAttr(getArgNo(), Attribute::NoAlias))
       indicateOptimisticFixpoint();
     Value &Val = getAssociatedValue();
     if (isa<ConstantPointerNull>(Val) &&
@@ -2335,32 +2334,32 @@
   /// \p OtherArgNo of \p ICS (= the underlying call site).
   bool mayAliasWithArgument(Attributor &A, AAResults *&AAR,
                             const AAMemoryBehavior &MemBehaviorAA,
-                            ImmutableCallSite ICS, unsigned OtherArgNo) {
+                            const CallBase &CB, unsigned OtherArgNo) {
     // We do not need to worry about aliasing with the underlying IRP.
     if (this->getArgNo() == (int)OtherArgNo)
       return false;
 
     // If it is not a pointer or pointer vector we do not alias.
-    const Value *ArgOp = ICS.getArgOperand(OtherArgNo);
+    const Value *ArgOp = CB.getArgOperand(OtherArgNo);
     if (!ArgOp->getType()->isPtrOrPtrVectorTy())
       return false;
 
-    auto &ICSArgMemBehaviorAA = A.getAAFor<AAMemoryBehavior>(
-        *this, IRPosition::callsite_argument(ICS, OtherArgNo),
+    auto &CBArgMemBehaviorAA = A.getAAFor<AAMemoryBehavior>(
+        *this, IRPosition::callsite_argument(CB, OtherArgNo),
         /* TrackDependence */ false);
 
     // If the argument is readnone, there is no read-write aliasing.
-    if (ICSArgMemBehaviorAA.isAssumedReadNone()) {
-      A.recordDependence(ICSArgMemBehaviorAA, *this, DepClassTy::OPTIONAL);
+    if (CBArgMemBehaviorAA.isAssumedReadNone()) {
+      A.recordDependence(CBArgMemBehaviorAA, *this, DepClassTy::OPTIONAL);
       return false;
     }
 
     // If the argument is readonly and the underlying value is readonly, there
     // is no read-write aliasing.
     bool IsReadOnly = MemBehaviorAA.isAssumedReadOnly();
-    if (ICSArgMemBehaviorAA.isAssumedReadOnly() && IsReadOnly) {
+    if (CBArgMemBehaviorAA.isAssumedReadOnly() && IsReadOnly) {
       A.recordDependence(MemBehaviorAA, *this, DepClassTy::OPTIONAL);
-      A.recordDependence(ICSArgMemBehaviorAA, *this, DepClassTy::OPTIONAL);
+      A.recordDependence(CBArgMemBehaviorAA, *this, DepClassTy::OPTIONAL);
       return false;
     }
 
@@ -2457,10 +2456,10 @@
     // Check there is no other pointer argument which could alias with the
     // value passed at this call site.
     // TODO: AbstractCallSite
-    ImmutableCallSite ICS(&getAnchorValue());
-    for (unsigned OtherArgNo = 0; OtherArgNo < ICS.getNumArgOperands();
+    const auto &CB = cast<CallBase>(getAnchorValue());
+    for (unsigned OtherArgNo = 0; OtherArgNo < CB.getNumArgOperands();
          OtherArgNo++)
-      if (mayAliasWithArgument(A, AAR, MemBehaviorAA, ICS, OtherArgNo))
+      if (mayAliasWithArgument(A, AAR, MemBehaviorAA, CB, OtherArgNo))
         return false;
 
     return true;
@@ -2511,8 +2510,7 @@
 
       /// For now, we can only deduce noalias if we have call sites.
       /// FIXME: add more support.
-      ImmutableCallSite ICS(&RV);
-      if (!ICS)
+      if (!isa<CallBase>(&RV))
         return false;
 
       const IRPosition &RVPos = IRPosition::value(RV);
@@ -2984,8 +2982,8 @@
     // is a performance optimization for blocks with calls to a lot of internal
     // functions. It can however cause dead functions to be treated as live.
     for (const Instruction &I : BB)
-      if (ImmutableCallSite ICS = ImmutableCallSite(&I))
-        if (const Function *F = ICS.getCalledFunction())
+      if (const auto *CB = dyn_cast<CallBase>(&I))
+        if (const Function *F = CB->getCalledFunction())
           if (F->hasLocalLinkage())
             A.markLiveInternalFunction(*F);
     return true;
@@ -3477,12 +3475,12 @@
   }
 
   MaybeAlign MA;
-  if (ImmutableCallSite ICS = ImmutableCallSite(I)) {
-    if (ICS.isBundleOperand(U) || ICS.isCallee(U))
+  if (const auto *CB = dyn_cast<CallBase>(I)) {
+    if (CB->isBundleOperand(U) || CB->isCallee(U))
       return 0;
 
-    unsigned ArgNo = ICS.getArgumentNo(U);
-    IRPosition IRP = IRPosition::callsite_argument(ICS, ArgNo);
+    unsigned ArgNo = CB->getArgOperandNo(U);
+    IRPosition IRP = IRPosition::callsite_argument(*CB, ArgNo);
     // As long as we only use known information there is no need to track
     // dependences here.
     auto &AlignAA = A.getAAFor<AAAlign>(QueryingAA, IRP,
@@ -3985,13 +3983,13 @@
 
     // For now we only use special logic for call sites. However, the tracker
     // itself knows about a lot of other non-capturing cases already.
-    CallSite CS(UInst);
-    if (!CS || !CS.isArgOperand(U))
+    auto *CB = dyn_cast<CallBase>(UInst);
+    if (!CB || !CB->isArgOperand(U))
       return isCapturedIn(/* Memory */ true, /* Integer */ true,
                           /* Return */ true);
 
-    unsigned ArgNo = CS.getArgumentNo(U);
-    const IRPosition &CSArgPos = IRPosition::callsite_argument(CS, ArgNo);
+    unsigned ArgNo = CB->getArgOperandNo(U);
+    const IRPosition &CSArgPos = IRPosition::callsite_argument(*CB, ArgNo);
     // If we have a abstract no-capture attribute for the argument we can use
     // it to justify a non-capture attribute here. This allows recursion!
     auto &ArgNoCaptureAA = A.getAAFor<AANoCapture>(NoCaptureAA, CSArgPos);
@@ -3999,7 +3997,7 @@
       return isCapturedIn(/* Memory */ false, /* Integer */ false,
                           /* Return */ false);
     if (ArgNoCaptureAA.isAssumedNoCaptureMaybeReturned()) {
-      addPotentialCopy(CS);
+      addPotentialCopy(*CB);
       return isCapturedIn(/* Memory */ false, /* Integer */ false,
                           /* Return */ false);
     }
@@ -4010,9 +4008,7 @@
   }
 
   /// Register \p CS as potential copy of the value we are checking.
-  void addPotentialCopy(CallSite CS) {
-    PotentialCopies.push_back(CS.getInstruction());
-  }
+  void addPotentialCopy(CallBase &CB) { PotentialCopies.push_back(&CB); }
 
   /// See CaptureTracker::shouldExplore(...).
   bool shouldExplore(const Use *U) override {
@@ -4992,10 +4988,9 @@
 
     // Helper to check if for the given call site the associated argument is
     // passed to a callback where the privatization would be different.
-    auto IsCompatiblePrivArgOfCallback = [&](CallSite CS) {
+    auto IsCompatiblePrivArgOfCallback = [&](CallBase &CB) {
       SmallVector<const Use *, 4> CallbackUses;
-      AbstractCallSite::getCallbackUses(cast<CallBase>(*CS.getInstruction()),
-                                        CallbackUses);
+      AbstractCallSite::getCallbackUses(CB, CallbackUses);
       for (const Use *U : CallbackUses) {
         AbstractCallSite CBACS(U);
         assert(CBACS && CBACS.isCallbackCall());
@@ -5012,7 +5007,7 @@
                 << CBArgNo << "@" << CBACS.getCalledFunction()->getName()
                 << ")\n[AAPrivatizablePtr] " << CBArg << " : "
                 << CBACS.getCallArgOperand(CBArg) << " vs "
-                << CS.getArgOperand(ArgNo) << "\n"
+                << CB.getArgOperand(ArgNo) << "\n"
                 << "[AAPrivatizablePtr] " << CBArg << " : "
                 << CBACS.getCallArgOperandNo(CBArg) << " vs " << ArgNo << "\n";
           });
@@ -5094,7 +5089,7 @@
     // here.
     auto IsCompatiblePrivArgOfOtherCallSite = [&](AbstractCallSite ACS) {
       if (ACS.isDirectCall())
-        return IsCompatiblePrivArgOfCallback(CallSite(ACS.getInstruction()));
+        return IsCompatiblePrivArgOfCallback(*ACS.getInstruction());
       if (ACS.isCallbackCall())
         return IsCompatiblePrivArgOfDirectCS(ACS);
       return false;
@@ -5727,9 +5722,9 @@
     // If the instruction has an own memory behavior state, use it to restrict
     // the local state. No further analysis is required as the other memory
     // state is as optimistic as it gets.
-    if (ImmutableCallSite ICS = ImmutableCallSite(&I)) {
+    if (const auto *CB = dyn_cast<CallBase>(&I)) {
       const auto &MemBehaviorAA = A.getAAFor<AAMemoryBehavior>(
-          *this, IRPosition::callsite_function(ICS));
+          *this, IRPosition::callsite_function(*CB));
       intersectAssumedBits(MemBehaviorAA.getAssumed());
       return !isAtFixpoint();
     }
@@ -5827,8 +5822,8 @@
 
   // By default we follow all uses assuming UserI might leak information on U,
   // we have special handling for call sites operands though.
-  ImmutableCallSite ICS(UserI);
-  if (!ICS || !ICS.isArgOperand(U))
+  const auto *CB = dyn_cast<CallBase>(UserI);
+  if (!CB || !CB->isArgOperand(U))
     return true;
 
   // If the use is a call argument known not to be captured, the users of
@@ -5838,9 +5833,9 @@
   // call might the argument "through return", which we allow and for which we
   // need to check call users.
   if (U->get()->getType()->isPointerTy()) {
-    unsigned ArgNo = ICS.getArgumentNo(U);
+    unsigned ArgNo = CB->getArgOperandNo(U);
     const auto &ArgNoCaptureAA = A.getAAFor<AANoCapture>(
-        *this, IRPosition::callsite_argument(ICS, ArgNo),
+        *this, IRPosition::callsite_argument(*CB, ArgNo),
         /* TrackDependence */ true, DepClassTy::OPTIONAL);
     return !ArgNoCaptureAA.isAssumedNoCapture();
   }
@@ -5874,17 +5869,17 @@
   case Instruction::Invoke: {
     // For call sites we look at the argument memory behavior attribute (this
     // could be recursive!) in order to restrict our own state.
-    ImmutableCallSite ICS(UserI);
+    const auto *CB = cast<CallBase>(UserI);
 
     // Give up on operand bundles.
-    if (ICS.isBundleOperand(U)) {
+    if (CB->isBundleOperand(U)) {
       indicatePessimisticFixpoint();
       return;
     }
 
     // Calling a function does read the function pointer, maybe write it if the
     // function is self-modifying.
-    if (ICS.isCallee(U)) {
+    if (CB->isCallee(U)) {
       removeAssumedBits(NO_READS);
       break;
     }
@@ -5893,9 +5888,9 @@
     // argument.
     IRPosition Pos;
     if (U->get()->getType()->isPointerTy())
-      Pos = IRPosition::callsite_argument(ICS, ICS.getArgumentNo(U));
+      Pos = IRPosition::callsite_argument(*CB, CB->getArgOperandNo(U));
     else
-      Pos = IRPosition::callsite_function(ICS);
+      Pos = IRPosition::callsite_function(*CB);
     const auto &MemBehaviorAA = A.getAAFor<AAMemoryBehavior>(
         *this, Pos,
         /* TrackDependence */ true, DepClassTy::OPTIONAL);
@@ -6184,9 +6179,9 @@
                                 Changed);
       return true;
     }
-    if (ImmutableCallSite ICS = ImmutableCallSite(&V)) {
+    if (const auto *CB = dyn_cast<CallBase>(&V)) {
       const auto &NoAliasAA =
-          A.getAAFor<AANoAlias>(*this, IRPosition::callsite_returned(ICS));
+          A.getAAFor<AANoAlias>(*this, IRPosition::callsite_returned(*CB));
       if (NoAliasAA.isAssumedNoAlias()) {
         updateStateAndAccessesMap(T, AccessKindAccessesMap, NO_MALLOCED_MEM, &I,
                                   &V, Changed);
@@ -6226,32 +6221,32 @@
   AAMemoryLocation::StateType AccessedLocs;
   AccessedLocs.intersectAssumedBits(NO_LOCATIONS);
 
-  if (ImmutableCallSite ICS = ImmutableCallSite(&I)) {
+  if (auto *CB = dyn_cast<CallBase>(&I)) {
 
     // First check if we assume any memory is access is visible.
-    const auto &ICSMemLocationAA =
-        A.getAAFor<AAMemoryLocation>(*this, IRPosition::callsite_function(ICS));
+    const auto &CBMemLocationAA =
+        A.getAAFor<AAMemoryLocation>(*this, IRPosition::callsite_function(*CB));
     LLVM_DEBUG(dbgs() << "[AAMemoryLocation] Categorize call site: " << I
-                      << " [" << ICSMemLocationAA << "]\n");
+                      << " [" << CBMemLocationAA << "]\n");
 
-    if (ICSMemLocationAA.isAssumedReadNone())
+    if (CBMemLocationAA.isAssumedReadNone())
       return NO_LOCATIONS;
 
-    if (ICSMemLocationAA.isAssumedInaccessibleMemOnly()) {
+    if (CBMemLocationAA.isAssumedInaccessibleMemOnly()) {
       updateStateAndAccessesMap(AccessedLocs, AccessKindAccessesMap,
                                 NO_INACCESSIBLE_MEM, &I, nullptr, Changed);
       return AccessedLocs.getAssumed();
     }
 
-    uint32_t ICSAssumedNotAccessedLocs =
-        ICSMemLocationAA.getAssumedNotAccessedLocation();
+    uint32_t CBAssumedNotAccessedLocs =
+        CBMemLocationAA.getAssumedNotAccessedLocation();
 
     // Set the argmemonly and global bit as we handle them separately below.
-    uint32_t ICSAssumedNotAccessedLocsNoArgMem =
-        ICSAssumedNotAccessedLocs | NO_ARGUMENT_MEM | NO_GLOBAL_MEM;
+    uint32_t CBAssumedNotAccessedLocsNoArgMem =
+        CBAssumedNotAccessedLocs | NO_ARGUMENT_MEM | NO_GLOBAL_MEM;
 
     for (MemoryLocationsKind CurMLK = 1; CurMLK < NO_LOCATIONS; CurMLK *= 2) {
-      if (ICSAssumedNotAccessedLocsNoArgMem & CurMLK)
+      if (CBAssumedNotAccessedLocsNoArgMem & CurMLK)
         continue;
       updateStateAndAccessesMap(AccessedLocs, AccessKindAccessesMap, CurMLK, &I,
                                 nullptr, Changed);
@@ -6259,7 +6254,7 @@
 
     // Now handle global memory if it might be accessed. This is slightly tricky
     // as NO_GLOBAL_MEM has multiple bits set.
-    bool HasGlobalAccesses = ((~ICSAssumedNotAccessedLocs) & NO_GLOBAL_MEM);
+    bool HasGlobalAccesses = ((~CBAssumedNotAccessedLocs) & NO_GLOBAL_MEM);
     if (HasGlobalAccesses) {
       auto AccessPred = [&](const Instruction *, const Value *Ptr,
                             AccessKind Kind, MemoryLocationsKind MLK) {
@@ -6267,7 +6262,7 @@
                                   Ptr, Changed);
         return true;
       };
-      if (!ICSMemLocationAA.checkForAllAccessesToMemoryKind(
+      if (!CBMemLocationAA.checkForAllAccessesToMemoryKind(
               AccessPred, inverseLocation(NO_GLOBAL_MEM, false, false)))
         return AccessedLocs.getWorstState();
     }
@@ -6277,18 +6272,18 @@
                << getMemoryLocationsAsStr(AccessedLocs.getAssumed()) << "\n");
 
     // Now handle argument memory if it might be accessed.
-    bool HasArgAccesses = ((~ICSAssumedNotAccessedLocs) & NO_ARGUMENT_MEM);
+    bool HasArgAccesses = ((~CBAssumedNotAccessedLocs) & NO_ARGUMENT_MEM);
     if (HasArgAccesses) {
-      for (unsigned ArgNo = 0, e = ICS.getNumArgOperands(); ArgNo < e;
+      for (unsigned ArgNo = 0, E = CB->getNumArgOperands(); ArgNo < E;
            ++ArgNo) {
 
         // Skip non-pointer arguments.
-        const Value *ArgOp = ICS.getArgOperand(ArgNo);
+        const Value *ArgOp = CB->getArgOperand(ArgNo);
         if (!ArgOp->getType()->isPtrOrPtrVectorTy())
           continue;
 
         // Skip readnone arguments.
-        const IRPosition &ArgOpIRP = IRPosition::callsite_argument(ICS, ArgNo);
+        const IRPosition &ArgOpIRP = IRPosition::callsite_argument(*CB, ArgNo);
         const auto &ArgOpMemLocationAA = A.getAAFor<AAMemoryBehavior>(
             *this, ArgOpIRP, /* TrackDependence */ true, DepClassTy::OPTIONAL);