diff --git a/llvm/lib/IR/AbstractCallSite.cpp b/llvm/lib/IR/AbstractCallSite.cpp
@@ -0,0 +1,135 @@
+//===-- AbstractCallSite.cpp - Implementation of abstract call sites ------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements abstract call sites which unify the interface for
+// direct, indirect, and callback call sites.
+//
+// For more information see:
+// https://llvm.org/devmtg/2018-10/talk-abstracts.html#talk20
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/StringSwitch.h"
+#include "llvm/IR/CallSite.h"
+#include "llvm/Support/Debug.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "abstract-call-sites"
+
+STATISTIC(NumCallbackCallSites, "Number of callback call sites created");
+STATISTIC(NumDirectAbstractCallSites,
+          "Number of direct abstract call sites created");
+STATISTIC(NumInvalidAbstractCallSitesUnknownUse,
+          "Number of invalid abstract call sites created (unknown use)");
+STATISTIC(NumInvalidAbstractCallSitesUnknownCallee,
+          "Number of invalid abstract call sites created (unknown callee)");
+STATISTIC(NumInvalidAbstractCallSitesNoCallback,
+          "Number of invalid abstract call sites created (no callback)");
+
+/// Create an abstract call site from a use.
+AbstractCallSite::AbstractCallSite(const Use *U) : CS(U->getUser()) {
+
+  // First handle unknown users.
+  if (!CS) {
+
+    // If the use is actually in a constant cast expression which itself
+    // has only one use, we look through the constant cast expression.
+    // This happens by updating the use @p U to the use of the constant
+    // cast expression and afterwards re-initializing CS accordingly.
+    if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U->getUser()))
+      if (CE->getNumUses() == 1 && CE->isCast()) {
+        U = &*CE->use_begin();
+        CS = CallSite(U->getUser());
+      }
+
+    if (!CS) {
+      NumInvalidAbstractCallSitesUnknownUse++;
+      return;
+    }
+  }
+
+  // Then handle direct or indirect calls. Thus, if U is the callee of the
+  // call site CS it is not a callback and we are done.
+  if (CS.isCallee(U)) {
+    NumDirectAbstractCallSites++;
+    return;
+  }
+
+  // If we cannot identify the broker function we cannot create a callback and
+  // invalidate the abstract call site.
+  Function *Callee = CS.getCalledFunction();
+  if (!Callee) {
+    NumInvalidAbstractCallSitesUnknownCallee++;
+    CS = CallSite();
+    return;
+  }
+
+  MDNode *CallbackMD = Callee->getMetadata(LLVMContext::MD_callback);
+  if (!CallbackMD) {
+    NumInvalidAbstractCallSitesNoCallback++;
+    CS = CallSite();
+    return;
+  }
+
+  unsigned UseIdx = CS.getArgumentNo(U);
+  MDNode *CallbackEncMD = nullptr;
+  for (const MDOperand &Op : CallbackMD->operands()) {
+    MDNode *OpMD = cast<MDNode>(Op.get());
+    auto *CBCalleeIdxAsCM = cast<ConstantAsMetadata>(OpMD->getOperand(0));
+    uint64_t CBCalleeIdx =
+        cast<ConstantInt>(CBCalleeIdxAsCM->getValue())->getZExtValue();
+    if (CBCalleeIdx != UseIdx)
+      continue;
+    CallbackEncMD = OpMD;
+    break;
+  }
+
+  if (!CallbackEncMD) {
+    NumInvalidAbstractCallSitesNoCallback++;
+    CS = CallSite();
+    return;
+  }
+
+  NumCallbackCallSites++;
+
+  assert(CallbackEncMD->getNumOperands() >= 2 && "Incomplete !callback metadata");
+
+  unsigned NumCallOperands = CS.getNumArgOperands();
+  // Skip the var-arg flag at the end when reading the metadata.
+  for (unsigned u = 0, e = CallbackEncMD->getNumOperands() - 1; u < e; u++) {
+    Metadata *OpAsM = CallbackEncMD->getOperand(u).get();
+    auto *OpAsCM = cast<ConstantAsMetadata>(OpAsM);
+    assert(OpAsCM->getType()->isIntegerTy(64) &&
+           "Malformed !callback metadata");
+
+    int64_t Idx = cast<ConstantInt>(OpAsCM->getValue())->getSExtValue();
+    assert(-1 <= Idx && Idx <= NumCallOperands &&
+           "Out-of-bounds !callback metadata index");
+
+    CI.ParameterEncoding.push_back(Idx);
+  }
+
+  if (!Callee->isVarArg())
+    return;
+
+  Metadata *VarArgFlagAsM =
+      CallbackEncMD->getOperand(CallbackEncMD->getNumOperands() - 1).get();
+  auto *VarArgFlagAsCM = cast<ConstantAsMetadata>(VarArgFlagAsM);
+  assert(VarArgFlagAsCM->getType()->isIntegerTy(1) &&
+         "Malformed !callback metadata var-arg flag");
+
+  if (VarArgFlagAsCM->getValue()->isNullValue())
+    return;
+
+  // Add all variadic arguments at the end.
+  for (unsigned u = Callee->arg_size(); u < NumCallOperands; u++)
+    CI.ParameterEncoding.push_back(u);
+}
diff --git a/llvm/lib/IR/CMakeLists.txt b/llvm/lib/IR/CMakeLists.txt
@@ -3,6 +3,7 @@ tablegen(LLVM AttributesCompatFunc.inc -gen-attrs)
 add_public_tablegen_target(AttributeCompatFuncTableGen)
 
 add_llvm_library(LLVMCore
+  AbstractCallSite.cpp
   AsmWriter.cpp
   Attributes.cpp
   AutoUpgrade.cpp

diff --git a/llvm/lib/IR/LLVMContext.cpp b/llvm/lib/IR/LLVMContext.cpp
@@ -62,6 +62,7 @@ LLVMContext::LLVMContext() : pImpl(new LLVMContextImpl(*this)) {
     {MD_callees, "callees"},
     {MD_irr_loop, "irr_loop"},
     {MD_access_group, "llvm.access.group"},
+    {MD_callback, "callback"},
   };
 
   for (auto &MDKind : MDKinds) {

diff --git a/llvm/lib/IR/MDBuilder.cpp b/llvm/lib/IR/MDBuilder.cpp
@@ -107,6 +107,50 @@ MDNode *MDBuilder::createCallees(ArrayRef<Function *> Callees) {
   return MDNode::get(Context, Ops);
 }
 
+MDNode *MDBuilder::createCallbackEncoding(unsigned CalleeArgNo,
+                                          ArrayRef<int> Arguments,
+                                          bool VarArgArePassed) {
+  SmallVector<Metadata *, 4> Ops;
+
+  Type *Int64 = Type::getInt64Ty(Context);
+  Ops.push_back(createConstant(ConstantInt::get(Int64, CalleeArgNo)));
+
+  for (int ArgNo : Arguments)
+    Ops.push_back(createConstant(ConstantInt::get(Int64, ArgNo, true)));
+
+  Type *Int1 = Type::getInt1Ty(Context);
+  Ops.push_back(createConstant(ConstantInt::get(Int1, VarArgArePassed)));
+
+  return MDNode::get(Context, Ops);
+}
+
+MDNode *MDBuilder::mergeCallbackEncodings(MDNode *ExistingCallbacks,
+                                          MDNode *NewCB) {
+  if (!ExistingCallbacks)
+    return MDNode::get(Context, {NewCB});
+
+  auto *NewCBCalleeIdxAsCM = cast<ConstantAsMetadata>(NewCB->getOperand(0));
+  uint64_t NewCBCalleeIdx =
+      cast<ConstantInt>(NewCBCalleeIdxAsCM->getValue())->getZExtValue();
+
+  SmallVector<Metadata *, 4> Ops;
+  unsigned NumExistingOps = ExistingCallbacks->getNumOperands();
+  Ops.resize(NumExistingOps + 1);
+
+  for (unsigned u = 0; u < NumExistingOps; u++) {
+    Ops[u] = ExistingCallbacks->getOperand(u);
+
+    auto *OldCBCalleeIdxAsCM = cast<ConstantAsMetadata>(Ops[u]);
+    uint64_t OldCBCalleeIdx =
+      cast<ConstantInt>(OldCBCalleeIdxAsCM->getValue())->getZExtValue();
+    assert(NewCBCalleeIdx != OldCBCalleeIdx &&
+           "Cannot map a callback callee index twice!");
+  }
+
+  Ops[NumExistingOps] = NewCB;
+  return MDNode::get(Context, Ops);
+}
+
 MDNode *MDBuilder::createAnonymousAARoot(StringRef Name, MDNode *Extra) {
   // To ensure uniqueness the root node is self-referential.
   auto Dummy = MDNode::getTemporary(Context, None);

diff --git a/llvm/test/ThinLTO/X86/lazyload_metadata.ll b/llvm/test/ThinLTO/X86/lazyload_metadata.ll
@@ -10,13 +10,13 @@
 ; RUN: llvm-lto -thinlto-action=import %t2.bc -thinlto-index=%t3.bc \
 ; RUN:          -o /dev/null -stats \
 ; RUN:  2>&1 | FileCheck %s -check-prefix=LAZY
-; LAZY: 57 bitcode-reader  - Number of Metadata records loaded
+; LAZY: 59 bitcode-reader  - Number of Metadata records loaded
 ; LAZY: 2 bitcode-reader  - Number of MDStrings loaded
 
 ; RUN: llvm-lto -thinlto-action=import %t2.bc -thinlto-index=%t3.bc \
 ; RUN:          -o /dev/null -disable-ondemand-mds-loading -stats \
 ; RUN:  2>&1 | FileCheck %s -check-prefix=NOTLAZY
-; NOTLAZY: 66 bitcode-reader  - Number of Metadata records loaded
+; NOTLAZY: 68 bitcode-reader  - Number of Metadata records loaded
 ; NOTLAZY: 7 bitcode-reader  - Number of MDStrings loaded