diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 7cfdb9e..84ae677 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -3480,6 +3480,144 @@
     D->addAttr(NewAttr);
 }
 
+/// Handle __attribute__((callback(CalleeIdx, PayloadIdx0, ...))) attributes.
+static void handleCallbackAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  // The index that identifies the callback callee is mandatory.
+  if (AL.getNumArgs() == 0) {
+    S.Diag(AL.getLoc(), diag::err_callback_attribute_no_callee)
+        << AL.getRange();
+    return;
+  }
+
+  bool HasImplicitThisParam = isInstanceMethod(D);
+  int32_t NumArgs = getFunctionOrMethodNumParams(D);
+
+  FunctionDecl *FD = D->getAsFunction();
+  assert(FD && "Expected a function declaration!");
+
+  llvm::StringMap<int> NameIdxMapping;
+  NameIdxMapping["__"] = -1;
+
+  NameIdxMapping["this"] = 0;
+
+  int Idx = 1;
+  for (const ParmVarDecl *PVD : FD->parameters())
+    NameIdxMapping[PVD->getName()] = Idx++;
+
+  auto UnknownName = NameIdxMapping.end();
+
+  SmallVector<int, 8> EncodingIndices;
+  for (unsigned I = 0, E = AL.getNumArgs(); I < E; ++I) {
+    SourceRange SR;
+    int32_t ArgIdx;
+
+    if (AL.isArgIdent(I)) {
+      IdentifierLoc *IdLoc = AL.getArgAsIdent(I);
+      auto It = NameIdxMapping.find(IdLoc->Ident->getName());
+      if (It == UnknownName) {
+        S.Diag(AL.getLoc(), diag::err_callback_attribute_argument_unknown)
+            << IdLoc->Ident << IdLoc->Loc;
+        return;
+      }
+
+      SR = SourceRange(IdLoc->Loc);
+      ArgIdx = It->second;
+    } else if (AL.isArgExpr(I)) {
+      Expr *IdxExpr = AL.getArgAsExpr(I);
+
+      // If the expression is not parseable as an int32_t we have a problem.
+      if (!checkUInt32Argument(S, AL, IdxExpr, (uint32_t &)ArgIdx, I + 1,
+                               false)) {
+        S.Diag(AL.getLoc(), diag::err_attribute_argument_out_of_bounds)
+            << AL << (I + 1) << IdxExpr->getSourceRange();
+        return;
+      }
+
+      // Check oob, excluding the special values, 0 and -1.
+      if (ArgIdx < -1 || ArgIdx > NumArgs) {
+        S.Diag(AL.getLoc(), diag::err_attribute_argument_out_of_bounds)
+            << AL << (I + 1) << IdxExpr->getSourceRange();
+        return;
+      }
+
+      SR = IdxExpr->getSourceRange();
+    } else {
+      llvm_unreachable("Unexpected ParsedAttr argument type!");
+    }
+
+    if (ArgIdx == 0 && !HasImplicitThisParam) {
+      S.Diag(AL.getLoc(), diag::err_callback_implicit_this_not_available)
+          << (I + 1) << SR;
+      return;
+    }
+
+    // Adjust for the case we do not have an implicit "this" parameter. In this
+    // case we decrease all positive values by 1 to get LLVM argument indices.
+    if (!HasImplicitThisParam && ArgIdx > 0)
+      ArgIdx -= 1;
+
+    EncodingIndices.push_back(ArgIdx);
+  }
+
+  int CalleeIdx = EncodingIndices.front();
+  // Check if the callee index is proper, thus not "this" and not "unknown".
+  if (CalleeIdx < HasImplicitThisParam) {
+    S.Diag(AL.getLoc(), diag::err_callback_attribute_invalid_callee)
+        << AL.getRange();
+    return;
+  }
+
+  // Get the callee type, note the index adjustment as the AST doesn't contain
+  // the this type (which the callee cannot reference anyway!).
+  const Type *CalleeType =
+      getFunctionOrMethodParamType(D, CalleeIdx - HasImplicitThisParam)
+          .getTypePtr();
+  if (!CalleeType || !CalleeType->isFunctionPointerType()) {
+    S.Diag(AL.getLoc(), diag::err_callback_callee_no_function_type)
+        << AL.getRange();
+    return;
+  }
+
+  const Type *CalleeFnType =
+      CalleeType->getPointeeType()->getUnqualifiedDesugaredType();
+
+  // TODO: Check the type of the callee arguments.
+
+  const auto *CalleeFnProtoType = dyn_cast<FunctionProtoType>(CalleeFnType);
+  if (!CalleeFnProtoType) {
+    S.Diag(AL.getLoc(), diag::err_callback_callee_no_function_type)
+        << AL.getRange();
+    return;
+  }
+
+  if (CalleeFnProtoType->getNumParams() > EncodingIndices.size() - 1) {
+    S.Diag(AL.getLoc(), diag::err_attribute_wrong_number_arguments)
+        << AL << (unsigned)(EncodingIndices.size() - 1);
+    return;
+  }
+
+  if (CalleeFnProtoType->getNumParams() < EncodingIndices.size() - 1) {
+    S.Diag(AL.getLoc(), diag::err_attribute_wrong_number_arguments)
+        << AL << (unsigned)(EncodingIndices.size() - 1);
+    return;
+  }
+
+  if (CalleeFnProtoType->isVariadic()) {
+    S.Diag(AL.getLoc(), diag::err_callback_callee_is_variadic) << AL.getRange();
+    return;
+  }
+
+  // Do not allow multiple callback attributes.
+  if (D->hasAttr<CallbackAttr>()) {
+    S.Diag(AL.getLoc(), diag::err_callback_attribute_multiple) << AL.getRange();
+    return;
+  }
+
+  D->addAttr(::new (S.Context) CallbackAttr(
+      AL.getRange(), S.Context, EncodingIndices.data(), EncodingIndices.size(),
+      AL.getAttributeSpellingListIndex()));
+}
+
 static void handleTransparentUnionAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   // Try to find the underlying union declaration.
   RecordDecl *RD = nullptr;
@@ -6451,6 +6589,9 @@
   case ParsedAttr::AT_FormatArg:
     handleFormatArgAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_Callback:
+    handleCallbackAttr(S, D, AL);
+    break;
   case ParsedAttr::AT_CUDAGlobal:
     handleGlobalAttr(S, D, AL);
     break;