diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2936,6 +2936,8 @@
   "string">;
 def err_callback_attribute_no_callee : Error<
   "'callback' attribute specifies no callback callee">;
+def err_callback_attribute_no_encoding_mode : Error<
+  "'callback' attribute specifies no parameter encoding mode">;
 def err_callback_attribute_invalid_callee : Error<
   "'callback' attribute specifies invalid callback callee">;
 def err_callback_attribute_multiple : Error<
diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -2126,19 +2126,23 @@
 
   if (const auto *CB = FD->getAttr<CallbackAttr>()) {
     // Annotate the callback behavior as metadata:
+    //  - The callback argument encoding mode (as argument number)
     //  - The callback callee (as argument number).
     //  - The callback payloads (as argument numbers).
     llvm::LLVMContext &Ctx = F->getContext();
     llvm::MDBuilder MDB(Ctx);
 
-    // The payload indices are all but the first one in the encoding. The first
-    // identifies the callback callee.
-    int CalleeIdx = *CB->encoding_begin();
-    ArrayRef<int> PayloadIndices(CB->encoding_begin() + 1, CB->encoding_end());
-    F->addMetadata(llvm::LLVMContext::MD_callback,
-                   *llvm::MDNode::get(Ctx, {MDB.createCallbackEncoding(
-                                               CalleeIdx, PayloadIndices,
-                                               /* VarArgsArePassed */ false)}));
+    // The payload indices are all but the first two in the encoding. The first
+    // identifies the callback argument encoding mode, and the second represents
+    // the callback callee.
+    int EncodingMode = *CB->encoding_begin();
+    int CalleeIdx = *(CB->encoding_begin() + 1);
+    ArrayRef<int> PayloadIndices(CB->encoding_begin() + 2, CB->encoding_end());
+    F->addMetadata(
+        llvm::LLVMContext::MD_callback,
+        *llvm::MDNode::get(Ctx, {MDB.createCallbackEncoding(
+                                    EncodingMode, CalleeIdx, PayloadIndices,
+                                    /* VarArgsArePassed */ false)}));
   }
 }
 
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -3555,8 +3555,15 @@
 
 /// 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.
+  // The parameter encoding mode is mandatory.
   if (AL.getNumArgs() == 0) {
+    S.Diag(AL.getLoc(), diag::err_callback_attribute_no_encoding_mode)
+        << AL.getRange();
+    return;
+  }
+
+  // The index that identifies the callback callee is mandatory.
+  if (AL.getNumArgs() == 1) {
     S.Diag(AL.getLoc(), diag::err_callback_attribute_no_callee)
         << AL.getRange();
     return;
@@ -3618,7 +3625,7 @@
       llvm_unreachable("Unexpected ParsedAttr argument type!");
     }
 
-    if (ArgIdx == 0 && !HasImplicitThisParam) {
+    if (ArgIdx == 0 && I != 0 && !HasImplicitThisParam) {
       S.Diag(AL.getLoc(), diag::err_callback_implicit_this_not_available)
           << (I + 1) << SR;
       return;
@@ -3632,7 +3639,9 @@
     EncodingIndices.push_back(ArgIdx);
   }
 
-  int CalleeIdx = EncodingIndices.front();
+  assert(EncodingIndices.size() >= 2);
+
+  int CalleeIdx = EncodingIndices[1];
   // Check if the callee index is proper, thus not "this" and not "unknown".
   // This means the "CalleeIdx" has to be non-negative if "HasImplicitThisParam"
   // is false and positive if "HasImplicitThisParam" is true.
@@ -3665,15 +3674,9 @@
     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) {
+  if (CalleeFnProtoType->getNumParams() != EncodingIndices.size() - 2) {
     S.Diag(AL.getLoc(), diag::err_attribute_wrong_number_arguments)
-        << AL << (unsigned)(EncodingIndices.size() - 1);
+        << AL << (unsigned)(CalleeFnProtoType->getNumParams() + 2);
     return;
   }
 
diff --git a/clang/test/CodeGen/attr-callback.c b/clang/test/CodeGen/attr-callback.c
--- a/clang/test/CodeGen/attr-callback.c
+++ b/clang/test/CodeGen/attr-callback.c
@@ -3,26 +3,26 @@
 void cb0(void);
 
 // CHECK-DAG: !callback ![[cid0:[0-9]+]] void @no_args
-__attribute__((callback(1))) void no_args(void (*callback)(void));
+__attribute__((callback(0, 1))) void no_args(void (*callback)(void));
 
 // CHECK-DAG: @args_1({{[^#]*#[0-9]+}} !callback ![[cid1:[0-9]+]]
-__attribute__((callback(1, 2, 3))) void args_1(void (*callback)(int, double), int a, double b) { no_args(cb0); }
+__attribute__((callback(0, 1, 2, 3))) void args_1(void (*callback)(int, double), int a, double b) { no_args(cb0); }
 
 // CHECK-DAG: !callback ![[cid2:[0-9]+]]  void @args_2a
-__attribute__((callback(2, 3, 3))) void args_2a(int a, void (*callback)(double, double), double b);
+__attribute__((callback(0, 2, 3, 3))) void args_2a(int a, void (*callback)(double, double), double b);
 // CHECK-DAG: !callback ![[cid2]]         void @args_2b
-__attribute__((callback(callback, b, b))) void args_2b(int a, void (*callback)(double, double), double b);
+__attribute__((callback(0, callback, b, b))) void args_2b(int a, void (*callback)(double, double), double b);
 
 // CHECK-DAG: void @args_3a({{[^#]*#[0-9]+}} !callback ![[cid3:[0-9]+]]
-__attribute__((callback(2, -1, -1))) void args_3a(int a, void (*callback)(double, double), double b) { args_2a(a, callback, b); }
+__attribute__((callback(0, 2, -1, -1))) void args_3a(int a, void (*callback)(double, double), double b) { args_2a(a, callback, b); }
 // CHECK-DAG: void @args_3b({{[^#]*#[0-9]+}} !callback ![[cid3]]
-__attribute__((callback(callback, __, __))) void args_3b(int a, void (*callback)(double, double), double b) { args_2b(a, callback, b); }
+__attribute__((callback(0, callback, __, __))) void args_3b(int a, void (*callback)(double, double), double b) { args_2b(a, callback, b); }
 
 // CHECK-DAG: ![[cid0]] = !{![[cid0b:[0-9]+]]}
-// CHECK-DAG: ![[cid0b]] = !{i64 0, i1 false}
+// CHECK-DAG: ![[cid0b]] = !{i64 0, i64 0, i1 false}
 // CHECK-DAG: ![[cid1]] = !{![[cid1b:[0-9]+]]}
-// CHECK-DAG: ![[cid1b]] = !{i64 0, i64 1, i64 2, i1 false}
+// CHECK-DAG: ![[cid1b]] = !{i64 0, i64 0, i64 1, i64 2, i1 false}
 // CHECK-DAG: ![[cid2]] = !{![[cid2b:[0-9]+]]}
-// CHECK-DAG: ![[cid2b]] = !{i64 1, i64 2, i64 2, i1 false}
+// CHECK-DAG: ![[cid2b]] = !{i64 0, i64 1, i64 2, i64 2, i1 false}
 // CHECK-DAG: ![[cid3]] = !{![[cid3b:[0-9]+]]}
-// CHECK-DAG: ![[cid3b]] = !{i64 1, i64 -1, i64 -1, i1 false}
+// CHECK-DAG: ![[cid3b]] = !{i64 0, i64 1, i64 -1, i64 -1, i1 false}
diff --git a/clang/test/CodeGen/callback_annotated.c b/clang/test/CodeGen/callback_annotated.c
--- a/clang/test/CodeGen/callback_annotated.c
+++ b/clang/test/CodeGen/callback_annotated.c
@@ -1,30 +1,30 @@
 // RUN: %clang_cc1 -triple i386-unknown-unknown -fopenmp -fno-experimental-new-pass-manager %s -emit-llvm -o - -disable-llvm-optzns | FileCheck %s --check-prefix=RUN1
 
 // RUN1-DAG: @broker0({{[^#]*#[0-9]+}} !callback ![[cid0:[0-9]+]]
-__attribute__((callback(1, 2))) void *broker0(void *(*callee)(void *), void *payload) {
+__attribute__((callback(0, 1, 2))) void *broker0(void *(*callee)(void *), void *payload) {
   return callee(payload);
 }
 
 // RUN1-DAG: @broker1({{[^#]*#[0-9]+}} !callback ![[cid1:[0-9]+]]
-__attribute__((callback(callee, payload))) void *broker1(void *payload, void *(*callee)(void *)) {
+__attribute__((callback(0, callee, payload))) void *broker1(void *payload, void *(*callee)(void *)) {
   return broker0(callee, payload);
 }
 
 void *broker2(void (*callee)(void));
 
 // RUN1-DAG: declare !callback ![[cid2:[0-9]+]] i8* @broker2
-__attribute__((callback(callee))) void *broker2(void (*callee)(void));
+__attribute__((callback(0, callee))) void *broker2(void (*callee)(void));
 
 void *broker2(void (*callee)(void));
 
 // RUN1-DAG: declare !callback ![[cid3:[0-9]+]] i8* @broker3
-__attribute__((callback(4, 1, 2, c))) void *broker3(int, int, int c, int (*callee)(int, int, int), int);
+__attribute__((callback(0, 4, 1, 2, c))) void *broker3(int, int, int c, int (*callee)(int, int, int), int);
 
 // RUN1-DAG: declare !callback ![[cid4:[0-9]+]] i8* @broker4
-__attribute__((callback(4, -1, a, __))) void *broker4(int a, int, int, int (*callee)(int, int, int), int);
+__attribute__((callback(0, 4, -1, a, __))) void *broker4(int a, int, int, int (*callee)(int, int, int), int);
 
 // RUN1-DAG: declare !callback ![[cid5:[0-9]+]] i8* @broker5
-__attribute__((callback(4, d, 5, 2))) void *broker5(int, int, int, int (*callee)(int, int, int), int d);
+__attribute__((callback(0, 4, d, 5, 2))) void *broker5(int, int, int, int (*callee)(int, int, int), int d);
 
 static void *VoidPtr2VoidPtr(void *payload) {
   return payload;
@@ -44,14 +44,14 @@
 }
 
 // RUN1-DAG: ![[cid0]] = !{![[cid0b:[0-9]+]]}
-// RUN1-DAG: ![[cid0b]] = !{i64 0, i64 1, i1 false}
+// RUN1-DAG: ![[cid0b]] = !{i64 0, i64 0, i64 1, i1 false}
 // RUN1-DAG: ![[cid1]] = !{![[cid1b:[0-9]+]]}
-// RUN1-DAG: ![[cid1b]] = !{i64 1, i64 0, i1 false}
+// RUN1-DAG: ![[cid1b]] = !{i64 0, i64 1, i64 0, i1 false}
 // RUN1-DAG: ![[cid2]] = !{![[cid2b:[0-9]+]]}
-// RUN1-DAG: ![[cid2b]] = !{i64 0, i1 false}
+// RUN1-DAG: ![[cid2b]] = !{i64 0, i64 0, i1 false}
 // RUN1-DAG: ![[cid3]] = !{![[cid3b:[0-9]+]]}
-// RUN1-DAG: ![[cid3b]] = !{i64 3, i64 0, i64 1, i64 2, i1 false}
+// RUN1-DAG: ![[cid3b]] = !{i64 0, i64 3, i64 0, i64 1, i64 2, i1 false}
 // RUN1-DAG: ![[cid4]] = !{![[cid4b:[0-9]+]]}
-// RUN1-DAG: ![[cid4b]] = !{i64 3, i64 -1, i64 0, i64 -1, i1 false}
+// RUN1-DAG: ![[cid4b]] = !{i64 0, i64 3, i64 -1, i64 0, i64 -1, i1 false}
 // RUN1-DAG: ![[cid5]] = !{![[cid5b:[0-9]+]]}
-// RUN1-DAG: ![[cid5b]] = !{i64 3, i64 4, i64 4, i64 1, i1 false}
+// RUN1-DAG: ![[cid5b]] = !{i64 0, i64 3, i64 4, i64 4, i64 1, i1 false}
diff --git a/clang/test/CodeGen/callback_openmp.c b/clang/test/CodeGen/callback_openmp.c
--- a/clang/test/CodeGen/callback_openmp.c
+++ b/clang/test/CodeGen/callback_openmp.c
@@ -3,7 +3,7 @@
 // CHECK: declare !callback ![[cid:[0-9]+]] void @__kmpc_fork_call
 // CHECK: declare !callback ![[cid]] void @__kmpc_fork_teams
 // CHECK: ![[cid]] = !{![[cidb:[0-9]+]]}
-// CHECK: ![[cidb]] = !{i64 2, i64 -1, i64 -1, i1 true}
+// CHECK: ![[cidb]] = !{i64 0, i64 2, i64 -1, i64 -1, i1 true}
 
 void work1(int, int);
 void work2(int, int);
diff --git a/clang/test/CodeGenCXX/attr-callback.cpp b/clang/test/CodeGenCXX/attr-callback.cpp
--- a/clang/test/CodeGenCXX/attr-callback.cpp
+++ b/clang/test/CodeGenCXX/attr-callback.cpp
@@ -3,28 +3,28 @@
 struct Base {
 
   void no_args_1(void (*callback)(void));
-  __attribute__((callback(1))) void no_args_2(void (*callback1)(void), void (*callback2)(void));
-  __attribute__((callback(callback1))) void no_args_3(void (*callback1)(void), void (*callback2)(void));
+  __attribute__((callback(0, 1))) void no_args_2(void (*callback1)(void), void (*callback2)(void));
+  __attribute__((callback(0, callback1))) void no_args_3(void (*callback1)(void), void (*callback2)(void));
 
   // TODO: There should probably be a warning or even an error for different
   //       callbacks on the same method.
-  __attribute__((callback(1))) virtual void
+  __attribute__((callback(0, 1))) virtual void
   virtual_1(void (*callback)(void));
 
-  __attribute__((callback(callback, this, __, this))) virtual void
+  __attribute__((callback(0, callback, this, __, this))) virtual void
   this_unknown_this(void (*callback)(Base *, Base *, Base *));
 };
 
 // CHECK-DAG:      define{{.*}} void @_ZN4Base9no_args_1EPFvvE({{[^!]*!callback}} ![[cid0:[0-9]+]]
-__attribute__((callback(1))) void
+__attribute__((callback(0, 1))) void
 Base::no_args_1(void (*callback)(void)) {
 }
 
 // CHECK-DAG:      define{{.*}} void @_ZN4Base9no_args_2EPFvvES1_({{[^!]*!callback}} ![[cid1:[0-9]+]]
-__attribute__((callback(2))) void Base::no_args_2(void (*callback1)(void), void (*callback2)(void)) {
+__attribute__((callback(0, 2))) void Base::no_args_2(void (*callback1)(void), void (*callback2)(void)) {
 }
 // CHECK-DAG:      define{{.*}} void @_ZN4Base9no_args_3EPFvvES1_({{[^!]*!callback}} ![[cid1]]
-__attribute__((callback(callback2))) void Base::no_args_3(void (*callback1)(void), void (*callback2)(void)) {
+__attribute__((callback(0, callback2))) void Base::no_args_3(void (*callback1)(void), void (*callback2)(void)) {
 }
 
 // CHECK-DAG:      define{{.*}} void @_ZN4Base17this_unknown_thisEPFvPS_S0_S0_E({{[^!]*!callback}} ![[cid2:[0-9]+]]
@@ -32,7 +32,7 @@
 }
 
 struct Derived_1 : public Base {
-  __attribute__((callback(1))) virtual void
+  __attribute__((callback(0, 1))) virtual void
   virtual_1(void (*callback)(void)) override;
 };
 
@@ -48,8 +48,8 @@
 void Derived_2::virtual_1(void (*callback)(void)) {}
 
 // CHECK-DAG: ![[cid0]] = !{![[cid0b:[0-9]+]]}
-// CHECK-DAG: ![[cid0b]] = !{i64 1, i1 false}
+// CHECK-DAG: ![[cid0b]] = !{i64 0, i64 1, i1 false}
 // CHECK-DAG: ![[cid1]] = !{![[cid1b:[0-9]+]]}
-// CHECK-DAG: ![[cid1b]] = !{i64 2, i1 false}
+// CHECK-DAG: ![[cid1b]] = !{i64 0, i64 2, i1 false}
 // CHECK-DAG: ![[cid2]] = !{![[cid2b:[0-9]+]]}
-// CHECK-DAG: ![[cid2b]] = !{i64 1, i64 0, i64 -1, i64 0, i1 false}
+// CHECK-DAG: ![[cid2b]] = !{i64 0, i64 1, i64 0, i64 -1, i64 0, i1 false}
diff --git a/clang/test/OpenMP/parallel_codegen.cpp b/clang/test/OpenMP/parallel_codegen.cpp
--- a/clang/test/OpenMP/parallel_codegen.cpp
+++ b/clang/test/OpenMP/parallel_codegen.cpp
@@ -182,5 +182,5 @@
 // ALL: attributes #[[FN_ATTRS]] = {{.+}} nounwind
 // ALL-DEBUG: attributes #[[FN_ATTRS]] = {{.+}} nounwind
 // ALL: ![[cbid]] = !{![[cbidb:[0-9]+]]}
-// ALL: ![[cbidb]] = !{i64 2, i64 -1, i64 -1, i1 true}
+// ALL: ![[cbidb]] = !{i64 0, i64 2, i64 -1, i64 -1, i1 true}
 #endif
diff --git a/clang/test/Sema/attr-callback-broken.c b/clang/test/Sema/attr-callback-broken.c
--- a/clang/test/Sema/attr-callback-broken.c
+++ b/clang/test/Sema/attr-callback-broken.c
@@ -1,75 +1,76 @@
 // RUN: %clang_cc1 %s -verify -fsyntax-only
 
-__attribute__((callback())) void no_callee(void (*callback)(void)); // expected-error {{'callback' attribute specifies no callback callee}}
+__attribute__((callback())) void no_encoding_mode(void (*callback)(void)); // expected-error {{'callback' attribute specifies no parameter encoding mode}}
+__attribute__((callback(0))) void no_callee(void (*callback)(void)); // expected-error {{'callback' attribute specifies no callback callee}}
 
-__attribute__((callback(1, 1))) void too_many_args_1(void (*callback)(void)) {}      // expected-error {{'callback' attribute takes one argument}}
-__attribute__((callback(1, -1))) void too_many_args_2(double (*callback)(void));     // expected-error {{'callback' attribute takes one argument}}
-__attribute__((callback(1, 2, 2))) void too_many_args_3(void (*callback)(int), int); // expected-error {{'callback' attribute requires exactly 2 arguments}}
+__attribute__((callback(0, 1, 1))) void too_many_args_1(void (*callback)(void)) {}      // expected-error {{'callback' attribute requires exactly 2 arguments}}
+__attribute__((callback(0, 1, -1))) void too_many_args_2(double (*callback)(void));     // expected-error {{'callback' attribute requires exactly 2 arguments}}
+__attribute__((callback(0, 1, 2, 2))) void too_many_args_3(void (*callback)(int), int); // expected-error {{'callback' attribute requires exactly 3 arguments}}
 
-__attribute__((callback(1, 2))) void too_few_args_1(void (*callback)(int, int), int); // expected-error {{'callback' attribute takes one argument}}
-__attribute__((callback(1))) void too_few_args_2(int (*callback)(int));               // expected-error {{'callback' attribute takes no arguments}}
-__attribute__((callback(1, -1))) void too_few_args_3(void (*callback)(int, int)) {}   // expected-error {{'callback' attribute takes one argument}}
+__attribute__((callback(0, 1, 2))) void too_few_args_1(void (*callback)(int, int), int); // expected-error {{'callback' attribute requires exactly 4 arguments}}
+__attribute__((callback(0, 1))) void too_few_args_2(int (*callback)(int));               // expected-error {{'callback' attribute requires exactly 3 arguments}}
+__attribute__((callback(0, 1, -1))) void too_few_args_3(void (*callback)(int, int)) {}   // expected-error {{'callback' attribute requires exactly 4 arguments}}
 
-__attribute__((callback(-1))) void oob_args_1(void (*callback)(void));         // expected-error {{'callback' attribute specifies invalid callback callee}}
-__attribute__((callback(2))) void oob_args_2(int *(*callback)(void)) {}        // expected-error {{'callback' attribute parameter 1 is out of bounds}}
-__attribute__((callback(1, 3))) void oob_args_3(short (*callback)(int), int);  // expected-error {{'callback' attribute parameter 2 is out of bounds}}
-__attribute__((callback(-2, 2))) void oob_args_4(void *(*callback)(int), int); // expected-error {{'callback' attribute parameter 1 is out of bounds}}
-__attribute__((callback(1, -2))) void oob_args_5(void *(*callback)(int), int); // expected-error {{'callback' attribute parameter 2 is out of bounds}}
-__attribute__((callback(1, 2))) void oob_args_6(void *(*callback)(int), ...);  // expected-error {{'callback' attribute parameter 2 is out of bounds}}
+__attribute__((callback(0, -1))) void oob_args_1(void (*callback)(void));         // expected-error {{'callback' attribute specifies invalid callback callee}}
+__attribute__((callback(0, 2))) void oob_args_2(int *(*callback)(void)) {}        // expected-error {{'callback' attribute parameter 2 is out of bounds}}
+__attribute__((callback(0, 1, 3))) void oob_args_3(short (*callback)(int), int);  // expected-error {{'callback' attribute parameter 3 is out of bounds}}
+__attribute__((callback(0, -2, 2))) void oob_args_4(void *(*callback)(int), int); // expected-error {{'callback' attribute parameter 2 is out of bounds}}
+__attribute__((callback(0, 1, -2))) void oob_args_5(void *(*callback)(int), int); // expected-error {{'callback' attribute parameter 3 is out of bounds}}
+__attribute__((callback(0, 1, 2))) void oob_args_6(void *(*callback)(int), ...);  // expected-error {{'callback' attribute parameter 3 is out of bounds}}
 
-__attribute__((callback(1))) __attribute__((callback(1))) void multiple_cb_1(void (*callback)(void));                           // expected-error {{multiple 'callback' attributes specified}}
-__attribute__((callback(1))) __attribute__((callback(2))) void multiple_cb_2(void (*callback1)(void), void (*callback2)(void)); // expected-error {{multiple 'callback' attributes specified}}
+__attribute__((callback(0, 1))) __attribute__((callback(0, 1))) void multiple_cb_1(void (*callback)(void));                           // expected-error {{multiple 'callback' attributes specified}}
+__attribute__((callback(0, 1))) __attribute__((callback(0, 2))) void multiple_cb_2(void (*callback1)(void), void (*callback2)(void)); // expected-error {{multiple 'callback' attributes specified}}
 
 #ifdef HAS_THIS
-__attribute__((callback(0))) void oob_args_0(void (*callback)(void)); // expected-error {{'callback' attribute specifies invalid callback callee}}
+__attribute__((callback(0, 0))) void oob_args_0(void (*callback)(void)); // expected-error {{'callback' attribute specifies invalid callback callee}}
 #else
-__attribute__((callback(0))) void oob_args_0(void (*callback)(void));                 // expected-error {{'callback' argument at position 1 references unavailable implicit 'this'}}
-__attribute__((callback(1, 0))) void no_this_1(void *(*callback)(void *));            // expected-error {{'callback' argument at position 2 references unavailable implicit 'this'}}
-__attribute__((callback(1, 0))) void no_this_2(void *(*callback)(int, void *));       // expected-error {{'callback' argument at position 2 references unavailable implicit 'this'}}
+__attribute__((callback(0, 0))) void oob_args_0(void (*callback)(void));                 // expected-error {{'callback' argument at position 2 references unavailable implicit 'this'}}
+__attribute__((callback(0, 1, 0))) void no_this_1(void *(*callback)(void *));            // expected-error {{'callback' argument at position 3 references unavailable implicit 'this'}}
+__attribute__((callback(0, 1, 0))) void no_this_2(void *(*callback)(int, void *));       // expected-error {{'callback' argument at position 3 references unavailable implicit 'this'}}
 #endif
 
 // We could allow the following declarations if we at some point need to:
 
-__attribute__((callback(1, -1))) void vararg_cb_1(void (*callback)(int, ...)) {}     // expected-error {{'callback' attribute callee may not be variadic}}
-__attribute__((callback(1, 1))) void vararg_cb_2(void (*callback)(int, ...), int a); // expected-error {{'callback' attribute callee may not be variadic}}
+__attribute__((callback(0, 1, -1))) void vararg_cb_1(void (*callback)(int, ...)) {}     // expected-error {{'callback' attribute callee may not be variadic}}
+__attribute__((callback(0, 1, 1))) void vararg_cb_2(void (*callback)(int, ...), int a); // expected-error {{'callback' attribute callee may not be variadic}}
 
-__attribute__((callback(1, -1, 1, 2, 3, 4, -1))) void varargs_1(void (*callback)(int, ...), int a, float b, double c) {}               // expected-error {{'callback' attribute requires exactly 6 arguments}}
-__attribute__((callback(1, -1, 4, 2, 3, 4, -1))) void varargs_2(void (*callback)(void *, double, int, ...), int a, float b, double c); // expected-error {{'callback' attribute requires exactly 6 arguments}}
+__attribute__((callback(0, 1, -1, 1, 2, 3, 4, -1))) void varargs_1(void (*callback)(int, ...), int a, float b, double c) {}               // expected-error {{'callback' attribute requires exactly 3 arguments}}
+__attribute__((callback(0, 1, -1, 4, 2, 3, 4, -1))) void varargs_2(void (*callback)(void *, double, int, ...), int a, float b, double c); // expected-error {{'callback' attribute requires exactly 5 arguments}}
 
-__attribute__((callback(1, -1, 1))) void self_arg_1(void (*callback)(int, ...)) {}          // expected-error {{'callback' attribute requires exactly 2 arguments}}
-__attribute__((callback(1, -1, 1, -1, -1, 1))) void self_arg_2(void (*callback)(int, ...)); // expected-error {{'callback' attribute requires exactly 5 arguments}}
+__attribute__((callback(0, 1, -1, 1))) void self_arg_1(void (*callback)(int, ...)) {}          // expected-error {{'callback' attribute requires exactly 3 arguments}}
+__attribute__((callback(0, 1, -1, 1, -1, -1, 1))) void self_arg_2(void (*callback)(int, ...)); // expected-error {{'callback' attribute requires exactly 3 arguments}}
 
-__attribute__((callback(cb))) void unknown_name1(void (*callback)(void)) {}     // expected-error {{'callback' attribute argument 'cb' is not a known function parameter}}
-__attribute__((callback(cb, ab))) void unknown_name2(void (*cb)(int), int a) {} // expected-error {{'callback' attribute argument 'ab' is not a known function parameter}}
+__attribute__((callback(0, cb))) void unknown_name1(void (*callback)(void)) {}     // expected-error {{'callback' attribute argument 'cb' is not a known function parameter}}
+__attribute__((callback(0, cb, ab))) void unknown_name2(void (*cb)(int), int a) {} // expected-error {{'callback' attribute argument 'ab' is not a known function parameter}}
 
-__attribute__((callback(callback, 1))) void too_many_args_1b(void (*callback)(void)) {}      // expected-error {{'callback' attribute takes one argument}}
-__attribute__((callback(callback, __))) void too_many_args_2b(double (*callback)(void));     // expected-error {{'callback' attribute takes one argument}}
-__attribute__((callback(callback, 2, 2))) void too_many_args_3b(void (*callback)(int), int); // expected-error {{'callback' attribute requires exactly 2 arguments}}
+__attribute__((callback(0, callback, 1))) void too_many_args_1b(void (*callback)(void)) {}      // expected-error {{'callback' attribute requires exactly 2 arguments}}
+__attribute__((callback(0, callback, __))) void too_many_args_2b(double (*callback)(void));     // expected-error {{'callback' attribute requires exactly 2 arguments}}
+__attribute__((callback(0, callback, 2, 2))) void too_many_args_3b(void (*callback)(int), int); // expected-error {{'callback' attribute requires exactly 3 arguments}}
 
-__attribute__((callback(callback, a))) void too_few_args_1b(void (*callback)(int, int), int a); // expected-error {{'callback' attribute takes one argument}}
-__attribute__((callback(callback))) void too_few_args_2b(int (*callback)(int));                 // expected-error {{'callback' attribute takes no arguments}}
-__attribute__((callback(callback, __))) void too_few_args_3b(void (*callback)(int, int)) {}     // expected-error {{'callback' attribute takes one argument}}
+__attribute__((callback(0, callback, a))) void too_few_args_1b(void (*callback)(int, int), int a); // expected-error {{'callback' attribute requires exactly 4 arguments}}
+__attribute__((callback(0, callback))) void too_few_args_2b(int (*callback)(int));                 // expected-error {{'callback' attribute requires exactly 3 arguments}}
+__attribute__((callback(0, callback, __))) void too_few_args_3b(void (*callback)(int, int)) {}     // expected-error {{'callback' attribute requires exactly 4 arguments}}
 
-__attribute__((callback(__))) void oob_args_1b(void (*callback)(void)); // expected-error {{'callback' attribute specifies invalid callback callee}}
+__attribute__((callback(0, __))) void oob_args_1b(void (*callback)(void)); // expected-error {{'callback' attribute specifies invalid callback callee}}
 
-__attribute__((callback(callback))) __attribute__((callback(callback))) void multiple_cb_1b(void (*callback)(void));                     // expected-error {{multiple 'callback' attributes specified}}
-__attribute__((callback(1))) __attribute__((callback(callback2))) void multiple_cb_2b(void (*callback1)(void), void (*callback2)(void)); // expected-error {{multiple 'callback' attributes specified}}
+__attribute__((callback(0, callback))) __attribute__((callback(0, callback))) void multiple_cb_1b(void (*callback)(void));                     // expected-error {{multiple 'callback' attributes specified}}
+__attribute__((callback(0, 1))) __attribute__((callback(0, callback2))) void multiple_cb_2b(void (*callback1)(void), void (*callback2)(void)); // expected-error {{multiple 'callback' attributes specified}}
 
 #ifdef HAS_THIS
-__attribute__((callback(this))) void oob_args_0b(void (*callback)(void)); // expected-error {{'callback' attribute specifies invalid callback callee}}
+__attribute__((callback(0, this))) void oob_args_0b(void (*callback)(void)); // expected-error {{'callback' attribute specifies invalid callback callee}}
 #else
-__attribute__((callback(this))) void oob_args_0b(void (*callback)(void));           // expected-error {{'callback' argument at position 1 references unavailable implicit 'this'}}
-__attribute__((callback(1, this))) void no_this_1b(void *(*callback)(void *));      // expected-error {{'callback' argument at position 2 references unavailable implicit 'this'}}
-__attribute__((callback(1, this))) void no_this_2b(void *(*callback)(int, void *)); // expected-error {{'callback' argument at position 2 references unavailable implicit 'this'}}
+__attribute__((callback(0, this))) void oob_args_0b(void (*callback)(void));           // expected-error {{'callback' argument at position 2 references unavailable implicit 'this'}}
+__attribute__((callback(0, 1, this))) void no_this_1b(void *(*callback)(void *));      // expected-error {{'callback' argument at position 3 references unavailable implicit 'this'}}
+__attribute__((callback(0, 1, this))) void no_this_2b(void *(*callback)(int, void *)); // expected-error {{'callback' argument at position 3 references unavailable implicit 'this'}}
 #endif
 
 // We could allow the following declarations if we at some point need to:
 
-__attribute__((callback(callback, __))) void vararg_cb_1b(void (*callback)(int, ...)) {} // expected-error {{'callback' attribute callee may not be variadic}}
-__attribute__((callback(1, a))) void vararg_cb_2b(void (*callback)(int, ...), int a);    // expected-error {{'callback' attribute callee may not be variadic}}
+__attribute__((callback(0, callback, __))) void vararg_cb_1b(void (*callback)(int, ...)) {} // expected-error {{'callback' attribute callee may not be variadic}}
+__attribute__((callback(0, 1, a))) void vararg_cb_2b(void (*callback)(int, ...), int a);    // expected-error {{'callback' attribute callee may not be variadic}}
 
-__attribute__((callback(callback, __, callback, a, b, c, __))) void varargs_1b(void (*callback)(int, ...), int a, float b, double c) {} // expected-error {{'callback' attribute requires exactly 6 arguments}}
-__attribute__((callback(1, __, c, a, b, c, -1))) void varargs_2b(void (*callback)(void *, double, int, ...), int a, float b, double c); // expected-error {{'callback' attribute requires exactly 6 arguments}}
+__attribute__((callback(0, callback, __, callback, a, b, c, __))) void varargs_1b(void (*callback)(int, ...), int a, float b, double c) {} // expected-error {{'callback' attribute requires exactly 3 arguments}}
+__attribute__((callback(0, 1, __, c, a, b, c, -1))) void varargs_2b(void (*callback)(void *, double, int, ...), int a, float b, double c); // expected-error {{'callback' attribute requires exactly 5 arguments}}
 
-__attribute__((callback(1, __, callback))) void self_arg_1b(void (*callback)(int, ...)) {}                        // expected-error {{'callback' attribute requires exactly 2 arguments}}
-__attribute__((callback(callback, __, callback, __, __, callback))) void self_arg_2b(void (*callback)(int, ...)); // expected-error {{'callback' attribute requires exactly 5 arguments}}
+__attribute__((callback(0, 1, __, callback))) void self_arg_1b(void (*callback)(int, ...)) {}                        // expected-error {{'callback' attribute requires exactly 3 arguments}}
+__attribute__((callback(0, callback, __, callback, __, __, callback))) void self_arg_2b(void (*callback)(int, ...)); // expected-error {{'callback' attribute requires exactly 3 arguments}}
diff --git a/clang/test/Sema/attr-callback.c b/clang/test/Sema/attr-callback.c
--- a/clang/test/Sema/attr-callback.c
+++ b/clang/test/Sema/attr-callback.c
@@ -2,13 +2,13 @@
 
 // expected-no-diagnostics
 
-__attribute__((callback(1))) void no_args(void (*callback)(void));
-__attribute__((callback(1, 2, 3))) void args_1(void (*callback)(int, double), int a, double b);
-__attribute__((callback(2, 3, 3))) void args_2(int a, void (*callback)(double, double), double b);
-__attribute__((callback(2, -1, -1))) void args_3(int a, void (*callback)(double, double), double b);
+__attribute__((callback(0, 1))) void no_args(void (*callback)(void));
+__attribute__((callback(0, 1, 2, 3))) void args_1(void (*callback)(int, double), int a, double b);
+__attribute__((callback(0, 2, 3, 3))) void args_2(int a, void (*callback)(double, double), double b);
+__attribute__((callback(0, 2, -1, -1))) void args_3(int a, void (*callback)(double, double), double b);
 
-__attribute__((callback(callback))) void no_argsb(void (*callback)(void));
-__attribute__((callback(callback, a, 3))) void args_1b(void (*callback)(int, double), int a, double b);
-__attribute__((callback(callback, b, b))) void args_2b(int a, void (*callback)(double, double), double b);
-__attribute__((callback(2, __, __))) void args_3b(int a, void (*callback)(double, double), double b);
-__attribute__((callback(callback, -1, __))) void args_3c(int a, void (*callback)(double, double), double b);
+__attribute__((callback(0, callback))) void no_argsb(void (*callback)(void));
+__attribute__((callback(0, callback, a, 3))) void args_1b(void (*callback)(int, double), int a, double b);
+__attribute__((callback(0, callback, b, b))) void args_2b(int a, void (*callback)(double, double), double b);
+__attribute__((callback(0, 2, __, __))) void args_3b(int a, void (*callback)(double, double), double b);
+__attribute__((callback(0, callback, -1, __))) void args_3c(int a, void (*callback)(double, double), double b);
diff --git a/clang/test/SemaCXX/attr-callback.cpp b/clang/test/SemaCXX/attr-callback.cpp
--- a/clang/test/SemaCXX/attr-callback.cpp
+++ b/clang/test/SemaCXX/attr-callback.cpp
@@ -9,26 +9,26 @@
 struct Base {
 
   void no_args_1(void (*callback)(void));
-  __attribute__((callback(1))) void no_args_2(void (*callback)(void));
-  __attribute__((callback(callback))) void no_args_3(void (*callback)(void)) {}
+  __attribute__((callback(0, 1))) void no_args_2(void (*callback)(void));
+  __attribute__((callback(0, callback))) void no_args_3(void (*callback)(void)) {}
 
-  __attribute__((callback(1, 0))) virtual void
+  __attribute__((callback(0, 1, 0))) virtual void
   this_tr(void (*callback)(Base *));
 
-  __attribute__((callback(1, this, __, this))) virtual void
+  __attribute__((callback(0, 1, this, __, this))) virtual void
   this_unknown_this(void (*callback)(Base *, Base *, Base *));
 
-  __attribute__((callback(1))) virtual void
+  __attribute__((callback(0, 1))) virtual void
   virtual_1(void (*callback)(void));
 
-  __attribute__((callback(callback))) virtual void
+  __attribute__((callback(0, callback))) virtual void
   virtual_2(void (*callback)(void));
 
-  __attribute__((callback(1))) virtual void
+  __attribute__((callback(0, 1))) virtual void
   virtual_3(void (*callback)(void));
 };
 
-__attribute__((callback(1))) void
+__attribute__((callback(0, 1))) void
 Base::no_args_1(void (*callback)(void)) {
 }
 
@@ -37,10 +37,10 @@
 
 struct Derived_1 : public Base {
 
-  __attribute__((callback(1, 0))) virtual void
+  __attribute__((callback(0, 1, 0))) virtual void
   this_tr(void (*callback)(Base *)) override;
 
-  __attribute__((callback(1))) virtual void
+  __attribute__((callback(0, 1))) virtual void
   virtual_1(void (*callback)(void)) override {}
 
   virtual void
@@ -49,7 +49,7 @@
 
 struct Derived_2 : public Base {
 
-  __attribute__((callback(callback))) virtual void
+  __attribute__((callback(0, callback))) virtual void
   virtual_1(void (*callback)(void)) override;
 
   virtual void
@@ -61,7 +61,7 @@
 
 void Derived_2::virtual_1(void (*callback)(void)) {}
 
-__attribute__((callback(1))) void
+__attribute__((callback(0, 1))) void
 Derived_2::virtual_2(void (*callback)(void)) {}
 
 void Derived_2::virtual_3(void (*callback)(void)) {}
diff --git a/llvm/include/llvm/IR/AbstractCallSite.h b/llvm/include/llvm/IR/AbstractCallSite.h
--- a/llvm/include/llvm/IR/AbstractCallSite.h
+++ b/llvm/include/llvm/IR/AbstractCallSite.h
@@ -52,6 +52,10 @@
 
   /// The encoding of a callback with regards to the underlying instruction.
   struct CallbackInfo {
+    enum ParameterEncodingModeTy : int64_t {
+      FLATTEN = 0,
+      STACK = 1
+    };
 
     /// For direct/indirect calls the parameter encoding is empty. If it is not,
     /// the abstract call site represents a callback. In that case, the first
@@ -65,7 +69,7 @@
     /// unknown values that are passed to the callee.
     using ParameterEncodingTy = SmallVector<int, 0>;
     ParameterEncodingTy ParameterEncoding;
-
+    ParameterEncodingModeTy ParameterEncodingMode;
   };
 
 private:
diff --git a/llvm/include/llvm/IR/MDBuilder.h b/llvm/include/llvm/IR/MDBuilder.h
--- a/llvm/include/llvm/IR/MDBuilder.h
+++ b/llvm/include/llvm/IR/MDBuilder.h
@@ -102,7 +102,8 @@
   //===------------------------------------------------------------------===//
 
   /// Return metadata describing a callback (see llvm::AbstractCallSite).
-  MDNode *createCallbackEncoding(unsigned CalleeArgNo, ArrayRef<int> Arguments,
+  MDNode *createCallbackEncoding(unsigned EncodeMode, unsigned CalleeArgNo,
+                                 ArrayRef<int> Arguments,
                                  bool VarArgsArePassed);
 
   /// Merge the new callback encoding \p NewCB into \p ExistingCallbacks.
diff --git a/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp b/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp
--- a/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp
+++ b/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp
@@ -94,10 +94,10 @@
         //  - The first two arguments of the callback callee are unknown (-1).
         //  - All variadic arguments to the runtime function are passed to the
         //    callback callee.
-        Fn->addMetadata(
-            LLVMContext::MD_callback,
-            *MDNode::get(Ctx, {MDB.createCallbackEncoding(
-                                  2, {-1, -1}, /* VarArgsArePassed */ true)}));
+        Fn->addMetadata(LLVMContext::MD_callback,
+                        *MDNode::get(Ctx, {MDB.createCallbackEncoding(
+                                              0, 2, {-1, -1},
+                                              /* VarArgsArePassed */ true)}));
       }
     }
 
@@ -596,7 +596,7 @@
       F->addMetadata(
           llvm::LLVMContext::MD_callback,
           *llvm::MDNode::get(
-              Ctx, {MDB.createCallbackEncoding(2, {-1, -1},
+              Ctx, {MDB.createCallbackEncoding(0, 2, {-1, -1},
                                                /* VarArgsArePassed */ true)}));
     }
   }
diff --git a/llvm/lib/IR/AbstractCallSite.cpp b/llvm/lib/IR/AbstractCallSite.cpp
--- a/llvm/lib/IR/AbstractCallSite.cpp
+++ b/llvm/lib/IR/AbstractCallSite.cpp
@@ -44,7 +44,7 @@
 
   for (const MDOperand &Op : CallbackMD->operands()) {
     MDNode *OpMD = cast<MDNode>(Op.get());
-    auto *CBCalleeIdxAsCM = cast<ConstantAsMetadata>(OpMD->getOperand(0));
+    auto *CBCalleeIdxAsCM = cast<ConstantAsMetadata>(OpMD->getOperand(1));
     uint64_t CBCalleeIdx =
         cast<ConstantInt>(CBCalleeIdxAsCM->getValue())->getZExtValue();
     if (CBCalleeIdx < CB.arg_size())
@@ -102,7 +102,7 @@
   MDNode *CallbackEncMD = nullptr;
   for (const MDOperand &Op : CallbackMD->operands()) {
     MDNode *OpMD = cast<MDNode>(Op.get());
-    auto *CBCalleeIdxAsCM = cast<ConstantAsMetadata>(OpMD->getOperand(0));
+    auto *CBCalleeIdxAsCM = cast<ConstantAsMetadata>(OpMD->getOperand(1));
     uint64_t CBCalleeIdx =
         cast<ConstantInt>(CBCalleeIdxAsCM->getValue())->getZExtValue();
     if (CBCalleeIdx != UseIdx)
@@ -119,11 +119,13 @@
 
   NumCallbackCallSites++;
 
-  assert(CallbackEncMD->getNumOperands() >= 2 && "Incomplete !callback metadata");
+  assert(CallbackEncMD->getNumOperands() >= 3 &&
+         "Incomplete !callback metadata");
 
   unsigned NumCallOperands = CB->getNumArgOperands();
-  // Skip the var-arg flag at the end when reading the metadata.
-  for (unsigned u = 0, e = CallbackEncMD->getNumOperands() - 1; u < e; u++) {
+  // Skip the var-arg flag at the end and mode at the beginning when reading the
+  // metadata.
+  for (unsigned u = 1, e = CallbackEncMD->getNumOperands() - 1; u < e; u++) {
     Metadata *OpAsM = CallbackEncMD->getOperand(u).get();
     auto *OpAsCM = cast<ConstantAsMetadata>(OpAsM);
     assert(OpAsCM->getType()->isIntegerTy(64) &&
@@ -151,4 +153,16 @@
   // Add all variadic arguments at the end.
   for (unsigned u = Callee->arg_size(); u < NumCallOperands; u++)
     CI.ParameterEncoding.push_back(u);
+
+  Metadata *EncodingModeAsM = CallbackEncMD->getOperand(0).get();
+  auto *EncodingModeAsCM = cast<ConstantAsMetadata>(EncodingModeAsM);
+  assert(EncodingModeAsCM->getType()->isIntegerTy(64) &&
+         "Malformed !callback metadata parameter encoding mode");
+  int64_t EncodingMode =
+      cast<ConstantInt>(EncodingModeAsCM->getValue())->getSExtValue();
+  assert((EncodingMode == CallbackInfo::ParameterEncodingModeTy::FLATTEN ||
+          EncodingMode == CallbackInfo::ParameterEncodingModeTy::STACK) &&
+         "Unnown !callback metadata parameter encoding mode");
+  CI.ParameterEncodingMode =
+      static_cast<CallbackInfo::ParameterEncodingModeTy>(EncodingMode);
 }
diff --git a/llvm/lib/IR/MDBuilder.cpp b/llvm/lib/IR/MDBuilder.cpp
--- a/llvm/lib/IR/MDBuilder.cpp
+++ b/llvm/lib/IR/MDBuilder.cpp
@@ -104,12 +104,14 @@
   return MDNode::get(Context, Ops);
 }
 
-MDNode *MDBuilder::createCallbackEncoding(unsigned CalleeArgNo,
+MDNode *MDBuilder::createCallbackEncoding(unsigned EncodeMode,
+                                          unsigned CalleeArgNo,
                                           ArrayRef<int> Arguments,
                                           bool VarArgArePassed) {
   SmallVector<Metadata *, 4> Ops;
 
   Type *Int64 = Type::getInt64Ty(Context);
+  Ops.push_back(createConstant(ConstantInt::get(Int64, EncodeMode)));
   Ops.push_back(createConstant(ConstantInt::get(Int64, CalleeArgNo)));
 
   for (int ArgNo : Arguments)
diff --git a/llvm/test/Analysis/CallGraph/callback-calls.ll b/llvm/test/Analysis/CallGraph/callback-calls.ll
--- a/llvm/test/Analysis/CallGraph/callback-calls.ll
+++ b/llvm/test/Analysis/CallGraph/callback-calls.ll
@@ -17,4 +17,4 @@
 declare !callback !0 void @broker(void (i32*)*, i32*)
 
 !0 = !{!1}
-!1 = !{i64 0, i64 1, i1 false}
+!1 = !{i64 0, i64 0, i64 1, i1 false}
diff --git a/llvm/test/Analysis/CallGraph/ignore-callback-uses.ll b/llvm/test/Analysis/CallGraph/ignore-callback-uses.ll
--- a/llvm/test/Analysis/CallGraph/ignore-callback-uses.ll
+++ b/llvm/test/Analysis/CallGraph/ignore-callback-uses.ll
@@ -48,4 +48,4 @@
 declare !callback !2 void @__kmpc_fork_call(%struct.ident_t*, i32, void (i32*, i32*, ...)*, ...) #2
 
 !2 = !{!3}
-!3 = !{i64 2, i64 -1, i64 -1, i1 true}
+!3 = !{i64 0, i64 2, i64 -1, i64 -1, i1 true}
diff --git a/llvm/test/Transforms/Attributor/IPConstantProp/multiple_callbacks.ll b/llvm/test/Transforms/Attributor/IPConstantProp/multiple_callbacks.ll
--- a/llvm/test/Transforms/Attributor/IPConstantProp/multiple_callbacks.ll
+++ b/llvm/test/Transforms/Attributor/IPConstantProp/multiple_callbacks.ll
@@ -146,7 +146,7 @@
 
 declare !callback !3 void @broker(i32 (i32)*, i32 (i32)*, i32 (i32)*, i32, i32)
 
-!0 = !{i64 0, i64 3, i1 false}
-!1 = !{i64 1, i64 4, i1 false}
-!2 = !{i64 2, i64 3, i1 false}
+!0 = !{i64 0, i64 0, i64 3, i1 false}
+!1 = !{i64 0, i64 1, i64 4, i1 false}
+!2 = !{i64 0, i64 2, i64 3, i1 false}
 !3 = !{!0, !2, !1}
diff --git a/llvm/test/Transforms/Attributor/IPConstantProp/openmp_parallel_for.ll b/llvm/test/Transforms/Attributor/IPConstantProp/openmp_parallel_for.ll
--- a/llvm/test/Transforms/Attributor/IPConstantProp/openmp_parallel_for.ll
+++ b/llvm/test/Transforms/Attributor/IPConstantProp/openmp_parallel_for.ll
@@ -285,5 +285,5 @@
 
 declare !callback !0 dso_local void @__kmpc_fork_call(%struct.ident_t*, i32, void (i32*, i32*, ...)*, ...)
 
-!1 = !{i64 2, i64 -1, i64 -1, i1 true}
+!1 = !{i64 0, i64 2, i64 -1, i64 -1, i1 true}
 !0 = !{!1}
diff --git a/llvm/test/Transforms/Attributor/IPConstantProp/pthreads.ll b/llvm/test/Transforms/Attributor/IPConstantProp/pthreads.ll
--- a/llvm/test/Transforms/Attributor/IPConstantProp/pthreads.ll
+++ b/llvm/test/Transforms/Attributor/IPConstantProp/pthreads.ll
@@ -134,5 +134,5 @@
   ret i8* %arg
 }
 
-!1 = !{i64 2, i64 3, i1 false}
+!1 = !{i64 0, i64 2, i64 3, i1 false}
 !0 = !{!1}
diff --git a/llvm/test/Transforms/Attributor/IPConstantProp/thread_local_acs.ll b/llvm/test/Transforms/Attributor/IPConstantProp/thread_local_acs.ll
--- a/llvm/test/Transforms/Attributor/IPConstantProp/thread_local_acs.ll
+++ b/llvm/test/Transforms/Attributor/IPConstantProp/thread_local_acs.ll
@@ -67,5 +67,5 @@
 
 declare !callback !0 dso_local void @broker(i32*, i32 (i32*, i32*)*, i32*)
 
-!1 = !{i64 1, i64 0, i64 2, i1 false}
+!1 = !{i64 0, i64 1, i64 0, i64 2, i1 false}
 !0 = !{!1}
diff --git a/llvm/test/Transforms/Attributor/callbacks.ll b/llvm/test/Transforms/Attributor/callbacks.ll
--- a/llvm/test/Transforms/Attributor/callbacks.ll
+++ b/llvm/test/Transforms/Attributor/callbacks.ll
@@ -417,4 +417,4 @@
 declare !callback !0 void @t3_callback_broker(i32* nocapture , i32* nocapture , void (i32*, i32*, ...)* nocapture, ...)
 
 !0 = !{!1}
-!1 = !{i64 2, i64 -1, i64 -1, i1 true}
+!1 = !{i64 0, i64 2, i64 -1, i64 -1, i1 true}
diff --git a/llvm/test/Transforms/Attributor/noundef.ll b/llvm/test/Transforms/Attributor/noundef.ll
--- a/llvm/test/Transforms/Attributor/noundef.ll
+++ b/llvm/test/Transforms/Attributor/noundef.ll
@@ -122,5 +122,5 @@
 }
 
 declare !callback !0 void @callback_broker(void (i8*)*, i8*)
-!1 = !{i64 0, i64 1, i1 false}
+!1 = !{i64 0, i64 0, i64 1, i1 false}
 !0 = !{!1}
diff --git a/llvm/test/Transforms/OpenMP/parallel_deletion.ll b/llvm/test/Transforms/OpenMP/parallel_deletion.ll
--- a/llvm/test/Transforms/OpenMP/parallel_deletion.ll
+++ b/llvm/test/Transforms/OpenMP/parallel_deletion.ll
@@ -442,7 +442,7 @@
 !0 = !{i32 1, !"wchar_size", i32 4}
 !1 = !{!"clang"}
 !2 = !{!3}
-!3 = !{i64 2, i64 -1, i64 -1, i1 true}
+!3 = !{i64 0, i64 2, i64 -1, i64 -1, i1 true}
 !4 = !{!5, !5, i64 0}
 !5 = !{!"int", !6, i64 0}
 !6 = !{!"omnipotent char", !7, i64 0}
diff --git a/llvm/test/Transforms/OpenMP/parallel_deletion_cg_update.ll b/llvm/test/Transforms/OpenMP/parallel_deletion_cg_update.ll
--- a/llvm/test/Transforms/OpenMP/parallel_deletion_cg_update.ll
+++ b/llvm/test/Transforms/OpenMP/parallel_deletion_cg_update.ll
@@ -9,7 +9,7 @@
 ; CHECK:   CS<None> calls function 'd'
 ;
 ; CHECK: Call graph node for function: '.omp_outlined..0'<<{{.*}}>>  #uses=1
-; 
+;
 ; CHECK: Call graph node for function: '.omp_outlined..1'<<{{.*}}>>  #uses=3
 ; CHECK:   CS<{{.*}}> calls function 'd'
 ;
@@ -89,4 +89,4 @@
 !0 = !{i32 1, !"wchar_size", i32 4}
 !1 = !{!"clang version 11.0.0"}
 !2 = !{!3}
-!3 = !{i64 2, i64 -1, i64 -1, i1 true}
+!3 = !{i64 0, i64 2, i64 -1, i64 -1, i1 true}
diff --git a/llvm/test/Transforms/OpenMP/parallel_deletion_remarks.ll b/llvm/test/Transforms/OpenMP/parallel_deletion_remarks.ll
--- a/llvm/test/Transforms/OpenMP/parallel_deletion_remarks.ll
+++ b/llvm/test/Transforms/OpenMP/parallel_deletion_remarks.ll
@@ -96,7 +96,7 @@
 !21 = !DILocation(line: 14, column: 1, scope: !15)
 !22 = !DILocation(line: 16, column: 1, scope: !15)
 !23 = !{!24}
-!24 = !{i64 2, i64 -1, i64 -1, i1 true}
+!24 = !{i64 0, i64 2, i64 -1, i64 -1, i1 true}
 !25 = distinct !DISubprogram(name: ".omp_outlined.", scope: !1, file: !1, line: 9, type: !26, scopeLine: 9, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagLocalToUnit | DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !33)
 !26 = !DISubroutineType(types: !27)
 !27 = !{null, !28, !28}
diff --git a/llvm/test/Transforms/OpenMP/parallel_region_merging.ll b/llvm/test/Transforms/OpenMP/parallel_region_merging.ll
--- a/llvm/test/Transforms/OpenMP/parallel_region_merging.ll
+++ b/llvm/test/Transforms/OpenMP/parallel_region_merging.ll
@@ -786,7 +786,7 @@
 
 !0 = !{i32 1, !"wchar_size", i32 4}
 !1 = !{!2}
-!2 = !{i64 2, i64 -1, i64 -1, i1 true}
+!2 = !{i64 0, i64 2, i64 -1, i64 -1, i1 true}
 ; CHECK-LABEL: define {{[^@]+}}@merge
 ; CHECK-SAME: (i32 [[A:%.*]]) local_unnamed_addr {
 ; CHECK-NEXT:  entry:
diff --git a/llvm/unittests/IR/AbstractCallSiteTest.cpp b/llvm/unittests/IR/AbstractCallSiteTest.cpp
--- a/llvm/unittests/IR/AbstractCallSiteTest.cpp
+++ b/llvm/unittests/IR/AbstractCallSiteTest.cpp
@@ -36,7 +36,7 @@
       "}\n"
       "declare !callback !0 void @broker(i32, void (i8*, ...)*, ...)\n"
       "!0 = !{!1}\n"
-      "!1 = !{i64 1, i64 -1, i1 true}";
+      "!1 = !{i64 0, i64 1, i64 -1, i1 true}";
 
   std::unique_ptr<Module> M = parseIR(C, IR);
   ASSERT_TRUE(M);
diff --git a/llvm/unittests/IR/LegacyPassManagerTest.cpp b/llvm/unittests/IR/LegacyPassManagerTest.cpp
--- a/llvm/unittests/IR/LegacyPassManagerTest.cpp
+++ b/llvm/unittests/IR/LegacyPassManagerTest.cpp
@@ -768,7 +768,7 @@
                        "}\n"
                        "\n"
                        "!0 = !{!1}\n"
-                       "!1 = !{i64 0, i64 1, i1 false}";
+                       "!1 = !{i64 0, i64 0, i64 1, i1 false}";
 
       SMDiagnostic Err;
       std::unique_ptr<Module> M = parseAssemblyString(IR, Err, Context);