diff --git a/clang/include/clang/Basic/Builtins.td b/clang/include/clang/Basic/Builtins.td
index b5b47ae274601..8c1d5e1c3e8ec 100644
--- a/clang/include/clang/Basic/Builtins.td
+++ b/clang/include/clang/Basic/Builtins.td
@@ -3347,10 +3347,12 @@ def VFork : LibBuiltin<"unistd.h"> {
 }
 
 // POSIX pthread.h
-// FIXME: This should be a GNULibBuiltin, but it's currently missing the prototype.
 
-def PthreadCreate : CustomEntry {
-  let Entry = "LIBBUILTIN(pthread_create, \"\",  \"fC<2,3>\", PTHREAD_H, ALL_GNU_LANGUAGES)";
+def PthreadCreate : GNULibBuiltin<"pthread.h"> {
+  let Spellings = ["pthread_create"];
+  let Attributes = [FunctionWithoutBuiltinPrefix, Callback<[2, 3]>];
+  // Note that we don't have an expressable prototype so we leave it empty.
+  let Prototype = "";
 }
 
 def SigSetJmp : LibBuiltin<"setjmp.h"> {
diff --git a/clang/include/clang/Basic/BuiltinsBase.td b/clang/include/clang/Basic/BuiltinsBase.td
index cff182f3f282c..8a3f024c20f44 100644
--- a/clang/include/clang/Basic/BuiltinsBase.td
+++ b/clang/include/clang/Basic/BuiltinsBase.td
@@ -17,6 +17,11 @@ class IndexedAttribute<string baseMangling, int I> : Attribute<baseMangling> {
   int Index = I;
 }
 
+class MultiIndexAttribute<string baseMangling, list<int> Is>
+    : Attribute<baseMangling> {
+  list<int> Indices = Is;
+}
+
 // Standard Attributes
 // -------------------
 def NoReturn : Attribute<"r">;
@@ -77,6 +82,10 @@ def Constexpr : Attribute<"E">;
 // Builtin is immediate and must be constant evaluated. Implies Constexpr, and will only be supported in C++20 mode.
 def Consteval : Attribute<"EG">;
 
+// Callback behavior: the first index argument is called with the arguments
+// indicated by the remaining indices.
+class Callback<list<int> ArgIndices> : MultiIndexAttribute<"C", ArgIndices>;
+
 // Builtin kinds
 // =============
 
@@ -90,10 +99,6 @@ class Builtin {
   bit RequiresUndef = 0;
 }
 
-class CustomEntry {
-  string Entry;
-}
-
 class AtomicBuiltin : Builtin;
 class TargetBuiltin : Builtin {
   string Features = "";
diff --git a/clang/utils/TableGen/ClangBuiltinsEmitter.cpp b/clang/utils/TableGen/ClangBuiltinsEmitter.cpp
index 6c3604adc92b9..f675dd154a7c9 100644
--- a/clang/utils/TableGen/ClangBuiltinsEmitter.cpp
+++ b/clang/utils/TableGen/ClangBuiltinsEmitter.cpp
@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "TableGenBackends.h"
+#include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/TableGen/Error.h"
 #include "llvm/TableGen/Record.h"
@@ -37,6 +38,14 @@ class PrototypeParser {
 private:
   void ParsePrototype(StringRef Prototype) {
     Prototype = Prototype.trim();
+
+    // Some builtins don't have an expressible prototype, simply emit an empty
+    // string for them.
+    if (Prototype.empty()) {
+      Type = "";
+      return;
+    }
+
     ParseTypes(Prototype);
   }
 
@@ -236,8 +245,15 @@ void PrintAttributes(const Record *Builtin, BuiltinType BT, raw_ostream &OS) {
 
   for (const auto *Attr : Builtin->getValueAsListOfDefs("Attributes")) {
     OS << Attr->getValueAsString("Mangling");
-    if (Attr->isSubClassOf("IndexedAttribute"))
+    if (Attr->isSubClassOf("IndexedAttribute")) {
       OS << ':' << Attr->getValueAsInt("Index") << ':';
+    } else if (Attr->isSubClassOf("MultiIndexAttribute")) {
+      OS << '<';
+      llvm::ListSeparator Sep(",");
+      for (int64_t Index : Attr->getValueAsListOfInts("Indices"))
+        OS << Sep << Index;
+      OS << '>';
+    }
   }
   OS << '\"';
 }
@@ -380,10 +396,6 @@ void clang::EmitClangBuiltins(const RecordKeeper &Records, raw_ostream &OS) {
     EmitBuiltin(OS, Builtin);
   }
 
-  for (const auto *Entry : Records.getAllDerivedDefinitions("CustomEntry")) {
-    OS << Entry->getValueAsString("Entry") << '\n';
-  }
-
   OS << R"c++(
 #undef ATOMIC_BUILTIN
 #undef BUILTIN