Remove the CustomEntry escape hatch from builtin TableGen

This was an especially challenging escape hatch because it directly
forced the use of a specific X-macro structure and prevented any other
form of TableGen emission.

The problematic feature that motivated this is a case where a builtin's
prototype can't be represented in the mini-language used by TableGen.
Instead of adding a complete custom entry for this, this PR just teaches
the prototype handling to do the same thing the X-macros did in this
case: emit an empty string and let the Clang builtin handling respond
appropriately.

This should produce identical results while preserving all the rest of
the structured representation in the builtin TableGen code.

Author: chandlerc

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"> {

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 = "";

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