TableGen: Introduce `llvm::TableGen::Emitter` to register backends

`Opt(flag, func, desc)` registers an option into `Action`.

`OptClass<EmitterC>` is also available if `EmitterC(RK).run(OS)` is capable.

`Action` is defined as `ManagedStatic<cl::opt>` to guarantee to be created
when each registration of emitter is invoked.

`llvm::TableGenMain(argv0, MainFn)` invokes `Action` instead of `MainFn`

Differential Revision: https://reviews.llvm.org/D144351

3 files changed, 25 insertions, 6 deletions

diff --git a/llvm/lib/TableGen/Main.cpp b/llvm/lib/TableGen/Main.cpp
index ee72b4b..9aee1f8 100644
--- a/llvm/lib/TableGen/Main.cpp
+++ b/llvm/lib/TableGen/Main.cpp
@@ -29,6 +29,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/TableGen/Error.h"
 #include "llvm/TableGen/Record.h"
+#include "llvm/TableGen/TableGenBackend.h"
 #include <memory>
 #include <string>
 #include <system_error>
@@ -131,7 +132,10 @@ int llvm::TableGenMain(const char *argv0,
   std::string OutString;
   raw_string_ostream Out(OutString);
   unsigned status = 0;
-  if (MainFn)
+  TableGen::Emitter::FnT ActionFn = TableGen::Emitter::Action->getValue();
+  if (ActionFn)
+    ActionFn(Records, Out);
+  else if (MainFn)
     status = MainFn(Out, Records);
   else
     return 1;
diff --git a/llvm/lib/TableGen/TableGenBackend.cpp b/llvm/lib/TableGen/TableGenBackend.cpp
index 252f126..135ec64 100644
--- a/llvm/lib/TableGen/TableGenBackend.cpp
+++ b/llvm/lib/TableGen/TableGenBackend.cpp
@@ -13,12 +13,21 @@
 #include "llvm/TableGen/TableGenBackend.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/raw_ostream.h"
+#include <algorithm>
 #include <cassert>
+#include <cstddef>
 
 using namespace llvm;
 
 const size_t MAX_LINE_LEN = 80U;
 
+namespace llvm::TableGen::Emitter {
+ManagedStatic<cl::opt<FnT>, OptCreatorT> Action;
+void *OptCreatorT::call() {
+  return new cl::opt<FnT>(cl::desc("Action to perform:"));
+}
+} // namespace llvm::TableGen::Emitter
+
 static void printLine(raw_ostream &OS, const Twine &Prefix, char Fill,
                       StringRef Suffix) {
   size_t Pos = (size_t)OS.tell();
diff --git a/llvm/lib/TableGen/TableGenBackendSkeleton.cpp b/llvm/lib/TableGen/TableGenBackendSkeleton.cpp
index 0ba00c8..2fde4a6 100644
--- a/llvm/lib/TableGen/TableGenBackendSkeleton.cpp
+++ b/llvm/lib/TableGen/TableGenBackendSkeleton.cpp
@@ -46,14 +46,20 @@ void SkeletonEmitter::run(raw_ostream &OS) {
   (void)Records; // To suppress unused variable warning; remove on use.
 }
 
-namespace llvm {
+// Choose either option A or B.
 
-// The only thing that should be in the llvm namespace is the
-// emitter entry point function.
+//===----------------------------------------------------------------------===//
+// Option A: Register the backed as class <SkeletonEmitter>
+static TableGen::Emitter::OptClass<SkeletonEmitter>
+    X("gen-skeleton-class", "Generate example skeleton class");
 
-void EmitSkeleton(RecordKeeper &RK, raw_ostream &OS) {
+//===----------------------------------------------------------------------===//
+// Option B: Register "EmitSkeleton" directly
+// The emitter entry may be private scope.
+static void EmitSkeleton(RecordKeeper &RK, raw_ostream &OS) {
   // Instantiate the emitter class and invoke run().
   SkeletonEmitter(RK).run(OS);
 }
 
-} // namespace llvm
+static TableGen::Emitter::Opt Y("gen-skeleton-entry", EmitSkeleton,
+                                "Generate example skeleton entry");