Remove error-prone mlir::ExecutionEngine::invoke overload.

[silvas]

Summary

I just spent a bunch of time debugging a mysterious bug that ended being due to my SmallVector getting passed to the Args&... overload instead of the MutableArrayRef overload, with disastrous results.

I appreciate the intent of this API, but for a function that does a bunch of unsafe casts, adding in potential overload confusion is just too much C++ footgun. If we end up needing this functionality, having something like a separate packArgs(Args&...) -> SmallVector overload would be preferable.

Turns out this API is unused and untested (even out of tree as far as I can tell, modulo the optional passing of no args to the other invoke as I fixed in this patch), so it's an easy fix -- just delete it and touch up the other overload.

mlir/include/mlir/ExecutionEngine/ExecutionEngine.h

create(ModuleOp m,
          ArrayRef<StringRef> sharedLibPaths = {}, bool enableObjectCache = true,
          bool enableGDBNotificationListener = true,
          bool enablePerfNotificationListener = true);
 
   /// Looks up a packed-argument function with the given name and returns a
   /// pointer to it.  Propagates errors in case of failure.
   llvm::Expected<void (*)(void **)> lookup(StringRef name) const;
 
-  /// Invokes the function with the given name passing it the list of arguments.
-  /// The arguments are accepted by lvalue-reference since the packed function
-  /// interface expects a list of non-null pointers.
-  template <typename... Args>
-  llvm::Error invoke(StringRef name, Args &... args);
-
   /// Invokes the function with the given name passing it the list of arguments
-  /// as a list of opaque pointers. This is the arity-agnostic equivalent of
-  /// the templated `invoke`.
-  llvm::Error invoke(StringRef name, MutableArrayRef<void *> args);
+  /// as a list of opaque pointers.
+  llvm::Error invoke(StringRef name, MutableArrayRef<void *> args = llvm::None);
 
   /// Set the target triple on the module. This is implicitly done when creating
   /// the engine.
   static bool setupTargetTriple(llvm::Module *llvmModule);
 
   /// Dump object code to output file `filename`.
   void dumpToObjectFile(StringRef filename);
 
private:
 
   /// GDB notification listener.
   llvm::JITEventListener *gdbListener;
 
   /// Perf notification listener.
   llvm::JITEventListener *perfListener;
 };
 
-template <typename... Args>
-llvm::Error ExecutionEngine::invoke(StringRef name, Args &... args) {
-  auto expectedFPtr = lookup(name);
-  if (!expectedFPtr)
-    return expectedFPtr.takeError();
-  auto fptr = *expectedFPtr;
-
-  SmallVector<void *, 8> packedArgs{static_cast<void *>(&args)...};
-  (*fptr)(packedArgs.data());
-
-  return llvm::Error::success();
-}
-
 } // end namespace mlir
 
 #endif // MLIR_EXECUTIONENGINE_EXECUTIONENGINE_H_