diff --git a/flang/lib/Lower/RTBuilder.h b/flang/lib/Lower/RTBuilder.h
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+++ b/flang/lib/Lower/RTBuilder.h
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+//===-- RTBuilder.h ---------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file defines some C++17 template classes that are used to convert the
+/// signatures of plain old C functions into a model that can be used to
+/// generate MLIR calls to those functions. This can be used to autogenerate
+/// tables at compiler compile-time to call runtime support code.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef FORTRAN_LOWER_RTBUILDER_H
+#define FORTRAN_LOWER_RTBUILDER_H
+
+#include "flang/Lower/ConvertType.h"
+#include "flang/Optimizer/Dialect/FIRType.h"
+#include "mlir/IR/MLIRContext.h"
+#include "mlir/IR/StandardTypes.h"
+#include "llvm/ADT/SmallVector.h"
+#include <functional>
+
+// List the runtime headers we want to be able to dissect
+#include "../../runtime/io-api.h"
+
+namespace Fortran::lower {
+
+using TypeBuilderFunc = mlir::Type (*)(mlir::MLIRContext *);
+using FuncTypeBuilderFunc = mlir::FunctionType (*)(mlir::MLIRContext *);
+
+//===----------------------------------------------------------------------===//
+// Type builder models
+//===----------------------------------------------------------------------===//
+
+/// Return a function that returns the type signature model for the type `T`
+/// when provided an MLIRContext*. This allows one to translate C(++) function
+/// signatures from runtime header files to MLIR signatures into a static table
+/// at compile-time.
+///
+/// For example, when `T` is `int`, return a function that returns the MLIR
+/// standard type `i32` when `sizeof(int)` is 4.
+template <typename T>
+static constexpr TypeBuilderFunc getModel();
+template <>
+constexpr TypeBuilderFunc getModel<int>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return mlir::IntegerType::get(8 * sizeof(int), context);
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<int &>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    TypeBuilderFunc f{getModel<int>()};
+    return fir::ReferenceType::get(f(context));
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<Fortran::runtime::io::Iostat>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return mlir::IntegerType::get(8 * sizeof(Fortran::runtime::io::Iostat),
+                                  context);
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<char *>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return fir::ReferenceType::get(mlir::IntegerType::get(8, context));
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<const char *>() {
+  return getModel<char *>();
+}
+template <>
+constexpr TypeBuilderFunc getModel<const char16_t *>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return fir::ReferenceType::get(mlir::IntegerType::get(16, context));
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<const char32_t *>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return fir::ReferenceType::get(mlir::IntegerType::get(32, context));
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<void **>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return fir::ReferenceType::get(
+        fir::PointerType::get(mlir::IntegerType::get(8, context)));
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<std::int64_t>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return mlir::IntegerType::get(64, context);
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<std::int64_t &>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    TypeBuilderFunc f{getModel<std::int64_t>()};
+    return fir::ReferenceType::get(f(context));
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<std::size_t>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return mlir::IntegerType::get(8 * sizeof(std::size_t), context);
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<Fortran::runtime::io::IoStatementState *>() {
+  return getModel<char *>();
+}
+template <>
+constexpr TypeBuilderFunc getModel<double>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return mlir::FloatType::getF64(context);
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<double &>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    TypeBuilderFunc f{getModel<double>()};
+    return fir::ReferenceType::get(f(context));
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<float>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return mlir::FloatType::getF32(context);
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<float &>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    TypeBuilderFunc f{getModel<float>()};
+    return fir::ReferenceType::get(f(context));
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<bool>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return mlir::IntegerType::get(1, context);
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<bool &>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    TypeBuilderFunc f{getModel<bool>()};
+    return fir::ReferenceType::get(f(context));
+  };
+}
+
+template <>
+constexpr TypeBuilderFunc getModel<float _Complex>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return fir::CplxType::get(context, sizeof(float));
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<double _Complex>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return fir::CplxType::get(context, sizeof(double));
+  };
+}
+
+template <>
+constexpr TypeBuilderFunc getModel<const Fortran::runtime::Descriptor &>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return fir::BoxType::get(mlir::NoneType::get(context));
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<const Fortran::runtime::NamelistGroup &>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    // FIXME: a namelist group must be some well-defined data structure, use a
+    // tuple as a proxy for the moment
+    return mlir::TupleType::get(llvm::None, context);
+  };
+}
+template <>
+constexpr TypeBuilderFunc getModel<void>() {
+  return [](mlir::MLIRContext *context) -> mlir::Type {
+    return mlir::NoneType::get(context);
+  };
+}
+
+template <typename...>
+struct RuntimeTableKey;
+template <typename RT, typename... ATs>
+struct RuntimeTableKey<RT(ATs...)> {
+  static constexpr FuncTypeBuilderFunc getTypeModel() {
+    return [](mlir::MLIRContext *ctxt) {
+      TypeBuilderFunc ret = getModel<RT>();
+      std::array<TypeBuilderFunc, sizeof...(ATs)> args = {getModel<ATs>()...};
+      mlir::Type retTy = ret(ctxt);
+      llvm::SmallVector<mlir::Type, sizeof...(ATs)> argTys;
+      for (auto f : args)
+        argTys.push_back(f(ctxt));
+      return mlir::FunctionType::get(argTys, {retTy}, ctxt);
+    };
+  }
+};
+
+//===----------------------------------------------------------------------===//
+// Runtime table building (constexpr folded)
+//===----------------------------------------------------------------------===//
+
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wgnu-string-literal-operator-template"
+#endif
+
+// clang++ generates warnings about usage of a GNU extension, ignore them
+template <char... Cs>
+using RuntimeIdentifier = std::integer_sequence<char, Cs...>;
+template <typename T, T... Cs>
+static constexpr RuntimeIdentifier<Cs...> operator""_rt_ident() {
+  return {};
+}
+
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
+template <typename...>
+struct RuntimeTableEntry;
+template <typename KT, char... Cs>
+struct RuntimeTableEntry<RuntimeTableKey<KT>, RuntimeIdentifier<Cs...>> {
+  static constexpr FuncTypeBuilderFunc getTypeModel() {
+    return RuntimeTableKey<KT>::getTypeModel();
+  }
+  static constexpr const char name[sizeof...(Cs) + 1] = {Cs..., '\0'};
+};
+
+#define QuoteKey(X) #X##_rt_ident
+#define ExpandKey(X) QuoteKey(X)
+#define mkKey(X)                                                               \
+  Fortran::lower::RuntimeTableEntry<                                           \
+      Fortran::lower::RuntimeTableKey<decltype(X)>, decltype(ExpandKey(X))>
+
+} // namespace Fortran::lower
+
+#endif // FORTRAN_LOWER_RTBUILDER_H