diff --git a/flang/lib/Lower/RTBuilder.h b/flang/lib/Lower/RTBuilder.h
deleted file mode 100644
--- a/flang/lib/Lower/RTBuilder.h
+++ /dev/null
@@ -1,287 +0,0 @@
-//===-- 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/BuiltinTypes.h"
-#include "mlir/IR/MLIRContext.h"
-#include "llvm/ADT/SmallVector.h"
-#include <functional>
-
-// List the runtime headers we want to be able to dissect
-#include "flang/Runtime/io-api.h"
-
-// Incomplete type indicating C99 complex ABI in interfaces. Beware, _Complex
-// and std::complex are layout compatible, but not compatible in all ABI call
-// interface (e.g. X86 32 bits). _Complex is not standard C++, so do not use
-// it here.
-struct c_float_complex_t;
-struct c_double_complex_t;
-
-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(context, 8 * sizeof(int));
-  };
-}
-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(context,
-                                  8 * sizeof(Fortran::runtime::io::Iostat));
-  };
-}
-template <>
-constexpr TypeBuilderFunc getModel<char *>() {
-  return [](mlir::MLIRContext *context) -> mlir::Type {
-    return fir::ReferenceType::get(mlir::IntegerType::get(context, 8));
-  };
-}
-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(context, 16));
-  };
-}
-template <>
-constexpr TypeBuilderFunc getModel<const char32_t *>() {
-  return [](mlir::MLIRContext *context) -> mlir::Type {
-    return fir::ReferenceType::get(mlir::IntegerType::get(context, 32));
-  };
-}
-template <>
-constexpr TypeBuilderFunc getModel<void **>() {
-  return [](mlir::MLIRContext *context) -> mlir::Type {
-    return fir::ReferenceType::get(
-        fir::PointerType::get(mlir::IntegerType::get(context, 8)));
-  };
-}
-template <>
-constexpr TypeBuilderFunc getModel<std::int64_t>() {
-  return [](mlir::MLIRContext *context) -> mlir::Type {
-    return mlir::IntegerType::get(context, 64);
-  };
-}
-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(context, 8 * sizeof(std::size_t));
-  };
-}
-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(context, 1);
-  };
-}
-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<c_float_complex_t>() {
-  return [](mlir::MLIRContext *context) -> mlir::Type {
-    return fir::ComplexType::get(context, sizeof(float));
-  };
-}
-template <>
-constexpr TypeBuilderFunc getModel<c_double_complex_t>() {
-  return [](mlir::MLIRContext *context) -> mlir::Type {
-    return fir::ComplexType::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::io::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(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(ctxt, argTys, {retTy});
-    };
-  }
-};
-
-//===----------------------------------------------------------------------===//
-// Runtime table building (constexpr folded)
-//===----------------------------------------------------------------------===//
-
-template <char... Cs>
-using RuntimeIdentifier = std::integer_sequence<char, Cs...>;
-
-namespace details {
-template <typename T, T... As, T... Bs>
-static constexpr std::integer_sequence<T, As..., Bs...>
-concat(std::integer_sequence<T, As...>, std::integer_sequence<T, Bs...>) {
-  return {};
-}
-template <typename T, T... As, T... Bs, typename... Cs>
-static constexpr auto concat(std::integer_sequence<T, As...>,
-                             std::integer_sequence<T, Bs...>, Cs...) {
-  return concat(std::integer_sequence<T, As..., Bs...>{}, Cs{}...);
-}
-template <typename T>
-static constexpr std::integer_sequence<T> concat(std::integer_sequence<T>) {
-  return {};
-}
-template <typename T, T a>
-static constexpr auto filterZero(std::integer_sequence<T, a>) {
-  if constexpr (a != 0) {
-    return std::integer_sequence<T, a>{};
-  } else {
-    return std::integer_sequence<T>{};
-  }
-}
-template <typename T, T... b>
-static constexpr auto filter(std::integer_sequence<T, b...>) {
-  if constexpr (sizeof...(b) > 0) {
-    return details::concat(filterZero(std::integer_sequence<T, b>{})...);
-  } else {
-    return std::integer_sequence<T>{};
-  }
-}
-} // namespace details
-
-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'};
-};
-
-#undef E
-#define E(L, I) (I < sizeof(L) / sizeof(*L) ? L[I] : 0)
-#define QuoteKey(X) #X
-#define MacroExpandKey(X)                                                      \
-  E(X, 0), E(X, 1), E(X, 2), E(X, 3), E(X, 4), E(X, 5), E(X, 6), E(X, 7),      \
-      E(X, 8), E(X, 9), E(X, 10), E(X, 11), E(X, 12), E(X, 13), E(X, 14),      \
-      E(X, 15), E(X, 16), E(X, 17), E(X, 18), E(X, 19), E(X, 20), E(X, 21),    \
-      E(X, 22), E(X, 23), E(X, 24), E(X, 25), E(X, 26), E(X, 27), E(X, 28),    \
-      E(X, 29), E(X, 30), E(X, 31), E(X, 32), E(X, 33), E(X, 34), E(X, 35),    \
-      E(X, 36), E(X, 37), E(X, 38), E(X, 39), E(X, 40), E(X, 41), E(X, 42),    \
-      E(X, 43), E(X, 44), E(X, 45), E(X, 46), E(X, 47), E(X, 48), E(X, 49)
-#define ExpandKey(X) MacroExpandKey(QuoteKey(X))
-#define FullSeq(X) std::integer_sequence<char, ExpandKey(X)>
-#define AsSequence(X) decltype(Fortran::lower::details::filter(FullSeq(X){}))
-#define mkKey(X)                                                               \
-  Fortran::lower::RuntimeTableEntry<                                           \
-      Fortran::lower::RuntimeTableKey<decltype(X)>, AsSequence(X)>
-
-} // namespace Fortran::lower
-
-#endif // FORTRAN_LOWER_RTBUILDER_H
diff --git a/flang/unittests/Optimizer/RTBuilder.cpp b/flang/unittests/Optimizer/RTBuilder.cpp
--- a/flang/unittests/Optimizer/RTBuilder.cpp
+++ b/flang/unittests/Optimizer/RTBuilder.cpp
@@ -6,7 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "../../lib/Lower/RTBuilder.h"
+#include "flang/Optimizer/Builder/Runtime/RTBuilder.h"
 #include "gtest/gtest.h"
 #include "flang/Optimizer/Support/InitFIR.h"
 
@@ -25,7 +25,7 @@
   mlir::MLIRContext ctx(registry);
   fir::support::loadDialects(ctx);
   mlir::Type c99_cacosf_signature{
-      Fortran::lower::RuntimeTableKey<decltype(c99_cacosf)>::getTypeModel()(
+      fir::runtime::RuntimeTableKey<decltype(c99_cacosf)>::getTypeModel()(
           &ctx)};
   auto c99_cacosf_funcTy = c99_cacosf_signature.cast<mlir::FunctionType>();
   EXPECT_EQ(c99_cacosf_funcTy.getNumInputs(), 1u);