[flang][Evaluate] Implement rewriting framework for evaluate::Expr

flang/include/flang/Evaluate/rewrite.h

@@ -0,0 +1,160 @@
+//===-- include/flang/Evaluate/rewrite.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
+//
+//===----------------------------------------------------------------------===//
+#ifndef FORTRAN_EVALUATE_REWRITE_H_
+#define FORTRAN_EVALUATE_REWRITE_H_
+
+#include "flang/Common/visit.h"
+#include "flang/Evaluate/expression.h"
+#include "flang/Support/Fortran.h"
+#include "llvm/ADT/STLExtras.h"
+
+#include <tuple>
+#include <type_traits>
+#include <utility>
+#include <variant>
+
+namespace Fortran::evaluate {
+namespace rewrite {
+namespace detail {
+template <typename, typename = void> //
+struct IsOperation {
+  static constexpr bool value{false};
+};
+
+template <typename T>
+struct IsOperation<T, std::void_t<decltype(T::operands)>> {
+  static constexpr bool value{true};
+};
+} // namespace detail
+
+template <typename T>
+constexpr bool is_operation_v{detail::IsOperation<T>::value};
+
+/// Individual Expr<T> rewriter that simply constructs an expression that is
+/// identical to the input. This is a suitable base class for all user-defined
+/// rewriters.
+struct Identity {
+  template <typename T, typename U>
+  Expr<T> operator()(Expr<T> &&x, const U &op) {
+    return std::move(x);
+  }
+};
+
+/// Bottom-up Expr<T> rewriter.
+///
+/// The Mutator traverses and reconstructs given Expr<T>. Going bottom-up,
+/// whenever the traversal visits a sub-node of type Expr<U> (for some U),
+/// it will invoke the user-provided rewriter via the () operator.
+///
+/// If x is of type Expr<U>, it will call (in pseudo-code):
+///   rewriter_(x, active_member_of(x.u))
+/// The second parameter is there to make it easier to overload the () operator
+/// for specific operations in Expr<...>.
+///
+/// The user rewriter is only invoked for Expr<U>, not for Operation, nor any
+/// other subobject.
+template <typename Rewriter> struct Mutator {
+  Mutator(Rewriter &rewriter) : rewriter_(rewriter) {}
+
+  template <typename T, typename U = llvm::remove_cvref_t<T>>
+  U operator()(T &&x) {
+    if constexpr (std::is_lvalue_reference_v<T>) {
+      return Mutate(U(x));
+    } else {
+      return Mutate(std::move(x));
+    }
+  }
+
+private:
+  template <typename T> struct LambdaWithRvalueCapture {
+    LambdaWithRvalueCapture(Rewriter &r, Expr<T> &&c)
+        : rewriter_(r), capture_(std::move(c)) {}
+    template <typename S> Expr<T> operator()(const S &s) {
+      return rewriter_(std::move(capture_), s);
+    }
+
+  private:
+    Rewriter &rewriter_;
+    Expr<T> &&capture_;
+  };
+
+  template <typename T, typename = std::enable_if_t<!is_operation_v<T>>>
+  T Mutate(T &&x) const {
+    return std::move(x);
+  }
+
+  template <typename D, typename = std::enable_if_t<is_operation_v<D>>>
+  D Mutate(D &&op, std::make_index_sequence<D::operands> t = {}) const {
+    return MutateOp(std::move(op), t);
+  }
+
+  template <typename T> //
+  Expr<T> Mutate(Expr<T> &&x) const {
+    // First construct the new expression with the rewritten op.
+    Expr<T> n{common::visit(
+        [&](auto &&s) { //
+          return Expr<T>(Mutate(std::move(s)));
+        },
+        std::move(x.u))};
+    // Return the rewritten expression. The second visit is to make sure
+    // that the second argument in the call to the rewriter is a part of
+    // the Expr<T> passed to it.
+    return common::visit(
+        LambdaWithRvalueCapture<T>(rewriter_, std::move(n)), std::move(n.u));
+  }
+
+  template <typename... Ts>
+  std::variant<Ts...> Mutate(std::variant<Ts...> &&u) const {
+    return common::visit(
+        [this](auto &&s) { return Mutate(std::move(s)); }, std::move(u));
+  }
+
+  template <typename... Ts>
+  std::tuple<Ts...> Mutate(std::tuple<Ts...> &&t) const {
+    return MutateTuple(std::move(t), std::index_sequence_for<Ts...>{});
+  }
+
+  template <typename... Ts, size_t... Is>
+  std::tuple<Ts...> MutateTuple(
+      std::tuple<Ts...> &&t, std::index_sequence<Is...>) const {
+    return std::make_tuple(Mutate(std::move(std::get<Is>(t))...));
+  }
+
+  template <typename D, size_t... Is>
+  D MutateOp(D &&op, std::index_sequence<Is...>) const {
+    return D(Mutate(std::move(op.template operand<Is>()))...);
+  }
+
+  template <typename T, size_t... Is>
+  Extremum<T> MutateOp(Extremum<T> &&op, std::index_sequence<Is...>) const {
+    return Extremum<T>(
+        op.ordering, Mutate(std::move(op.template operand<Is>()))...);
+  }
+
+  template <int K, size_t... Is>
+  ComplexComponent<K> MutateOp(
+      ComplexComponent<K> &&op, std::index_sequence<Is...>) const {
+    return ComplexComponent<K>(
+        op.isImaginaryPart, Mutate(std::move(op.template operand<Is>()))...);
+  }
+
+  template <int K, size_t... Is>
+  LogicalOperation<K> MutateOp(
+      LogicalOperation<K> &&op, std::index_sequence<Is...>) const {
+    return LogicalOperation<K>(
+        op.logicalOperator, Mutate(std::move(op.template operand<Is>()))...);
+  }
+
+  Rewriter &rewriter_;
+};
+
+template <typename Rewriter> Mutator(Rewriter &) -> Mutator<Rewriter>;
+} // namespace rewrite
+} // namespace Fortran::evaluate
+
+#endif // FORTRAN_EVALUATE_REWRITE_H_