[ADT] Add LLVM_MARK_AS_BITMASK_ENUM, used to enable bitwise operations on enums without static_cast.

Summary: Normally when you do a bitwise operation on an enum value, you
get back an instance of the underlying type (e.g. int).  But using this
macro, bitwise ops on your enum will return you back instances of the
enum.  This is particularly useful for enums which represent a
combination of flags.

Suppose you have a function which takes an int and a set of flags.  One
way to do this would be to take two numeric params:

  enum SomeFlags { F1 = 1, F2 = 2, F3 = 4, ... };
  void Fn(int Num, int Flags);

  void foo() {
    Fn(42, F2 | F3);
  }

But now if you get the order of arguments wrong, you won't get an error.

You might try to fix this by changing the signature of Fn so it accepts
a SomeFlags arg:

  enum SomeFlags { F1 = 1, F2 = 2, F3 = 4, ... };
  void Fn(int Num, SomeFlags Flags);

  void foo() {
    Fn(42, static_cast<SomeFlags>(F2 | F3));
  }

But now we need a static cast after doing "F2 | F3" because the result
of that computation is the enum's underlying type.

This patch adds a mechanism which gives us the safety of the second
approach with the brevity of the first.

  enum SomeFlags {
    F1 = 1, F2 = 2, F3 = 4, ..., F_MAX = 128,
    LLVM_MARK_AS_BITMASK_ENUM(F_MAX)
  };

  void Fn(int Num, SomeFlags Flags);

  void foo() {
    Fn(42, F2 | F3);  // No static_cast.
  }

The LLVM_MARK_AS_BITMASK_ENUM macro enables overloads for bitwise
operators on SomeFlags.  Critically, these operators return the enum
type, not its underlying type, so you don't need any static_casts.

An advantage of this solution over the previously-proposed BitMask class
[0, 1] is that we don't need any wrapper classes -- we can operate
directly on the enum itself.

The approach here is somewhat similar to OpenOffice's typed_flags_set
[2].  But we skirt the need for a wrapper class (and a good deal of
complexity) by judicious use of enable_if.  We SFINAE on the presence of
a particular enumerator (added by the LLVM_MARK_AS_BITMASK_ENUM macro)
instead of using a traits class so that it's impossible to use the enum
before the overloads are present.  The solution here also seamlessly
works across multiple namespaces.

[0] http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150622/283369.html
[1] http://lists.llvm.org/pipermail/llvm-commits/attachments/20150623/073434b6/attachment.obj
[2] https://cgit.freedesktop.org/libreoffice/core/tree/include/o3tl/typed_flags_set.hxx

Reviewers: chandlerc, rsmith

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D22279

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275292 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/BitmaskEnum.h

@@ -0,0 +1,152 @@
+//===-- llvm/ADT/BitmaskEnum.h ----------------------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_BITMASKENUM_H
+#define LLVM_ADT_BITMASKENUM_H
+
+#include <cassert>
+#include <type_traits>
+#include <utility>
+
+#include "llvm/Support/MathExtras.h"
+
+/// LLVM_MARK_AS_BITMASK_ENUM lets you opt in an individual enum type so you can
+/// perform bitwise operations on it without putting static_cast everywhere.
+///
+/// \code
+///   enum MyEnum {
+///     E1 = 1, E2 = 2, E3 = 4, E4 = 8,
+///     LLVM_MARK_AS_BITMASK_ENUM(/* LargestValue = */ E4)
+///   };
+///
+///   void Foo() {
+///     MyEnum A = (E1 | E2) & E3 ^ ~E4; // Look, ma: No static_cast!
+///   }
+/// \endcode
+///
+/// Normally when you do a bitwise operation on an enum value, you get back an
+/// instance of the underlying type (e.g. int).  But using this macro, bitwise
+/// ops on your enum will return you back instances of the enum.  This is
+/// particularly useful for enums which represent a combination of flags.
+///
+/// The parameter to LLVM_MARK_AS_BITMASK_ENUM should be the largest individual
+/// value in your enum.
+///
+/// All of the enum's values must be non-negative.
+#define LLVM_MARK_AS_BITMASK_ENUM(LargestValue)                                \
+  LLVM_BITMASK_LARGEST_ENUMERATOR = LargestValue
+
+/// LLVM_ENABLE_BITMASK_ENUMS_IN_NAMESPACE() pulls the operator overloads used
+/// by LLVM_MARK_AS_BITMASK_ENUM into the current namespace.
+///
+/// Suppose you have an enum foo::bar::MyEnum.  Before using
+/// LLVM_MARK_AS_BITMASK_ENUM on MyEnum, you must put
+/// LLVM_ENABLE_BITMASK_ENUMS_IN_NAMESPACE() somewhere inside namespace foo or
+/// namespace foo::bar.  This allows the relevant operator overloads to be found
+/// by ADL.
+///
+/// You don't need to use this macro in namespace llvm; it's done at the bottom
+/// of this file.
+#define LLVM_ENABLE_BITMASK_ENUMS_IN_NAMESPACE()                               \
+  using ::llvm::BitmaskEnumDetail::operator~;                                  \
+  using ::llvm::BitmaskEnumDetail::operator|;                                  \
+  using ::llvm::BitmaskEnumDetail::operator&;                                  \
+  using ::llvm::BitmaskEnumDetail::operator^;                                  \
+  using ::llvm::BitmaskEnumDetail::operator|=;                                 \
+  using ::llvm::BitmaskEnumDetail::operator&=;                                 \
+  using ::llvm::BitmaskEnumDetail::operator^=;
+
+namespace llvm {
+
+/// Traits class to determine whether an enum has a
+/// LLVM_BITMASK_LARGEST_ENUMERATOR enumerator.
+template <typename E, typename Enable = void>
+struct is_bitmask_enum : std::false_type {};
+
+template <typename E>
+struct is_bitmask_enum<
+    E, typename std::enable_if<sizeof(E::LLVM_BITMASK_LARGEST_ENUMERATOR) >=
+                               0>::type> : std::true_type {};
+namespace BitmaskEnumDetail {
+
+/// Get a bitmask with 1s in all places up to the high-order bit of E's largest
+/// value.
+template <typename E> typename std::underlying_type<E>::type Mask() {
+  // On overflow, NextPowerOf2 returns zero with the type uint64_t, so
+  // subtracting 1 gives us the mask with all bits set, like we want.
+  return NextPowerOf2(static_cast<typename std::underlying_type<E>::type>(
+             E::LLVM_BITMASK_LARGEST_ENUMERATOR)) -
+         1;
+}
+
+/// Check that Val is in range for E, and return Val cast to E's underlying
+/// type.
+template <typename E> typename std::underlying_type<E>::type Underlying(E Val) {
+  auto U = static_cast<typename std::underlying_type<E>::type>(Val);
+  assert(U >= 0 && "Negative enum values are not allowed.");
+  assert(U <= Mask<E>() && "Enum value too large (or largest val too small?)");
+  return U;
+}
+
+template <typename E,
+          typename = typename std::enable_if<is_bitmask_enum<E>::value>::type>
+E operator~(E Val) {
+  return static_cast<E>(~Underlying(Val) & Mask<E>());
+}
+
+template <typename E,
+          typename = typename std::enable_if<is_bitmask_enum<E>::value>::type>
+E operator|(E LHS, E RHS) {
+  return static_cast<E>(Underlying(LHS) | Underlying(RHS));
+}
+
+template <typename E,
+          typename = typename std::enable_if<is_bitmask_enum<E>::value>::type>
+E operator&(E LHS, E RHS) {
+  return static_cast<E>(Underlying(LHS) & Underlying(RHS));
+}
+
+template <typename E,
+          typename = typename std::enable_if<is_bitmask_enum<E>::value>::type>
+E operator^(E LHS, E RHS) {
+  return static_cast<E>(Underlying(LHS) ^ Underlying(RHS));
+}
+
+// |=, &=, and ^= return a reference to LHS, to match the behavior of the
+// operators on builtin types.
+
+template <typename E,
+          typename = typename std::enable_if<is_bitmask_enum<E>::value>::type>
+E &operator|=(E &LHS, E RHS) {
+  LHS = LHS | RHS;
+  return LHS;
+}
+
+template <typename E,
+          typename = typename std::enable_if<is_bitmask_enum<E>::value>::type>
+E &operator&=(E &LHS, E RHS) {
+  LHS = LHS & RHS;
+  return LHS;
+}
+
+template <typename E,
+          typename = typename std::enable_if<is_bitmask_enum<E>::value>::type>
+E &operator^=(E &LHS, E RHS) {
+  LHS = LHS ^ RHS;
+  return LHS;
+}
+
+} // namespace BitmaskEnumDetail
+
+// Enable bitmask enums in namespace ::llvm and all nested namespaces.
+LLVM_ENABLE_BITMASK_ENUMS_IN_NAMESPACE();
+
+} // namespace llvm
+
+#endif

unittests/ADT/BitmaskEnumTest.cpp

@@ -0,0 +1,134 @@
+//===- llvm/unittest/ADT/BitmaskEnumTest.cpp - BitmaskEnum unit tests -----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/BitmaskEnum.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+namespace {
+enum Flags {
+  F0 = 0,
+  F1 = 1,
+  F2 = 2,
+  F3 = 4,
+  F4 = 8,
+  LLVM_MARK_AS_BITMASK_ENUM(F4)
+};
+
+TEST(BitmaskEnumTest, BitwiseOr) {
+  Flags f = F1 | F2;
+  EXPECT_EQ(3, f);
+
+  f = f | F3;
+  EXPECT_EQ(7, f);
+}
+
+TEST(BitmaskEnumTest, BitwiseOrEquals) {
+  Flags f = F1;
+  f |= F3;
+  EXPECT_EQ(5, f);
+
+  // |= should return a reference to the LHS.
+  f = F2;
+  (f |= F3) = F1;
+  EXPECT_EQ(F1, f);
+}
+
+TEST(BitmaskEnumTest, BitwiseAnd) {
+  Flags f = static_cast<Flags>(3) & F2;
+  EXPECT_EQ(F2, f);
+
+  f = (f | F3) & (F1 | F2 | F3);
+  EXPECT_EQ(6, f);
+}
+
+TEST(BitmaskEnumTest, BitwiseAndEquals) {
+  Flags f = F1 | F2 | F3;
+  f &= F1 | F2;
+  EXPECT_EQ(3, f);
+
+  // &= should return a reference to the LHS.
+  (f &= F1) = F3;
+  EXPECT_EQ(F3, f);
+}
+
+TEST(BitmaskEnumTest, BitwiseXor) {
+  Flags f = (F1 | F2) ^ (F2 | F3);
+  EXPECT_EQ(5, f);
+
+  f = f ^ F1;
+  EXPECT_EQ(4, f);
+}
+
+TEST(BitmaskEnumTest, BitwiseXorEquals) {
+  Flags f = (F1 | F2);
+  f ^= (F2 | F4);
+  EXPECT_EQ(9, f);
+
+  // ^= should return a reference to the LHS.
+  (f ^= F4) = F3;
+  EXPECT_EQ(F3, f);
+}
+
+TEST(BitmaskEnumTest, BitwiseNot) {
+  Flags f = ~F1;
+  EXPECT_EQ(14, f); // Largest value for f is 15.
+  EXPECT_EQ(15, ~F0);
+}
+
+enum class FlagsClass {
+  F0 = 0,
+  F1 = 1,
+  F2 = 2,
+  F3 = 4,
+  LLVM_MARK_AS_BITMASK_ENUM(F3)
+};
+
+TEST(BitmaskEnumTest, ScopedEnum) {
+  FlagsClass f = (FlagsClass::F1 & ~FlagsClass::F0) | FlagsClass::F2;
+  f |= FlagsClass::F3;
+  EXPECT_EQ(7, static_cast<int>(f));
+}
+
+struct Container {
+  enum Flags { F0 = 0, F1 = 1, F2 = 2, F3 = 4, LLVM_MARK_AS_BITMASK_ENUM(F3) };
+
+  static Flags getFlags() {
+    Flags f = F0 | F1;
+    f |= F2;
+    return f;
+  }
+};
+
+TEST(BitmaskEnumTest, EnumInStruct) { EXPECT_EQ(3, Container::getFlags()); }
+
+} // namespace
+
+namespace foo {
+namespace bar {
+namespace {
+enum FlagsInNamespace {
+  F0 = 0,
+  F1 = 1,
+  F2 = 2,
+  F3 = 4,
+  LLVM_MARK_AS_BITMASK_ENUM(F3)
+};
+} // namespace
+} // namespace foo
+} // namespace bar
+
+namespace {
+TEST(BitmaskEnumTest, EnumInNamespace) {
+  foo::bar::FlagsInNamespace f = ~foo::bar::F0 & (foo::bar::F1 | foo::bar::F2);
+  f |= foo::bar::F3;
+  EXPECT_EQ(7, f);
+}
+} // namespace

unittests/ADT/CMakeLists.txt

@@ -7,6 +7,7 @@ set(ADTSources
   APIntTest.cpp
   APSIntTest.cpp
   ArrayRefTest.cpp
+  BitmaskEnumTest.cpp
   BitVectorTest.cpp
   DAGDeltaAlgorithmTest.cpp
   DeltaAlgorithmTest.cpp