- b9a479321581cd0293f124041bf5c06f456afec1 Adds exception safety tests for absl::make_unique<T>(...) by Abseil Team <absl-team@google.com>

- 78c61364007f6ab66155c151d0061bbec89c3dbd Update variadic visitation to use a switch statement when... by Matt Calabrese <calabrese@google.com>
  - b62eb9546087e0001307a741fcdf023b2d156966 Merge GitHub PR #130 - Add MIPS support to GetProgramCoun... by Derek Mauro <dmauro@google.com>
  - 09ab5739de33c8f1bebab2bb70bf7d4331348f05 Update ABSL_ASSERT to silence clang-tidy warnings about c... by Matt Calabrese <calabrese@google.com>
  - e73ee389ce8fe1a90738973c219ebbb19bb389f3 Update unary visitation to use a switch statement when th... by Matt Calabrese <calabrese@google.com>
  - c8734ccf475b856c95220f21a5ec4f44302cb5ce Work around a MSVC bug for absl::variant, by making `Acce... by Xiaoyi Zhang <zhangxy@google.com>

GitOrigin-RevId: b9a479321581cd0293f124041bf5c06f456afec1
Change-Id: Idb6fc906087c0a4e6fc5c75a391c7f73101c613e

absl/base/macros.h

@@ -195,7 +195,8 @@ enum LinkerInitialized {
 #define ABSL_ASSERT(expr) (false ? (void)(expr) : (void)0)
 #else
 #define ABSL_ASSERT(expr)              \
-  (ABSL_PREDICT_TRUE((expr)) ? (void)0 : [] { assert(false && #expr); }())
+  (ABSL_PREDICT_TRUE((expr)) ? (void)0 \
+                             : [] { assert(false && #expr); }())  // NOLINT
 #endif
 
 #endif  // ABSL_BASE_MACROS_H_

absl/memory/BUILD.bazel

@@ -18,6 +18,7 @@ load(
     "//absl:copts.bzl",
     "ABSL_DEFAULT_COPTS",
     "ABSL_TEST_COPTS",
+    "ABSL_EXCEPTIONS_FLAG",
 )
 
 package(default_visibility = ["//visibility:public"])
@@ -45,3 +46,16 @@ cc_test(
         "@com_google_googletest//:gtest_main",
     ],
 )
+
+cc_test(
+    name = "memory_exception_safety_test",
+    srcs = [
+        "memory_exception_safety_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
+    deps = [
+        ":memory",
+        "//absl/base:exception_safety_testing",
+        "@com_google_googletest//:gtest_main",
+    ],
+)

absl/memory/CMakeLists.txt

@@ -49,4 +49,23 @@ absl_test(
 )
 
 
+# test memory_exception_safety_test
+set(MEMORY_EXCEPTION_SAFETY_TEST_SRC "memory_exception_safety_test.cc")
+set(MEMORY_EXCEPTION_SAFETY_TEST_PUBLIC_LIBRARIES
+  absl::memory
+  absl_base_internal_exception_safety_testing
+)
+
+absl_test(
+  TARGET
+    memory_exception_safety_test
+  SOURCES
+    ${MEMORY_EXCEPTION_SAFETY_TEST_SRC}
+  PUBLIC_LIBRARIES
+    ${MEMORY_EXCEPTION_SAFETY_TEST_PUBLIC_LIBRARIES}
+  PRIVATE_COMPILE_FLAGS
+    ${ABSL_EXCEPTIONS_FLAG}
+)
+
+
 

absl/memory/memory_exception_safety_test.cc

@@ -0,0 +1,49 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/memory/memory.h"
+
+#include "gtest/gtest.h"
+#include "absl/base/internal/exception_safety_testing.h"
+
+namespace absl {
+namespace {
+
+using Thrower = ::testing::ThrowingValue<>;
+
+TEST(MakeUnique, CheckForLeaks) {
+  constexpr int kValue = 321;
+  constexpr size_t kLength = 10;
+  auto tester = testing::MakeExceptionSafetyTester()
+                    .WithInitialValue(Thrower(kValue))
+                    // Ensures make_unique does not modify the input. The real
+                    // test, though, is ConstructorTracker checking for leaks.
+                    .WithInvariants(testing::strong_guarantee);
+
+  EXPECT_TRUE(tester.Test([](Thrower* thrower) {
+    static_cast<void>(absl::make_unique<Thrower>(*thrower));
+  }));
+
+  EXPECT_TRUE(tester.Test([](Thrower* thrower) {
+    static_cast<void>(absl::make_unique<Thrower>(std::move(*thrower)));
+  }));
+
+  // Test T[n] overload
+  EXPECT_TRUE(tester.Test([&](Thrower*) {
+    static_cast<void>(absl::make_unique<Thrower[]>(kLength));
+  }));
+}
+
+}  // namespace
+}  // namespace absl

absl/types/BUILD.bazel

@@ -247,6 +247,19 @@ cc_test(
     ],
 )
 
+cc_test(
+    name = "variant_benchmark",
+    srcs = [
+        "variant_benchmark.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":variant",
+        "//absl/utility",
+        "@com_github_google_benchmark//:benchmark_main",
+    ],
+)
+
 cc_test(
     name = "variant_exception_safety_test",
     size = "small",

absl/types/internal/variant.h

@@ -19,15 +19,19 @@
 #ifndef ABSL_TYPES_variant_internal_H_
 #define ABSL_TYPES_variant_internal_H_
 
+#include <cassert>
 #include <cstddef>
+#include <cstdlib>
 #include <memory>
 #include <stdexcept>
 #include <tuple>
 #include <type_traits>
 
+#include "absl/base/config.h"
 #include "absl/base/internal/identity.h"
 #include "absl/base/internal/inline_variable.h"
 #include "absl/base/internal/invoke.h"
+#include "absl/base/macros.h"
 #include "absl/base/optimization.h"
 #include "absl/meta/type_traits.h"
 #include "absl/types/bad_variant_access.h"
@@ -119,6 +123,8 @@ using GiveQualsToT = typename GiveQualsTo<T, U>::type;
 template <std::size_t I>
 using SizeT = std::integral_constant<std::size_t, I>;
 
+using NPos = SizeT<variant_npos>;
+
 template <class Variant, class T, class = void>
 struct IndexOfConstructedType {};
 
@@ -248,19 +254,270 @@ struct MakeVisitationMatrix<ReturnType, FunctionObject,
           ReturnType, FunctionObject, index_sequence<TailEndIndices...>,
           absl::make_index_sequence<HeadEndIndex>, BoundIndices...> {};
 
-template <std::size_t... EndIndices, class Op, class... SizeT>
+struct UnreachableSwitchCase {
-VisitIndicesResultT<Op, SizeT...> visit_indices(Op&& op, SizeT... indices) {
+  template <class Op>
-  return AccessSimpleArray(
+  [[noreturn]] static VisitIndicesResultT<Op, std::size_t> Run(
-      MakeVisitationMatrix<VisitIndicesResultT<Op, SizeT...>, Op,
+      Op&& /*ignored*/) {
-                           index_sequence<(EndIndices + 1)...>>::Run(),
+#if ABSL_HAVE_BUILTIN(__builtin_unreachable) || \
-      (indices + 1)...)(absl::forward<Op>(op));
+    (defined(__GNUC__) && !defined(__clang__))
-}
+    __builtin_unreachable();
+#elif defined(_MSC_VER)
+    __assume(false);
+#else
+    // Try to use assert of false being identified as an unreachable intrinsic.
+    // NOTE: We use assert directly to increase chances of exploiting an assume
+    //       intrinsic.
+    assert(false);  // NOLINT
+
+    // Hack to silence potential no return warning -- cause an infinite loop.
+    return Run(absl::forward<Op>(op));
+#endif  // Checks for __builtin_unreachable
+  }
+};
+
+template <class Op, std::size_t I>
+struct ReachableSwitchCase {
+  static VisitIndicesResultT<Op, std::size_t> Run(Op&& op) {
+    return absl::base_internal::Invoke(absl::forward<Op>(op), SizeT<I>());
+  }
+};
+
+// The number 33 is just a guess at a reasonable maximum to our switch. It is
+// not based on any analysis. The reason it is a power of 2 plus 1 instead of a
+// power of 2 is because the number was picked to correspond to a power of 2
+// amount of "normal" alternatives, plus one for the possibility of the user
+// providing "monostate" in addition to the more natural alternatives.
+ABSL_INTERNAL_INLINE_CONSTEXPR(std::size_t, MaxUnrolledVisitCases, 33);
+
+// Note: The default-definition is for unreachable cases.
+template <bool IsReachable>
+struct PickCaseImpl {
+  template <class Op, std::size_t I>
+  using Apply = UnreachableSwitchCase;
+};
+
+template <>
+struct PickCaseImpl</*IsReachable =*/true> {
+  template <class Op, std::size_t I>
+  using Apply = ReachableSwitchCase<Op, I>;
+};
+
+// Note: This form of dance with template aliases is to make sure that we
+//       instantiate a number of templates proportional to the number of variant
+//       alternatives rather than a number of templates proportional to our
+//       maximum unrolled amount of visitation cases (aliases are effectively
+//       "free" whereas other template instantiations are costly).
+template <class Op, std::size_t I, std::size_t EndIndex>
+using PickCase = typename PickCaseImpl<(I < EndIndex)>::template Apply<Op, I>;
 
 template <class ReturnType>
 [[noreturn]] ReturnType TypedThrowBadVariantAccess() {
   absl::variant_internal::ThrowBadVariantAccess();
 }
 
+// Given N variant sizes, determine the number of cases there would need to be
+// in a single switch-statement that would cover every possibility in the
+// corresponding N-ary visit operation.
+template <std::size_t... NumAlternatives>
+struct NumCasesOfSwitch;
+
+template <std::size_t HeadNumAlternatives, std::size_t... TailNumAlternatives>
+struct NumCasesOfSwitch<HeadNumAlternatives, TailNumAlternatives...> {
+  static constexpr std::size_t value =
+      (HeadNumAlternatives + 1) *
+      NumCasesOfSwitch<TailNumAlternatives...>::value;
+};
+
+template <>
+struct NumCasesOfSwitch<> {
+  static constexpr std::size_t value = 1;
+};
+
+// A switch statement optimizes better than the table of function pointers.
+template <std::size_t EndIndex>
+struct VisitIndicesSwitch {
+  static_assert(EndIndex <= MaxUnrolledVisitCases,
+                "Maximum unrolled switch size exceeded.");
+
+  template <class Op>
+  static VisitIndicesResultT<Op, std::size_t> Run(Op&& op, std::size_t i) {
+    switch (i) {
+      case 0:
+        return PickCase<Op, 0, EndIndex>::Run(absl::forward<Op>(op));
+      case 1:
+        return PickCase<Op, 1, EndIndex>::Run(absl::forward<Op>(op));
+      case 2:
+        return PickCase<Op, 2, EndIndex>::Run(absl::forward<Op>(op));
+      case 3:
+        return PickCase<Op, 3, EndIndex>::Run(absl::forward<Op>(op));
+      case 4:
+        return PickCase<Op, 4, EndIndex>::Run(absl::forward<Op>(op));
+      case 5:
+        return PickCase<Op, 5, EndIndex>::Run(absl::forward<Op>(op));
+      case 6:
+        return PickCase<Op, 6, EndIndex>::Run(absl::forward<Op>(op));
+      case 7:
+        return PickCase<Op, 7, EndIndex>::Run(absl::forward<Op>(op));
+      case 8:
+        return PickCase<Op, 8, EndIndex>::Run(absl::forward<Op>(op));
+      case 9:
+        return PickCase<Op, 9, EndIndex>::Run(absl::forward<Op>(op));
+      case 10:
+        return PickCase<Op, 10, EndIndex>::Run(absl::forward<Op>(op));
+      case 11:
+        return PickCase<Op, 11, EndIndex>::Run(absl::forward<Op>(op));
+      case 12:
+        return PickCase<Op, 12, EndIndex>::Run(absl::forward<Op>(op));
+      case 13:
+        return PickCase<Op, 13, EndIndex>::Run(absl::forward<Op>(op));
+      case 14:
+        return PickCase<Op, 14, EndIndex>::Run(absl::forward<Op>(op));
+      case 15:
+        return PickCase<Op, 15, EndIndex>::Run(absl::forward<Op>(op));
+      case 16:
+        return PickCase<Op, 16, EndIndex>::Run(absl::forward<Op>(op));
+      case 17:
+        return PickCase<Op, 17, EndIndex>::Run(absl::forward<Op>(op));
+      case 18:
+        return PickCase<Op, 18, EndIndex>::Run(absl::forward<Op>(op));
+      case 19:
+        return PickCase<Op, 19, EndIndex>::Run(absl::forward<Op>(op));
+      case 20:
+        return PickCase<Op, 20, EndIndex>::Run(absl::forward<Op>(op));
+      case 21:
+        return PickCase<Op, 21, EndIndex>::Run(absl::forward<Op>(op));
+      case 22:
+        return PickCase<Op, 22, EndIndex>::Run(absl::forward<Op>(op));
+      case 23:
+        return PickCase<Op, 23, EndIndex>::Run(absl::forward<Op>(op));
+      case 24:
+        return PickCase<Op, 24, EndIndex>::Run(absl::forward<Op>(op));
+      case 25:
+        return PickCase<Op, 25, EndIndex>::Run(absl::forward<Op>(op));
+      case 26:
+        return PickCase<Op, 26, EndIndex>::Run(absl::forward<Op>(op));
+      case 27:
+        return PickCase<Op, 27, EndIndex>::Run(absl::forward<Op>(op));
+      case 28:
+        return PickCase<Op, 28, EndIndex>::Run(absl::forward<Op>(op));
+      case 29:
+        return PickCase<Op, 29, EndIndex>::Run(absl::forward<Op>(op));
+      case 30:
+        return PickCase<Op, 30, EndIndex>::Run(absl::forward<Op>(op));
+      case 31:
+        return PickCase<Op, 31, EndIndex>::Run(absl::forward<Op>(op));
+      case 32:
+        return PickCase<Op, 32, EndIndex>::Run(absl::forward<Op>(op));
+      default:
+        ABSL_ASSERT(i == variant_npos);
+        return absl::base_internal::Invoke(absl::forward<Op>(op), NPos());
+    }
+  }
+};
+
+template <std::size_t... EndIndices>
+struct VisitIndicesFallback {
+  template <class Op, class... SizeT>
+  static VisitIndicesResultT<Op, SizeT...> Run(Op&& op, SizeT... indices) {
+    return AccessSimpleArray(
+        MakeVisitationMatrix<VisitIndicesResultT<Op, SizeT...>, Op,
+                             index_sequence<(EndIndices + 1)...>>::Run(),
+        (indices + 1)...)(absl::forward<Op>(op));
+  }
+};
+
+// Take an N-dimensional series of indices and convert them into a single index
+// without loss of information. The purpose of this is to be able to convert an
+// N-ary visit operation into a single switch statement.
+template <std::size_t...>
+struct FlattenIndices;
+
+template <std::size_t HeadSize, std::size_t... TailSize>
+struct FlattenIndices<HeadSize, TailSize...> {
+  template<class... SizeType>
+  static constexpr std::size_t Run(std::size_t head, SizeType... tail) {
+    return head + HeadSize * FlattenIndices<TailSize...>::Run(tail...);
+  }
+};
+
+template <>
+struct FlattenIndices<> {
+  static constexpr std::size_t Run() { return 0; }
+};
+
+// Take a single "flattened" index (flattened by FlattenIndices) and determine
+// the value of the index of one of the logically represented dimensions.
+template <std::size_t I, std::size_t IndexToGet, std::size_t HeadSize,
+          std::size_t... TailSize>
+struct UnflattenIndex {
+  static constexpr std::size_t value =
+      UnflattenIndex<I / HeadSize, IndexToGet - 1, TailSize...>::value;
+};
+
+template <std::size_t I, std::size_t HeadSize, std::size_t... TailSize>
+struct UnflattenIndex<I, 0, HeadSize, TailSize...> {
+  static constexpr std::size_t value = (I % HeadSize);
+};
+
+// The backend for converting an N-ary visit operation into a unary visit.
+template <class IndexSequence, std::size_t... EndIndices>
+struct VisitIndicesVariadicImpl;
+
+template <std::size_t... N, std::size_t... EndIndices>
+struct VisitIndicesVariadicImpl<absl::index_sequence<N...>, EndIndices...> {
+  // A type that can take an N-ary function object and converts it to a unary
+  // function object that takes a single, flattened index, and "unflattens" it
+  // into its individual dimensions when forwarding to the wrapped object.
+  template <class Op>
+  struct FlattenedOp {
+    template <std::size_t I>
+    VisitIndicesResultT<Op, decltype(EndIndices)...> operator()(
+        SizeT<I> /*index*/) && {
+      return base_internal::Invoke(
+          absl::forward<Op>(op),
+          SizeT<UnflattenIndex<I, N, (EndIndices + 1)...>::value -
+                std::size_t{1}>()...);
+    }
+
+    Op&& op;
+  };
+
+  template <class Op, class... SizeType>
+  static VisitIndicesResultT<Op, decltype(EndIndices)...> Run(
+      Op&& op, SizeType... i) {
+    return VisitIndicesSwitch<NumCasesOfSwitch<EndIndices...>::value>::Run(
+        FlattenedOp<Op>{absl::forward<Op>(op)},
+        FlattenIndices<(EndIndices + std::size_t{1})...>::Run(
+            (i + std::size_t{1})...));
+  }
+};
+
+template <std::size_t... EndIndices>
+struct VisitIndicesVariadic
+    : VisitIndicesVariadicImpl<absl::make_index_sequence<sizeof...(EndIndices)>,
+                               EndIndices...> {};
+
+// This implementation will flatten N-ary visit operations into a single switch
+// statement when the number of cases would be less than our maximum specified
+// switch-statement size.
+// TODO(calabrese)
+//   Based on benchmarks, determine whether the function table approach actually
+//   does optimize better than a chain of switch statements and possibly update
+//   the implementation accordingly. Also consider increasing the maximum switch
+//   size.
+template <std::size_t... EndIndices>
+struct VisitIndices
+    : absl::conditional_t<(NumCasesOfSwitch<EndIndices...>::value <=
+                           MaxUnrolledVisitCases),
+                          VisitIndicesVariadic<EndIndices...>,
+                          VisitIndicesFallback<EndIndices...>> {};
+
+template <std::size_t EndIndex>
+struct VisitIndices<EndIndex>
+    : absl::conditional_t<(EndIndex <= MaxUnrolledVisitCases),
+                          VisitIndicesSwitch<EndIndex>,
+                          VisitIndicesFallback<EndIndex>> {};
+
 // Suppress bogus warning on MSVC: MSVC complains that the `reinterpret_cast`
 // below is returning the address of a temporary or local object.
 #ifdef _MSC_VER
@@ -270,8 +527,10 @@ template <class ReturnType>
 
 // TODO(calabrese) std::launder
 // TODO(calabrese) constexpr
+// NOTE: DO NOT REMOVE the `inline` keyword as it is necessary to work around a
+// MSVC bug. See https://github.com/abseil/abseil-cpp/issues/129 for details.
 template <class Self, std::size_t I>
-VariantAccessResult<I, Self> AccessUnion(Self&& self, SizeT<I> /*i*/) {
+inline VariantAccessResult<I, Self> AccessUnion(Self&& self, SizeT<I> /*i*/) {
   return reinterpret_cast<VariantAccessResult<I, Self>>(self);
 }
 
@@ -313,7 +572,7 @@ struct VariantCoreAccess {
 
   template <class Variant>
   static void InitFrom(Variant& self, Variant&& other) {  // NOLINT
-    variant_internal::visit_indices<absl::variant_size<Variant>::value>(
+    VisitIndices<absl::variant_size<Variant>::value>::Run(
         InitFromVisitor<Variant, Variant&&>{&self,
                                             std::forward<Variant>(other)},
         other.index());
@@ -1049,9 +1308,7 @@ class VariantStateBaseDestructorNontrivial : protected VariantStateBase<T...> {
     VariantStateBaseDestructorNontrivial* self;
   };
 
-  void destroy() {
+  void destroy() { VisitIndices<sizeof...(T)>::Run(Destroyer{this}, index_); }
-    variant_internal::visit_indices<sizeof...(T)>(Destroyer{this}, index_);
-  }
 
   ~VariantStateBaseDestructorNontrivial() { destroy(); }
 
@@ -1087,8 +1344,7 @@ class VariantMoveBaseNontrivial : protected VariantStateBaseDestructor<T...> {
   VariantMoveBaseNontrivial(VariantMoveBaseNontrivial&& other) noexcept(
       absl::conjunction<std::is_nothrow_move_constructible<T>...>::value)
       : Base(NoopConstructorTag()) {
-    variant_internal::visit_indices<sizeof...(T)>(Construct{this, &other},
+    VisitIndices<sizeof...(T)>::Run(Construct{this, &other}, other.index_);
-                                                  other.index_);
     index_ = other.index_;
   }
 
@@ -1131,8 +1387,7 @@ class VariantCopyBaseNontrivial : protected VariantMoveBase<T...> {
 
   VariantCopyBaseNontrivial(VariantCopyBaseNontrivial const& other)
       : Base(NoopConstructorTag()) {
-    variant_internal::visit_indices<sizeof...(T)>(Construct{this, &other},
+    VisitIndices<sizeof...(T)>::Run(Construct{this, &other}, other.index_);
-                                                  other.index_);
     index_ = other.index_;
   }
 
@@ -1166,7 +1421,7 @@ class VariantMoveAssignBaseNontrivial : protected VariantCopyBase<T...> {
     operator=(VariantMoveAssignBaseNontrivial&& other) noexcept(
         absl::conjunction<std::is_nothrow_move_constructible<T>...,
                           std::is_nothrow_move_assignable<T>...>::value) {
-      variant_internal::visit_indices<sizeof...(T)>(
+      VisitIndices<sizeof...(T)>::Run(
           VariantCoreAccess::MakeMoveAssignVisitor(this, &other), other.index_);
       return *this;
     }
@@ -1195,7 +1450,7 @@ class VariantCopyAssignBaseNontrivial : protected VariantMoveAssignBase<T...> {
 
     VariantCopyAssignBaseNontrivial& operator=(
         const VariantCopyAssignBaseNontrivial& other) {
-      variant_internal::visit_indices<sizeof...(T)>(
+      VisitIndices<sizeof...(T)>::Run(
           VariantCoreAccess::MakeCopyAssignVisitor(this, other), other.index_);
       return *this;
     }
@@ -1336,7 +1591,7 @@ struct Swap {
   template <std::size_t Wi>
   void operator()(SizeT<Wi> /*w_i*/) {
     if (v->index() == Wi) {
-      visit_indices<sizeof...(Types)>(SwapSameIndex<Types...>{v, w}, Wi);
+      VisitIndices<sizeof...(Types)>::Run(SwapSameIndex<Types...>{v, w}, Wi);
     } else {
       generic_swap();
     }
@@ -1370,8 +1625,7 @@ struct VariantHashBase<Variant,
     if (var.valueless_by_exception()) {
       return 239799884;
     }
-    size_t result =
+    size_t result = VisitIndices<variant_size<Variant>::value>::Run(
-        variant_internal::visit_indices<variant_size<Variant>::value>(
         PerformVisitation<VariantHashVisitor, const Variant&>{
             std::forward_as_tuple(var), VariantHashVisitor{}},
         var.index());

absl/types/variant.h

@@ -416,8 +416,8 @@ constexpr absl::add_pointer_t<const T> get_if(
 template <typename Visitor, typename... Variants>
 variant_internal::VisitResult<Visitor, Variants...> visit(Visitor&& vis,
                                                           Variants&&... vars) {
-  return variant_internal::visit_indices<
+  return variant_internal::
-      variant_size<absl::decay_t<Variants>>::value...>(
+      VisitIndices<variant_size<absl::decay_t<Variants> >::value...>::Run(
           variant_internal::PerformVisitation<Visitor, Variants...>{
               std::forward_as_tuple(absl::forward<Variants>(vars)...),
               absl::forward<Visitor>(vis)},
@@ -573,7 +573,7 @@ class variant<T0, Tn...> : private variant_internal::VariantBase<T0, Tn...> {
   variant& operator=(T&& t) noexcept(
       std::is_nothrow_assignable<Tj&, T>::value&&
           std::is_nothrow_constructible<Tj, T>::value) {
-    variant_internal::visit_indices<sizeof...(Tn) + 1>(
+    variant_internal::VisitIndices<sizeof...(Tn) + 1>::Run(
         variant_internal::VariantCoreAccess::MakeConversionAssignVisitor(
             this, absl::forward<T>(t)),
         index());
@@ -682,7 +682,7 @@ class variant<T0, Tn...> : private variant_internal::VariantBase<T0, Tn...> {
   //   true and `is_nothrow_swappable()` is same as `std::is_trivial()`.
   void swap(variant& rhs) noexcept(
       absl::conjunction<std::is_trivial<T0>, std::is_trivial<Tn>...>::value) {
-    return variant_internal::visit_indices<sizeof...(Tn) + 1>(
+    return variant_internal::VisitIndices<sizeof...(Tn) + 1>::Run(
         variant_internal::Swap<T0, Tn...>{this, &rhs}, rhs.index());
   }
 };
@@ -722,7 +722,7 @@ template <typename... Types>
 constexpr variant_internal::RequireAllHaveEqualT<Types...> operator==(
     const variant<Types...>& a, const variant<Types...>& b) {
   return (a.index() == b.index()) &&
-         variant_internal::visit_indices<sizeof...(Types)>(
+         variant_internal::VisitIndices<sizeof...(Types)>::Run(
              variant_internal::EqualsOp<Types...>{&a, &b}, a.index());
 }
 
@@ -731,7 +731,7 @@ template <typename... Types>
 constexpr variant_internal::RequireAllHaveNotEqualT<Types...> operator!=(
     const variant<Types...>& a, const variant<Types...>& b) {
   return (a.index() != b.index()) ||
-         variant_internal::visit_indices<sizeof...(Types)>(
+         variant_internal::VisitIndices<sizeof...(Types)>::Run(
              variant_internal::NotEqualsOp<Types...>{&a, &b}, a.index());
 }
 
@@ -741,7 +741,7 @@ constexpr variant_internal::RequireAllHaveLessThanT<Types...> operator<(
     const variant<Types...>& a, const variant<Types...>& b) {
   return (a.index() != b.index())
              ? (a.index() + 1) < (b.index() + 1)
-             : variant_internal::visit_indices<sizeof...(Types)>(
+             : variant_internal::VisitIndices<sizeof...(Types)>::Run(
                    variant_internal::LessThanOp<Types...>{&a, &b}, a.index());
 }
 
@@ -751,7 +751,7 @@ constexpr variant_internal::RequireAllHaveGreaterThanT<Types...> operator>(
     const variant<Types...>& a, const variant<Types...>& b) {
   return (a.index() != b.index())
              ? (a.index() + 1) > (b.index() + 1)
-             : variant_internal::visit_indices<sizeof...(Types)>(
+             : variant_internal::VisitIndices<sizeof...(Types)>::Run(
                    variant_internal::GreaterThanOp<Types...>{&a, &b},
                    a.index());
 }
@@ -762,7 +762,7 @@ constexpr variant_internal::RequireAllHaveLessThanOrEqualT<Types...> operator<=(
     const variant<Types...>& a, const variant<Types...>& b) {
   return (a.index() != b.index())
              ? (a.index() + 1) < (b.index() + 1)
-             : variant_internal::visit_indices<sizeof...(Types)>(
+             : variant_internal::VisitIndices<sizeof...(Types)>::Run(
                    variant_internal::LessThanOrEqualsOp<Types...>{&a, &b},
                    a.index());
 }
@@ -773,7 +773,7 @@ constexpr variant_internal::RequireAllHaveGreaterThanOrEqualT<Types...>
 operator>=(const variant<Types...>& a, const variant<Types...>& b) {
   return (a.index() != b.index())
              ? (a.index() + 1) > (b.index() + 1)
-             : variant_internal::visit_indices<sizeof...(Types)>(
+             : variant_internal::VisitIndices<sizeof...(Types)>::Run(
                    variant_internal::GreaterThanOrEqualsOp<Types...>{&a, &b},
                    a.index());
 }

absl/types/variant_benchmark.cc

@@ -0,0 +1,220 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Unit tests for the variant template. The 'is' and 'IsEmpty' methods
+// of variant are not explicitly tested because they are used repeatedly
+// in building other tests. All other public variant methods should have
+// explicit tests.
+
+#include "absl/types/variant.h"
+
+#include <cstddef>
+#include <cstdlib>
+#include <string>
+#include <tuple>
+
+#include "benchmark/benchmark.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+namespace {
+
+template <std::size_t I>
+struct VariantAlternative {
+  char member;
+};
+
+template <class Indices>
+struct VariantOfAlternativesImpl;
+
+template <std::size_t... Indices>
+struct VariantOfAlternativesImpl<absl::index_sequence<Indices...>> {
+  using type = absl::variant<VariantAlternative<Indices>...>;
+};
+
+template <std::size_t NumAlternatives>
+using VariantOfAlternatives = typename VariantOfAlternativesImpl<
+    absl::make_index_sequence<NumAlternatives>>::type;
+
+struct Empty {};
+
+template <class... T>
+void Ignore(T...) noexcept {}
+
+template <class T>
+Empty DoNotOptimizeAndReturnEmpty(T&& arg) noexcept {
+  benchmark::DoNotOptimize(arg);
+  return {};
+}
+
+struct VisitorApplier {
+  struct Visitor {
+    template <class... T>
+    void operator()(T&&... args) const noexcept {
+      Ignore(DoNotOptimizeAndReturnEmpty(args)...);
+    }
+  };
+
+  template <class... Vars>
+  void operator()(const Vars&... vars) const noexcept {
+    absl::visit(Visitor(), vars...);
+  }
+};
+
+template <std::size_t NumIndices, std::size_t CurrIndex = NumIndices - 1>
+struct MakeWithIndex {
+  using Variant = VariantOfAlternatives<NumIndices>;
+
+  static Variant Run(std::size_t index) {
+    return index == CurrIndex
+               ? Variant(absl::in_place_index_t<CurrIndex>())
+               : MakeWithIndex<NumIndices, CurrIndex - 1>::Run(index);
+  }
+};
+
+template <std::size_t NumIndices>
+struct MakeWithIndex<NumIndices, 0> {
+  using Variant = VariantOfAlternatives<NumIndices>;
+
+  static Variant Run(std::size_t /*index*/) { return Variant(); }
+};
+
+template <std::size_t NumIndices, class Dimensions>
+struct MakeVariantTuple;
+
+template <class T, std::size_t /*I*/>
+using always_t = T;
+
+template <std::size_t NumIndices>
+VariantOfAlternatives<NumIndices> MakeVariant(std::size_t dimension,
+                                              std::size_t index) {
+  return dimension == 0
+             ? MakeWithIndex<NumIndices>::Run(index % NumIndices)
+             : MakeVariant<NumIndices>(dimension - 1, index / NumIndices);
+}
+
+template <std::size_t NumIndices, std::size_t... Dimensions>
+struct MakeVariantTuple<NumIndices, absl::index_sequence<Dimensions...>> {
+  using VariantTuple =
+      std::tuple<always_t<VariantOfAlternatives<NumIndices>, Dimensions>...>;
+
+  static VariantTuple Run(int index) {
+    return std::make_tuple(MakeVariant<NumIndices>(Dimensions, index)...);
+  }
+};
+
+constexpr std::size_t integral_pow(std::size_t base, std::size_t power) {
+  return power == 0 ? 1 : base * integral_pow(base, power - 1);
+}
+
+template <std::size_t End, std::size_t I = 0>
+struct VisitTestBody {
+  template <class Vars, class State>
+  static bool Run(Vars& vars, State& state) {
+    if (state.KeepRunning()) {
+      absl::apply(VisitorApplier(), vars[I]);
+      return VisitTestBody<End, I + 1>::Run(vars, state);
+    }
+    return false;
+  }
+};
+
+template <std::size_t End>
+struct VisitTestBody<End, End> {
+  template <class Vars, class State>
+  static bool Run(Vars& /*vars*/, State& /*state*/) {
+    return true;
+  }
+};
+
+// Visit operations where branch prediction is likely to give a boost.
+template <std::size_t NumIndices, std::size_t NumDimensions = 1>
+void BM_RedundantVisit(benchmark::State& state) {
+  auto vars =
+      MakeVariantTuple<NumIndices, absl::make_index_sequence<NumDimensions>>::
+          Run(static_cast<std::size_t>(state.range(0)));
+
+  for (auto _ : state) {  // NOLINT
+    benchmark::DoNotOptimize(vars);
+    absl::apply(VisitorApplier(), vars);
+  }
+}
+
+// Visit operations where branch prediction is unlikely to give a boost.
+template <std::size_t NumIndices, std::size_t NumDimensions = 1>
+void BM_Visit(benchmark::State& state) {
+  constexpr std::size_t num_possibilities =
+      integral_pow(NumIndices, NumDimensions);
+
+  using VariantTupleMaker =
+      MakeVariantTuple<NumIndices, absl::make_index_sequence<NumDimensions>>;
+  using Tuple = typename VariantTupleMaker::VariantTuple;
+
+  Tuple vars[num_possibilities];
+  for (std::size_t i = 0; i < num_possibilities; ++i)
+    vars[i] = VariantTupleMaker::Run(i);
+
+  while (VisitTestBody<num_possibilities>::Run(vars, state)) {
+  }
+}
+
+// Visitation
+//   Each visit is on a different variant with a different active alternative)
+
+// Unary visit
+BENCHMARK_TEMPLATE(BM_Visit, 1);
+BENCHMARK_TEMPLATE(BM_Visit, 2);
+BENCHMARK_TEMPLATE(BM_Visit, 3);
+BENCHMARK_TEMPLATE(BM_Visit, 4);
+BENCHMARK_TEMPLATE(BM_Visit, 5);
+BENCHMARK_TEMPLATE(BM_Visit, 6);
+BENCHMARK_TEMPLATE(BM_Visit, 7);
+BENCHMARK_TEMPLATE(BM_Visit, 8);
+BENCHMARK_TEMPLATE(BM_Visit, 16);
+BENCHMARK_TEMPLATE(BM_Visit, 32);
+BENCHMARK_TEMPLATE(BM_Visit, 64);
+
+// Binary visit
+BENCHMARK_TEMPLATE(BM_Visit, 1, 2);
+BENCHMARK_TEMPLATE(BM_Visit, 2, 2);
+BENCHMARK_TEMPLATE(BM_Visit, 3, 2);
+BENCHMARK_TEMPLATE(BM_Visit, 4, 2);
+BENCHMARK_TEMPLATE(BM_Visit, 5, 2);
+
+// Ternary visit
+BENCHMARK_TEMPLATE(BM_Visit, 1, 3);
+BENCHMARK_TEMPLATE(BM_Visit, 2, 3);
+BENCHMARK_TEMPLATE(BM_Visit, 3, 3);
+
+// Quaternary visit
+BENCHMARK_TEMPLATE(BM_Visit, 1, 4);
+BENCHMARK_TEMPLATE(BM_Visit, 2, 4);
+
+// Redundant Visitation
+//   Each visit consistently has the same alternative active
+
+// Unary visit
+BENCHMARK_TEMPLATE(BM_RedundantVisit, 1)->Arg(0);
+BENCHMARK_TEMPLATE(BM_RedundantVisit, 2)->DenseRange(0, 1);
+BENCHMARK_TEMPLATE(BM_RedundantVisit, 8)->DenseRange(0, 7);
+
+// Binary visit
+BENCHMARK_TEMPLATE(BM_RedundantVisit, 1, 2)->Arg(0);
+BENCHMARK_TEMPLATE(BM_RedundantVisit, 2, 2)
+    ->DenseRange(0, integral_pow(2, 2) - 1);
+BENCHMARK_TEMPLATE(BM_RedundantVisit, 4, 2)
+    ->DenseRange(0, integral_pow(4, 2) - 1);
+
+}  // namespace
+}  // namespace absl