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-rw-r--r--absl/types/internal/variant.h349
1 files changed, 298 insertions, 51 deletions
diff --git a/absl/types/internal/variant.h b/absl/types/internal/variant.h
index 61c56ddf..3b884647 100644
--- a/absl/types/internal/variant.h
+++ b/absl/types/internal/variant.h
@@ -19,21 +19,26 @@
#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"
#include "absl/utility/utility.h"
namespace absl {
+inline namespace lts_2018_06_20 {
template <class... Types>
class variant;
@@ -119,6 +124,8 @@ 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 +255,270 @@ struct MakeVisitationMatrix<ReturnType, FunctionObject,
ReturnType, FunctionObject, index_sequence<TailEndIndices...>,
absl::make_index_sequence<HeadEndIndex>, BoundIndices...> {};
-template <std::size_t... EndIndices, class Op, class... SizeT>
-VisitIndicesResultT<Op, SizeT...> visit_indices(Op&& op, SizeT... indices) {
- return AccessSimpleArray(
- MakeVisitationMatrix<VisitIndicesResultT<Op, SizeT...>, Op,
- index_sequence<(EndIndices + 1)...>>::Run(),
- (indices + 1)...)(absl::forward<Op>(op));
-}
+struct UnreachableSwitchCase {
+ template <class Op>
+ [[noreturn]] static VisitIndicesResultT<Op, std::size_t> Run(
+ Op&& /*ignored*/) {
+#if ABSL_HAVE_BUILTIN(__builtin_unreachable) || \
+ (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 +528,10 @@ 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 +573,7 @@ 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());
@@ -841,49 +1101,40 @@ using EqualResult = decltype(std::declval<T>() == std::declval<T>());
template <class T>
using NotEqualResult = decltype(std::declval<T>() != std::declval<T>());
-template <class T>
-using HasLessThan = is_detected_convertible<bool, LessThanResult, T>;
-
-template <class T>
-using HasGreaterThan = is_detected_convertible<bool, GreaterThanResult, T>;
-
-template <class T>
-using HasLessThanOrEqual =
- is_detected_convertible<bool, LessThanOrEqualResult, T>;
-
-template <class T>
-using HasGreaterThanOrEqual =
- is_detected_convertible<bool, GreaterThanOrEqualResult, T>;
-
-template <class T>
-using HasEqual = is_detected_convertible<bool, EqualResult, T>;
-
-template <class T>
-using HasNotEqual = is_detected_convertible<bool, NotEqualResult, T>;
-
template <class... T>
-using RequireAllHaveEqualT =
- absl::enable_if_t<absl::conjunction<HasEqual<T>...>::value, bool>;
+using RequireAllHaveEqualT = absl::enable_if_t<
+ absl::conjunction<is_detected_convertible<bool, EqualResult, T>...>::value,
+ bool>;
template <class... T>
using RequireAllHaveNotEqualT =
- absl::enable_if_t<absl::conjunction<HasEqual<T>...>::value, bool>;
+ absl::enable_if_t<absl::conjunction<is_detected_convertible<
+ bool, NotEqualResult, T>...>::value,
+ bool>;
template <class... T>
using RequireAllHaveLessThanT =
- absl::enable_if_t<absl::conjunction<HasLessThan<T>...>::value, bool>;
+ absl::enable_if_t<absl::conjunction<is_detected_convertible<
+ bool, LessThanResult, T>...>::value,
+ bool>;
template <class... T>
using RequireAllHaveLessThanOrEqualT =
- absl::enable_if_t<absl::conjunction<HasLessThan<T>...>::value, bool>;
+ absl::enable_if_t<absl::conjunction<is_detected_convertible<
+ bool, LessThanOrEqualResult, T>...>::value,
+ bool>;
template <class... T>
using RequireAllHaveGreaterThanOrEqualT =
- absl::enable_if_t<absl::conjunction<HasLessThan<T>...>::value, bool>;
+ absl::enable_if_t<absl::conjunction<is_detected_convertible<
+ bool, GreaterThanOrEqualResult, T>...>::value,
+ bool>;
template <class... T>
using RequireAllHaveGreaterThanT =
- absl::enable_if_t<absl::conjunction<HasLessThan<T>...>::value, bool>;
+ absl::enable_if_t<absl::conjunction<is_detected_convertible<
+ bool, GreaterThanResult, T>...>::value,
+ bool>;
// Helper template containing implementations details of variant that can't go
// in the private section. For convenience, this takes the variant type as a
@@ -1049,9 +1300,7 @@ class VariantStateBaseDestructorNontrivial : protected VariantStateBase<T...> {
VariantStateBaseDestructorNontrivial* self;
};
- void destroy() {
- variant_internal::visit_indices<sizeof...(T)>(Destroyer{this}, index_);
- }
+ void destroy() { VisitIndices<sizeof...(T)>::Run(Destroyer{this}, index_); }
~VariantStateBaseDestructorNontrivial() { destroy(); }
@@ -1087,8 +1336,7 @@ 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},
- other.index_);
+ VisitIndices<sizeof...(T)>::Run(Construct{this, &other}, other.index_);
index_ = other.index_;
}
@@ -1131,8 +1379,7 @@ class VariantCopyBaseNontrivial : protected VariantMoveBase<T...> {
VariantCopyBaseNontrivial(VariantCopyBaseNontrivial const& other)
: Base(NoopConstructorTag()) {
- variant_internal::visit_indices<sizeof...(T)>(Construct{this, &other},
- other.index_);
+ VisitIndices<sizeof...(T)>::Run(Construct{this, &other}, other.index_);
index_ = other.index_;
}
@@ -1166,7 +1413,7 @@ 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 +1442,7 @@ 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 +1583,7 @@ 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();
}
@@ -1370,11 +1617,10 @@ struct VariantHashBase<Variant,
if (var.valueless_by_exception()) {
return 239799884;
}
- size_t result =
- variant_internal::visit_indices<variant_size<Variant>::value>(
- PerformVisitation<VariantHashVisitor, const Variant&>{
- std::forward_as_tuple(var), VariantHashVisitor{}},
- var.index());
+ size_t result = VisitIndices<variant_size<Variant>::value>::Run(
+ PerformVisitation<VariantHashVisitor, const Variant&>{
+ std::forward_as_tuple(var), VariantHashVisitor{}},
+ var.index());
// Combine the index and the hash result in order to distinguish
// std::variant<int, int> holding the same value as different alternative.
return result ^ var.index();
@@ -1382,6 +1628,7 @@ struct VariantHashBase<Variant,
};
} // namespace variant_internal
+} // inline namespace lts_2018_06_20
} // namespace absl
#endif // ABSL_TYPES_variant_internal_H_
generated by cgit v1.2.3 (git 2.39.1) at 2024-05-14 06:50:32 +0000