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[libc++] Shrink variant's index type when possible

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Summary:
Currently `std::variant` always uses an unsigned int to store the variant index. However this isn't nessesary and causes `std::variant` to be larger than it needs to be in most cases.

This patch changes the index type to be `unsigned char` when possible, and `unsigned short` or `unsigned int` otherwise, depending on the size (Although it's questionable if it's even possible to create a variant with 65535 elements.

Unfortunately this change is an ABI break, and as such is only enabled in ABI v2.

Reviewers: mpark

Reviewed By: mpark

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D40210

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@318621 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Eric Fiselier
2017-11-19 04:19:44 +00:00
parent 115071a08e
commit e7f8cd476d
3 changed files with 103 additions and 8 deletions
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39
include/variant
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@@ -207,6 +207,7 @@ namespace std {
#include <tuple>
#include <type_traits>
#include <utility>
#include <limits>
#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
@@ -283,7 +284,28 @@ struct _LIBCPP_TEMPLATE_VIS variant_alternative<_Ip, variant<_Types...>> {
};
constexpr size_t variant_npos = static_cast<size_t>(-1);
constexpr unsigned int __variant_npos = static_cast<unsigned int>(-1);
constexpr int __choose_index_type(unsigned int __num_elem) {
if (__num_elem < std::numeric_limits<unsigned char>::max())
return 0;
if (__num_elem < std::numeric_limits<unsigned short>::max())
return 1;
return 2;
}
template <size_t _NumAlts>
using __variant_index_t =
#ifndef _LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION
unsigned int;
#else
std::tuple_element_t<
__choose_index_type(_NumAlts),
std::tuple<unsigned char, unsigned short, unsigned int>
>;
#endif
template <class _IndexType>
constexpr _IndexType __variant_npos = static_cast<_IndexType>(-1);
namespace __find_detail {
@@ -647,9 +669,11 @@ _LIBCPP_VARIANT_UNION(_Trait::_Unavailable, ~__union() = delete;);
template <_Trait _DestructibleTrait, class... _Types>
class _LIBCPP_TEMPLATE_VIS __base {
public:
using __index_t = __variant_index_t<sizeof...(_Types)>;
inline _LIBCPP_INLINE_VISIBILITY
explicit constexpr __base(__valueless_t tag) noexcept
: __data(tag), __index(__variant_npos) {}
: __data(tag), __index(__variant_npos<__index_t>) {}
template <size_t _Ip, class... _Args>
inline _LIBCPP_INLINE_VISIBILITY
@@ -665,7 +689,7 @@ public:
inline _LIBCPP_INLINE_VISIBILITY
constexpr size_t index() const noexcept {
return __index == __variant_npos ? variant_npos : __index;
return __index == __variant_npos<__index_t> ? variant_npos : __index;
}
protected:
@@ -685,7 +709,7 @@ protected:
static constexpr size_t __size() { return sizeof...(_Types); }
__union<_DestructibleTrait, 0, _Types...> __data;
unsigned int __index;
__index_t __index;
friend struct __access::__base;
friend struct __visitation::__base;
@@ -696,10 +720,11 @@ class _LIBCPP_TEMPLATE_VIS __destructor;
#define _LIBCPP_VARIANT_DESTRUCTOR(destructible_trait, destructor, destroy) \
template <class... _Types> \
class _LIBCPP_TEMPLATE_VIS __destructor<__traits<_Types...>, \
class _LIBCPP_TEMPLATE_VIS __destructor<__traits<_Types...>, \
destructible_trait> \
: public __base<destructible_trait, _Types...> { \
using __base_type = __base<destructible_trait, _Types...>; \
using __index_t = typename __base_type::__index_t; \
\
public: \
using __base_type::__base_type; \
@@ -719,7 +744,7 @@ class _LIBCPP_TEMPLATE_VIS __destructor;
_LIBCPP_VARIANT_DESTRUCTOR(
_Trait::_TriviallyAvailable,
~__destructor() = default;,
void __destroy() noexcept { this->__index = __variant_npos; });
void __destroy() noexcept { this->__index = __variant_npos<__index_t>; });
_LIBCPP_VARIANT_DESTRUCTOR(
_Trait::_Available,
@@ -733,7 +758,7 @@ _LIBCPP_VARIANT_DESTRUCTOR(
},
*this);
}
this->__index = __variant_npos;
this->__index = __variant_npos<__index_t>;
});
_LIBCPP_VARIANT_DESTRUCTOR(