[libc++] Shrink variant's index type when possible
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
llvm-svn: 318621
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 0ed525382f733390e6400a058d5dead3c39449bb
diff --git a/include/variant b/include/variant
index f8d3e28..13450dd 100644
--- a/include/variant
+++ b/include/variant
@@ -207,6 +207,7 @@
#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 @@
};
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 @@
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 @@
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 @@
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 @@
#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 @@
_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 @@
},
*this);
}
- this->__index = __variant_npos;
+ this->__index = __variant_npos<__index_t>;
});
_LIBCPP_VARIANT_DESTRUCTOR(