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Make any_internal::FastTypeId() and IdForType() constexpr

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This means removing all side effects from FastTypeId(). So rather than instantiate dummy_var in the first call to FastTypeId(), move this responsibility to the linker, and only read its address during execution - guaranteed to never change. This allows for more optimization opportunities, with more explicit uses of constexpr
This commit is contained in:
Bronek Kozicki 2017-10-01 13:14:12 +01:00
parent 962e9931d5
commit 89f1f531d3
1 changed files with 29 additions and 21 deletions
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50
absl/types/any.h
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@ -94,23 +94,20 @@ namespace absl {
namespace any_internal {
// FastTypeId<Type>() evaluates at compile/link-time to a unique integer for the
// passed in type. Their values are neither contiguous nor small, making them
// unfit for using as an index into a vector, but a good match for keys into
// maps or straight up comparisons.
// Note that on 64-bit (unix) systems size_t is 64-bit while int is 32-bit and
// the compiler will happily and quietly assign such a 64-bit value to a
// 32-bit integer. While a client should never do that it SHOULD still be safe,
// assuming the BSS segment doesn't span more than 4GiB.
template<typename Type>
inline size_t FastTypeId() {
static_assert(sizeof(char*) <= sizeof(size_t),
"ptr size too large for size_t");
template <typename Type>
struct TypeTag {
constexpr static char dummy_var = 0;
};
// This static variable isn't actually used, only its address, so there are
// no concurrency issues.
static char dummy_var;
return reinterpret_cast<size_t>(&dummy_var);
template <typename Type>
constexpr char TypeTag<Type>::dummy_var;
// FastTypeId<Type>() evaluates at compile/link-time to a unique pointer for the
// passed in type. These are meant to be good match for keys into maps or straight
// up comparisons.
template<typename Type>
constexpr inline const void* FastTypeId() {
return &TypeTag<Type>::dummy_var;
}
} // namespace any_internal
@ -382,10 +379,20 @@ class any {
public:
virtual ~ObjInterface() = default;
virtual std::unique_ptr<ObjInterface> Clone() const = 0;
virtual size_t type_id() const noexcept = 0;
virtual const void* ObjTypeId() const noexcept = 0;
#if ABSL_ANY_DETAIL_HAS_RTTI
virtual const std::type_info& Type() const noexcept = 0;
#endif // ABSL_ANY_DETAIL_HAS_RTTI
// Note that on 64-bit (unix) systems size_t is 64-bit while int is 32-bit and
// the compiler will happily and quietly assign such a 64-bit value to a
// 32-bit integer. While a client should never do that it SHOULD still be safe,
// assuming the BSS segment doesn't span more than 4GiB.
size_t type_id() const noexcept {
static_assert(sizeof(void*) <= sizeof(size_t),
"ptr size too large for size_t");
return reinterpret_cast<size_t>(ObjTypeId());
}
};
// Hold a value of some queryable type, with an ability to Clone it.
@ -400,7 +407,7 @@ class any {
return std::unique_ptr<ObjInterface>(new Obj(in_place, value));
}
size_t type_id() const noexcept final { return IdForType<T>(); }
const void* ObjTypeId() const noexcept final { return IdForType<T>(); }
#if ABSL_ANY_DETAIL_HAS_RTTI
const std::type_info& Type() const noexcept final { return typeid(T); }
@ -415,7 +422,7 @@ class any {
}
template <typename T>
static size_t IdForType() {
constexpr static const void* IdForType() {
// Note: This type dance is to make the behavior consistent with typeid.
using NormalizedType =
typename std::remove_cv<typename std::remove_reference<T>::type>::type;
@ -423,8 +430,9 @@ class any {
return any_internal::FastTypeId<NormalizedType>();
}
size_t GetObjTypeId() const {
return obj_ == nullptr ? any_internal::FastTypeId<void>() : obj_->type_id();
const void* GetObjTypeId() const {
return obj_ == nullptr ? any_internal::FastTypeId<void>()
: obj_->ObjTypeId();
}
// `absl::any` nonmember functions //