diff options
Diffstat (limited to 'absl/container/internal/layout.h')
| -rw-r--r-- | absl/container/internal/layout.h | 264 |
1 files changed, 190 insertions, 74 deletions
diff --git a/absl/container/internal/layout.h b/absl/container/internal/layout.h index a4ba6101..384929af 100644 --- a/absl/container/internal/layout.h +++ b/absl/container/internal/layout.h @@ -81,9 +81,30 @@ // } // // The layout we used above combines fixed-size with dynamically-sized fields. -// This is quite common. Layout is optimized for this use case and generates -// optimal code. All computations that can be performed at compile time are -// indeed performed at compile time. +// This is quite common. Layout is optimized for this use case and attempts to +// generate optimal code. To help the compiler do that in more cases, you can +// specify the fixed sizes using `WithStaticSizes`. This ensures that all +// computations that can be performed at compile time are indeed performed at +// compile time. Note that sometimes the `template` keyword is needed. E.g.: +// +// using SL = L::template WithStaticSizes<1, 1>; +// +// void Use(unsigned char* p) { +// // First, extract N and M. +// // Using `prefix` we can access the first three arrays but not more. +// // +// // More details: The first element always has offset 0. `SL` +// // has offsets for the second and third array based on sizes of +// // the first and second array, specified via `WithStaticSizes`. +// constexpr auto prefix = SL::Partial(); +// size_t n = *prefix.Pointer<0>(p); +// size_t m = *prefix.Pointer<1>(p); +// +// // Now we can get a pointer to the final payload. +// const SL layout(n, m); +// double* a = layout.Pointer<double>(p); +// int* b = layout.Pointer<int>(p); +// } // // Efficiency tip: The order of fields matters. In `Layout<T1, ..., TN>` try to // ensure that `alignof(T1) >= ... >= alignof(TN)`. This way you'll have no @@ -107,7 +128,7 @@ // CompactString(const char* s = "") { // const size_t size = strlen(s); // // size_t[1] followed by char[size + 1]. -// const L layout(1, size + 1); +// const L layout(size + 1); // p_.reset(new unsigned char[layout.AllocSize()]); // // If running under ASAN, mark the padding bytes, if any, to catch // // memory errors. @@ -125,14 +146,13 @@ // // const char* c_str() const { // // Equivalent to reinterpret_cast<char*>(p.get() + sizeof(size_t)). -// // The argument in Partial(1) specifies that we have size_t[1] in front -// // of the characters. -// return L::Partial(1).Pointer<char>(p_.get()); +// return L::Partial().Pointer<char>(p_.get()); // } // // private: -// // Our heap allocation contains a size_t followed by an array of chars. -// using L = Layout<size_t, char>; +// // Our heap allocation contains a single size_t followed by an array of +// // chars. +// using L = Layout<size_t, char>::WithStaticSizes<1>; // std::unique_ptr<unsigned char[]> p_; // }; // @@ -146,11 +166,12 @@ // // The interface exported by this file consists of: // - class `Layout<>` and its public members. -// - The public members of class `internal_layout::LayoutImpl<>`. That class -// isn't intended to be used directly, and its name and template parameter -// list are internal implementation details, but the class itself provides -// most of the functionality in this file. See comments on its members for -// detailed documentation. +// - The public members of classes `internal_layout::LayoutWithStaticSizes<>` +// and `internal_layout::LayoutImpl<>`. Those classes aren't intended to be +// used directly, and their name and template parameter list are internal +// implementation details, but the classes themselves provide most of the +// functionality in this file. See comments on their members for detailed +// documentation. // // `Layout<T1,... Tn>::Partial(count1,..., countm)` (where `m` <= `n`) returns a // `LayoutImpl<>` object. `Layout<T1,..., Tn> layout(count1,..., countn)` @@ -164,13 +185,14 @@ #include <stddef.h> #include <stdint.h> -#include <ostream> +#include <array> #include <string> #include <tuple> #include <type_traits> #include <typeinfo> #include <utility> +#include "absl/base/attributes.h" #include "absl/base/config.h" #include "absl/debugging/internal/demangle.h" #include "absl/meta/type_traits.h" @@ -209,9 +231,6 @@ struct NotAligned<const Aligned<T, N>> { template <size_t> using IntToSize = size_t; -template <class> -using TypeToSize = size_t; - template <class T> struct Type : NotAligned<T> { using type = T; @@ -308,7 +327,8 @@ using IsLegalElementType = std::integral_constant< !std::is_volatile<typename Type<T>::type>::value && adl_barrier::IsPow2(AlignOf<T>::value)>; -template <class Elements, class SizeSeq, class OffsetSeq> +template <class Elements, class StaticSizeSeq, class RuntimeSizeSeq, + class SizeSeq, class OffsetSeq> class LayoutImpl; // Public base class of `Layout` and the result type of `Layout::Partial()`. @@ -316,31 +336,49 @@ class LayoutImpl; // `Elements...` contains all template arguments of `Layout` that created this // instance. // -// `SizeSeq...` is `[0, NumSizes)` where `NumSizes` is the number of arguments -// passed to `Layout::Partial()` or `Layout::Layout()`. +// `StaticSizeSeq...` is an index_sequence containing the sizes specified at +// compile-time. +// +// `RuntimeSizeSeq...` is `[0, NumRuntimeSizes)`, where `NumRuntimeSizes` is the +// number of arguments passed to `Layout::Partial()` or `Layout::Layout()`. +// +// `SizeSeq...` is `[0, NumSizes)` where `NumSizes` is `NumRuntimeSizes` plus +// the number of sizes in `StaticSizeSeq`. // // `OffsetSeq...` is `[0, NumOffsets)` where `NumOffsets` is // `Min(sizeof...(Elements), NumSizes + 1)` (the number of arrays for which we // can compute offsets). -template <class... Elements, size_t... SizeSeq, size_t... OffsetSeq> -class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>, - absl::index_sequence<OffsetSeq...>> { +template <class... Elements, size_t... StaticSizeSeq, size_t... RuntimeSizeSeq, + size_t... SizeSeq, size_t... OffsetSeq> +class LayoutImpl< + std::tuple<Elements...>, absl::index_sequence<StaticSizeSeq...>, + absl::index_sequence<RuntimeSizeSeq...>, absl::index_sequence<SizeSeq...>, + absl::index_sequence<OffsetSeq...>> { private: static_assert(sizeof...(Elements) > 0, "At least one field is required"); static_assert(absl::conjunction<IsLegalElementType<Elements>...>::value, "Invalid element type (see IsLegalElementType)"); + static_assert(sizeof...(StaticSizeSeq) <= sizeof...(Elements), + "Too many static sizes specified"); enum { NumTypes = sizeof...(Elements), + NumStaticSizes = sizeof...(StaticSizeSeq), + NumRuntimeSizes = sizeof...(RuntimeSizeSeq), NumSizes = sizeof...(SizeSeq), NumOffsets = sizeof...(OffsetSeq), }; // These are guaranteed by `Layout`. + static_assert(NumStaticSizes + NumRuntimeSizes == NumSizes, "Internal error"); + static_assert(NumSizes <= NumTypes, "Internal error"); static_assert(NumOffsets == adl_barrier::Min(NumTypes, NumSizes + 1), "Internal error"); static_assert(NumTypes > 0, "Internal error"); + static constexpr std::array<size_t, sizeof...(StaticSizeSeq)> kStaticSizes = { + StaticSizeSeq...}; + // Returns the index of `T` in `Elements...`. Results in a compilation error // if `Elements...` doesn't contain exactly one instance of `T`. template <class T> @@ -363,7 +401,7 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>, template <size_t N> using ElementType = typename std::tuple_element<N, ElementTypes>::type; - constexpr explicit LayoutImpl(IntToSize<SizeSeq>... sizes) + constexpr explicit LayoutImpl(IntToSize<RuntimeSizeSeq>... sizes) : size_{sizes...} {} // Alignment of the layout, equal to the strictest alignment of all elements. @@ -389,7 +427,7 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>, constexpr size_t Offset() const { static_assert(N < NumOffsets, "Index out of bounds"); return adl_barrier::Align( - Offset<N - 1>() + SizeOf<ElementType<N - 1>>::value * size_[N - 1], + Offset<N - 1>() + SizeOf<ElementType<N - 1>>::value * Size<N - 1>(), ElementAlignment<N>::value); } @@ -411,8 +449,7 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>, return {{Offset<OffsetSeq>()...}}; } - // The number of elements in the Nth array. This is the Nth argument of - // `Layout::Partial()` or `Layout::Layout()` (zero-based). + // The number of elements in the Nth array (zero-based). // // // int[3], 4 bytes of padding, double[4]. // Layout<int, double> x(3, 4); @@ -420,10 +457,15 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>, // assert(x.Size<1>() == 4); // // Requires: `N < NumSizes`. - template <size_t N> + template <size_t N, EnableIf<(N < NumStaticSizes)> = 0> + constexpr size_t Size() const { + return kStaticSizes[N]; + } + + template <size_t N, EnableIf<(N >= NumStaticSizes)> = 0> constexpr size_t Size() const { static_assert(N < NumSizes, "Index out of bounds"); - return size_[N]; + return size_[N - NumStaticSizes]; } // The number of elements in the array with the specified element type. @@ -500,13 +542,8 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>, // std::tie(ints, doubles) = x.Pointers(p); // // Requires: `p` is aligned to `Alignment()`. - // - // Note: We're not using ElementType alias here because it does not compile - // under MSVC. template <class Char> - std::tuple<CopyConst< - Char, typename std::tuple_element<OffsetSeq, ElementTypes>::type>*...> - Pointers(Char* p) const { + auto Pointers(Char* p) const { return std::tuple<CopyConst<Char, ElementType<OffsetSeq>>*...>( Pointer<OffsetSeq>(p)...); } @@ -559,15 +596,10 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>, // // Requires: `p` is aligned to `Alignment()`. // - // Note: We're not using ElementType alias here because it does not compile - // under MSVC. + // Note: We mark the parameter as unused because GCC detects it is not used + // when `SizeSeq` is empty [-Werror=unused-but-set-parameter]. template <class Char> - std::tuple<SliceType<CopyConst< - Char, typename std::tuple_element<SizeSeq, ElementTypes>::type>>...> - Slices(Char* p) const { - // Workaround for https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63875 (fixed - // in 6.1). - (void)p; + auto Slices(ABSL_ATTRIBUTE_UNUSED Char* p) const { return std::tuple<SliceType<CopyConst<Char, ElementType<SizeSeq>>>...>( Slice<SizeSeq>(p)...); } @@ -582,7 +614,7 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>, constexpr size_t AllocSize() const { static_assert(NumTypes == NumSizes, "You must specify sizes of all fields"); return Offset<NumTypes - 1>() + - SizeOf<ElementType<NumTypes - 1>>::value * size_[NumTypes - 1]; + SizeOf<ElementType<NumTypes - 1>>::value * Size<NumTypes - 1>(); } // If built with --config=asan, poisons padding bytes (if any) in the @@ -606,7 +638,7 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>, // The `if` is an optimization. It doesn't affect the observable behaviour. if (ElementAlignment<N - 1>::value % ElementAlignment<N>::value) { size_t start = - Offset<N - 1>() + SizeOf<ElementType<N - 1>>::value * size_[N - 1]; + Offset<N - 1>() + SizeOf<ElementType<N - 1>>::value * Size<N - 1>(); ASAN_POISON_MEMORY_REGION(p + start, Offset<N>() - start); } #endif @@ -635,47 +667,66 @@ class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>, adl_barrier::TypeName<ElementType<OffsetSeq>>()...}; std::string res = absl::StrCat("@0", types[0], "(", sizes[0], ")"); for (size_t i = 0; i != NumOffsets - 1; ++i) { - absl::StrAppend(&res, "[", size_[i], "]; @", offsets[i + 1], types[i + 1], - "(", sizes[i + 1], ")"); + absl::StrAppend(&res, "[", DebugSize(i), "]; @", offsets[i + 1], + types[i + 1], "(", sizes[i + 1], ")"); } // NumSizes is a constant that may be zero. Some compilers cannot see that // inside the if statement "size_[NumSizes - 1]" must be valid. int last = static_cast<int>(NumSizes) - 1; if (NumTypes == NumSizes && last >= 0) { - absl::StrAppend(&res, "[", size_[last], "]"); + absl::StrAppend(&res, "[", DebugSize(static_cast<size_t>(last)), "]"); } return res; } private: + size_t DebugSize(size_t n) const { + if (n < NumStaticSizes) { + return kStaticSizes[n]; + } else { + return size_[n - NumStaticSizes]; + } + } + // Arguments of `Layout::Partial()` or `Layout::Layout()`. - size_t size_[NumSizes > 0 ? NumSizes : 1]; + size_t size_[NumRuntimeSizes > 0 ? NumRuntimeSizes : 1]; }; -template <size_t NumSizes, class... Ts> -using LayoutType = LayoutImpl< - std::tuple<Ts...>, absl::make_index_sequence<NumSizes>, - absl::make_index_sequence<adl_barrier::Min(sizeof...(Ts), NumSizes + 1)>>; +// Defining a constexpr static class member variable is redundant and deprecated +// in C++17, but required in C++14. +template <class... Elements, size_t... StaticSizeSeq, size_t... RuntimeSizeSeq, + size_t... SizeSeq, size_t... OffsetSeq> +constexpr std::array<size_t, sizeof...(StaticSizeSeq)> LayoutImpl< + std::tuple<Elements...>, absl::index_sequence<StaticSizeSeq...>, + absl::index_sequence<RuntimeSizeSeq...>, absl::index_sequence<SizeSeq...>, + absl::index_sequence<OffsetSeq...>>::kStaticSizes; -} // namespace internal_layout +template <class StaticSizeSeq, size_t NumRuntimeSizes, class... Ts> +using LayoutType = LayoutImpl< + std::tuple<Ts...>, StaticSizeSeq, + absl::make_index_sequence<NumRuntimeSizes>, + absl::make_index_sequence<NumRuntimeSizes + StaticSizeSeq::size()>, + absl::make_index_sequence<adl_barrier::Min( + sizeof...(Ts), NumRuntimeSizes + StaticSizeSeq::size() + 1)>>; + +template <class StaticSizeSeq, class... Ts> +class LayoutWithStaticSizes + : public LayoutType<StaticSizeSeq, + sizeof...(Ts) - adl_barrier::Min(sizeof...(Ts), + StaticSizeSeq::size()), + Ts...> { + private: + using Super = + LayoutType<StaticSizeSeq, + sizeof...(Ts) - + adl_barrier::Min(sizeof...(Ts), StaticSizeSeq::size()), + Ts...>; -// Descriptor of arrays of various types and sizes laid out in memory one after -// another. See the top of the file for documentation. -// -// Check out the public API of internal_layout::LayoutImpl above. The type is -// internal to the library but its methods are public, and they are inherited -// by `Layout`. -template <class... Ts> -class Layout : public internal_layout::LayoutType<sizeof...(Ts), Ts...> { public: - static_assert(sizeof...(Ts) > 0, "At least one field is required"); - static_assert( - absl::conjunction<internal_layout::IsLegalElementType<Ts>...>::value, - "Invalid element type (see IsLegalElementType)"); - // The result type of `Partial()` with `NumSizes` arguments. template <size_t NumSizes> - using PartialType = internal_layout::LayoutType<NumSizes, Ts...>; + using PartialType = + internal_layout::LayoutType<StaticSizeSeq, NumSizes, Ts...>; // `Layout` knows the element types of the arrays we want to lay out in // memory but not the number of elements in each array. @@ -701,14 +752,18 @@ class Layout : public internal_layout::LayoutType<sizeof...(Ts), Ts...> { // Note: The sizes of the arrays must be specified in number of elements, // not in bytes. // - // Requires: `sizeof...(Sizes) <= sizeof...(Ts)`. + // Requires: `sizeof...(Sizes) + NumStaticSizes <= sizeof...(Ts)`. // Requires: all arguments are convertible to `size_t`. template <class... Sizes> static constexpr PartialType<sizeof...(Sizes)> Partial(Sizes&&... sizes) { - static_assert(sizeof...(Sizes) <= sizeof...(Ts), ""); - return PartialType<sizeof...(Sizes)>(absl::forward<Sizes>(sizes)...); + static_assert(sizeof...(Sizes) + StaticSizeSeq::size() <= sizeof...(Ts), + ""); + return PartialType<sizeof...(Sizes)>( + static_cast<size_t>(std::forward<Sizes>(sizes))...); } + // Inherit LayoutType's constructor. + // // Creates a layout with the sizes of all arrays specified. If you know // only the sizes of the first N arrays (where N can be zero), you can use // `Partial()` defined above. The constructor is essentially equivalent to @@ -717,8 +772,69 @@ class Layout : public internal_layout::LayoutType<sizeof...(Ts), Ts...> { // // Note: The sizes of the arrays must be specified in number of elements, // not in bytes. - constexpr explicit Layout(internal_layout::TypeToSize<Ts>... sizes) - : internal_layout::LayoutType<sizeof...(Ts), Ts...>(sizes...) {} + // + // Implementation note: we do this via a `using` declaration instead of + // defining our own explicit constructor because the signature of LayoutType's + // constructor depends on RuntimeSizeSeq, which we don't have access to here. + // If we defined our own constructor here, it would have to use a parameter + // pack and then cast the arguments to size_t when calling the superclass + // constructor, similar to what Partial() does. But that would suffer from the + // same problem that Partial() has, which is that the parameter types are + // inferred from the arguments, which may be signed types, which must then be + // cast to size_t. This can lead to negative values being silently (i.e. with + // no compiler warnings) cast to an unsigned type. Having a constructor with + // size_t parameters helps the compiler generate better warnings about + // potential bad casts, while avoiding false warnings when positive literal + // arguments are used. If an argument is a positive literal integer (e.g. + // `1`), the compiler will understand that it can be safely converted to + // size_t, and hence not generate a warning. But if a negative literal (e.g. + // `-1`) or a variable with signed type is used, then it can generate a + // warning about a potentially unsafe implicit cast. It would be great if we + // could do this for Partial() too, but unfortunately as of C++23 there seems + // to be no way to define a function with a variable number of parameters of a + // certain type, a.k.a. homogeneous function parameter packs. So we're forced + // to choose between explicitly casting the arguments to size_t, which + // suppresses all warnings, even potentially valid ones, or implicitly casting + // them to size_t, which generates bogus warnings whenever literal arguments + // are used, even if they're positive. + using Super::Super; +}; + +} // namespace internal_layout + +// Descriptor of arrays of various types and sizes laid out in memory one after +// another. See the top of the file for documentation. +// +// Check out the public API of internal_layout::LayoutWithStaticSizes and +// internal_layout::LayoutImpl above. Those types are internal to the library +// but their methods are public, and they are inherited by `Layout`. +template <class... Ts> +class Layout : public internal_layout::LayoutWithStaticSizes< + absl::make_index_sequence<0>, Ts...> { + private: + using Super = + internal_layout::LayoutWithStaticSizes<absl::make_index_sequence<0>, + Ts...>; + + public: + // If you know the sizes of some or all of the arrays at compile time, you can + // use `WithStaticSizes` or `WithStaticSizeSequence` to create a `Layout` type + // with those sizes baked in. This can help the compiler generate optimal code + // for calculating array offsets and AllocSize(). + // + // Like `Partial()`, the N sizes you specify are for the first N arrays, and + // they specify the number of elements in each array, not the number of bytes. + template <class StaticSizeSeq> + using WithStaticSizeSequence = + internal_layout::LayoutWithStaticSizes<StaticSizeSeq, Ts...>; + + template <size_t... StaticSizes> + using WithStaticSizes = + WithStaticSizeSequence<std::index_sequence<StaticSizes...>>; + + // Inherit LayoutWithStaticSizes's constructor, which requires you to specify + // all the array sizes. + using Super::Super; }; } // namespace container_internal |