diff options
Diffstat (limited to 'absl/strings/internal')
-rw-r--r-- | absl/strings/internal/cord_internal.h | 82 | ||||
-rw-r--r-- | absl/strings/internal/string_constant.h | 70 | ||||
-rw-r--r-- | absl/strings/internal/string_constant_test.cc | 60 |
3 files changed, 200 insertions, 12 deletions
diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h index 00b9baa9..aa91a691 100644 --- a/absl/strings/internal/cord_internal.h +++ b/absl/strings/internal/cord_internal.h @@ -33,14 +33,17 @@ namespace cord_internal { // Wraps std::atomic for reference counting. class Refcount { public: - Refcount() : count_{1} {} - ~Refcount() {} + constexpr Refcount() : count_{kRefIncrement} {} + struct Immortal {}; + explicit constexpr Refcount(Immortal) : count_(kImmortalTag) {} - // Increments the reference count by 1. Imposes no memory ordering. - inline void Increment() { count_.fetch_add(1, std::memory_order_relaxed); } + // Increments the reference count. Imposes no memory ordering. + inline void Increment() { + count_.fetch_add(kRefIncrement, std::memory_order_relaxed); + } // Asserts that the current refcount is greater than 0. If the refcount is - // greater than 1, decrements the reference count by 1. + // greater than 1, decrements the reference count. // // Returns false if there are no references outstanding; true otherwise. // Inserts barriers to ensure that state written before this method returns @@ -48,19 +51,24 @@ class Refcount { // false. inline bool Decrement() { int32_t refcount = count_.load(std::memory_order_acquire); - assert(refcount > 0); - return refcount != 1 && count_.fetch_sub(1, std::memory_order_acq_rel) != 1; + assert(refcount > 0 || refcount & kImmortalTag); + return refcount != kRefIncrement && + count_.fetch_sub(kRefIncrement, std::memory_order_acq_rel) != + kRefIncrement; } // Same as Decrement but expect that refcount is greater than 1. inline bool DecrementExpectHighRefcount() { - int32_t refcount = count_.fetch_sub(1, std::memory_order_acq_rel); - assert(refcount > 0); - return refcount != 1; + int32_t refcount = + count_.fetch_sub(kRefIncrement, std::memory_order_acq_rel); + assert(refcount > 0 || refcount & kImmortalTag); + return refcount != kRefIncrement; } // Returns the current reference count using acquire semantics. - inline int32_t Get() const { return count_.load(std::memory_order_acquire); } + inline int32_t Get() const { + return count_.load(std::memory_order_acquire) >> kImmortalShift; + } // Returns whether the atomic integer is 1. // If the reference count is used in the conventional way, a @@ -70,9 +78,27 @@ class Refcount { // performs the memory barrier needed for the owning thread // to act on the object, knowing that it has exclusive access to the // object. - inline bool IsOne() { return count_.load(std::memory_order_acquire) == 1; } + inline bool IsOne() { + return count_.load(std::memory_order_acquire) == kRefIncrement; + } + + bool IsImmortal() const { + return (count_.load(std::memory_order_relaxed) & kImmortalTag) != 0; + } private: + // We reserve the bottom bit to tag a reference count as immortal. + // By making it `1` we ensure that we never reach `0` when adding/subtracting + // `2`, thus it never looks as if it should be destroyed. + // These are used for the StringConstant constructor where we do not increase + // the refcount at construction time (due to constinit requirements) but we + // will still decrease it at destruction time to avoid branching on Unref. + enum { + kImmortalShift = 1, + kRefIncrement = 1 << kImmortalShift, + kImmortalTag = kRefIncrement - 1 + }; + std::atomic<int32_t> count_; }; @@ -97,6 +123,10 @@ enum CordRepKind { }; struct CordRep { + CordRep() = default; + constexpr CordRep(Refcount::Immortal immortal, size_t l) + : length(l), refcount(immortal), tag(EXTERNAL), data{} {} + // The following three fields have to be less than 32 bytes since // that is the smallest supported flat node size. size_t length; @@ -135,6 +165,12 @@ using ExternalReleaserInvoker = void (*)(CordRepExternal*); // External CordReps are allocated together with a type erased releaser. The // releaser is stored in the memory directly following the CordRepExternal. struct CordRepExternal : public CordRep { + CordRepExternal() = default; + explicit constexpr CordRepExternal(absl::string_view str) + : CordRep(Refcount::Immortal{}, str.size()), + base(str.data()), + releaser_invoker(nullptr) {} + const char* base; // Pointer to function that knows how to call and destroy the releaser. ExternalReleaserInvoker releaser_invoker; @@ -178,6 +214,14 @@ struct CordRepExternalImpl } }; +template <typename Str> +struct ConstInitExternalStorage { + ABSL_CONST_INIT static CordRepExternal value; +}; + +template <typename Str> +CordRepExternal ConstInitExternalStorage<Str>::value(Str::value); + enum { kMaxInline = 15, // Tag byte & kMaxInline means we are storing a pointer. @@ -195,9 +239,23 @@ struct AsTree { char padding[kMaxInline + 1 - sizeof(absl::cord_internal::CordRep*) - 1]; char tagged_size; }; + +constexpr char GetOrNull(absl::string_view data, size_t pos) { + return pos < data.size() ? data[pos] : '\0'; +} + union InlineData { constexpr InlineData() : as_chars{} {} explicit constexpr InlineData(AsTree tree) : as_tree(tree) {} + explicit constexpr InlineData(absl::string_view chars) + : as_chars{GetOrNull(chars, 0), GetOrNull(chars, 1), + GetOrNull(chars, 2), GetOrNull(chars, 3), + GetOrNull(chars, 4), GetOrNull(chars, 5), + GetOrNull(chars, 6), GetOrNull(chars, 7), + GetOrNull(chars, 8), GetOrNull(chars, 9), + GetOrNull(chars, 10), GetOrNull(chars, 11), + GetOrNull(chars, 12), GetOrNull(chars, 13), + GetOrNull(chars, 14), static_cast<char>(chars.size())} {} AsTree as_tree; char as_chars[kMaxInline + 1]; diff --git a/absl/strings/internal/string_constant.h b/absl/strings/internal/string_constant.h new file mode 100644 index 00000000..b15f1d9b --- /dev/null +++ b/absl/strings/internal/string_constant.h @@ -0,0 +1,70 @@ +// Copyright 2020 The Abseil Authors. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// https://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +#ifndef ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_ +#define ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_ + +#include "absl/meta/type_traits.h" +#include "absl/strings/string_view.h" + +namespace absl { +ABSL_NAMESPACE_BEGIN +namespace strings_internal { + +// StringConstant<T> represents a compile time string constant. +// It can be accessed via its `absl::string_view value` static member. +// It is guaranteed that the `string_view` returned has constant `.data()`, +// constant `.size()` and constant `value[i]` for all `0 <= i < .size()` +// +// The `T` is an opaque type. It is guaranteed that different string constants +// will have different values of `T`. This allows users to associate the string +// constant with other static state at compile time. +// +// Instances should be made using the `MakeStringConstant()` factory function +// below. +template <typename T> +struct StringConstant { + private: + // Returns true if `view` points to constant data. + // Otherwise, it can't be constant evaluated. + static constexpr bool ValidateConstant(absl::string_view view) { + return view.empty() || 2 * view[0] != 1; + } + + public: + static constexpr absl::string_view value = T{}(); + constexpr absl::string_view operator()() const { return value; } + + static_assert(ValidateConstant(value), + "The input string_view must point to constant data."); +}; + +template <typename T> +constexpr absl::string_view StringConstant<T>::value; // NOLINT + +// Factory function for `StringConstant` instances. +// It supports callables that have a constexpr default constructor and a +// constexpr operator(). +// It must return an `absl::string_view` or `const char*` pointing to constant +// data. This is validated at compile time. +template <typename T> +constexpr StringConstant<T> MakeStringConstant(T) { + return {}; +} + +} // namespace strings_internal +ABSL_NAMESPACE_END +} // namespace absl + +#endif // ABSL_STRINGS_INTERNAL_STRING_CONSTANT_H_ diff --git a/absl/strings/internal/string_constant_test.cc b/absl/strings/internal/string_constant_test.cc new file mode 100644 index 00000000..392833cf --- /dev/null +++ b/absl/strings/internal/string_constant_test.cc @@ -0,0 +1,60 @@ +// Copyright 2020 The Abseil Authors. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// https://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +#include "absl/strings/internal/string_constant.h" + +#include "absl/meta/type_traits.h" +#include "gmock/gmock.h" +#include "gtest/gtest.h" + +namespace { + +using absl::strings_internal::MakeStringConstant; + +struct Callable { + constexpr absl::string_view operator()() const { + return absl::string_view("Callable", 8); + } +}; + +TEST(StringConstant, Traits) { + constexpr auto str = MakeStringConstant(Callable{}); + using T = decltype(str); + + EXPECT_TRUE(std::is_empty<T>::value); + EXPECT_TRUE(std::is_trivial<T>::value); + EXPECT_TRUE(absl::is_trivially_default_constructible<T>::value); + EXPECT_TRUE(absl::is_trivially_copy_constructible<T>::value); + EXPECT_TRUE(absl::is_trivially_move_constructible<T>::value); + EXPECT_TRUE(absl::is_trivially_destructible<T>::value); +} + +TEST(StringConstant, MakeFromCallable) { + constexpr auto str = MakeStringConstant(Callable{}); + using T = decltype(str); + EXPECT_EQ(Callable{}(), T::value); + EXPECT_EQ(Callable{}(), str()); +} + +TEST(StringConstant, MakeFromStringConstant) { + // We want to make sure the StringConstant itself is a valid input to the + // factory function. + constexpr auto str = MakeStringConstant(Callable{}); + constexpr auto str2 = MakeStringConstant(str); + using T = decltype(str2); + EXPECT_EQ(Callable{}(), T::value); + EXPECT_EQ(Callable{}(), str2()); +} + +} // namespace |