diff options
Diffstat (limited to 'absl/strings/internal/cord_rep_flat.h')
-rw-r--r-- | absl/strings/internal/cord_rep_flat.h | 79 |
1 files changed, 60 insertions, 19 deletions
diff --git a/absl/strings/internal/cord_rep_flat.h b/absl/strings/internal/cord_rep_flat.h index a98aa9df..e3e27fcd 100644 --- a/absl/strings/internal/cord_rep_flat.h +++ b/absl/strings/internal/cord_rep_flat.h @@ -20,6 +20,8 @@ #include <cstdint> #include <memory> +#include "absl/base/config.h" +#include "absl/base/macros.h" #include "absl/strings/internal/cord_internal.h" namespace absl { @@ -42,23 +44,45 @@ static constexpr size_t kMinFlatSize = 32; static constexpr size_t kMaxFlatSize = 4096; static constexpr size_t kMaxFlatLength = kMaxFlatSize - kFlatOverhead; static constexpr size_t kMinFlatLength = kMinFlatSize - kFlatOverhead; +static constexpr size_t kMaxLargeFlatSize = 256 * 1024; +static constexpr size_t kMaxLargeFlatLength = kMaxLargeFlatSize - kFlatOverhead; +// kTagBase should make the Size <--> Tag computation resilient +// against changes to the value of FLAT when we add a new tag.. +static constexpr uint8_t kTagBase = FLAT - 4; + +// Converts the provided rounded size to the corresponding tag constexpr uint8_t AllocatedSizeToTagUnchecked(size_t size) { - return static_cast<uint8_t>((size <= 1024) ? size / 8 - : 128 + size / 32 - 1024 / 32); + return static_cast<uint8_t>(size <= 512 ? kTagBase + size / 8 + : size <= 8192 + ? kTagBase + 512 / 8 + size / 64 - 512 / 64 + : kTagBase + 512 / 8 + ((8192 - 512) / 64) + + size / 4096 - 8192 / 4096); +} + +// Converts the provided tag to the corresponding allocated size +constexpr size_t TagToAllocatedSize(uint8_t tag) { + return (tag <= kTagBase + 512 / 8) ? tag * 8 - kTagBase * 8 + : (tag <= kTagBase + (512 / 8) + ((8192 - 512) / 64)) + ? 512 + tag * 64 - kTagBase * 64 - 512 / 8 * 64 + : 8192 + tag * 4096 - kTagBase * 4096 - + ((512 / 8) + ((8192 - 512) / 64)) * 4096; } -static_assert(kMinFlatSize / 8 >= FLAT, ""); -static_assert(AllocatedSizeToTagUnchecked(kMaxFlatSize) <= MAX_FLAT_TAG, ""); +static_assert(AllocatedSizeToTagUnchecked(kMinFlatSize) == FLAT, ""); +static_assert(AllocatedSizeToTagUnchecked(kMaxLargeFlatSize) == MAX_FLAT_TAG, + ""); -// Helper functions for rounded div, and rounding to exact sizes. -constexpr size_t DivUp(size_t n, size_t m) { return (n + m - 1) / m; } -constexpr size_t RoundUp(size_t n, size_t m) { return DivUp(n, m) * m; } +// RoundUp logically performs `((n + m - 1) / m) * m` to round up to the nearest +// multiple of `m`, optimized for the invariant that `m` is a power of 2. +constexpr size_t RoundUp(size_t n, size_t m) { + return (n + m - 1) & (0 - m); +} // Returns the size to the nearest equal or larger value that can be // expressed exactly as a tag value. inline size_t RoundUpForTag(size_t size) { - return RoundUp(size, (size <= 1024) ? 8 : 32); + return RoundUp(size, (size <= 512) ? 8 : (size <= 8192 ? 64 : 4096)); } // Converts the allocated size to a tag, rounding down if the size @@ -71,26 +95,26 @@ inline uint8_t AllocatedSizeToTag(size_t size) { return tag; } -// Converts the provided tag to the corresponding allocated size -constexpr size_t TagToAllocatedSize(uint8_t tag) { - return (tag <= 128) ? (tag * 8) : (1024 + (tag - 128) * 32); -} - // Converts the provided tag to the corresponding available data length constexpr size_t TagToLength(uint8_t tag) { return TagToAllocatedSize(tag) - kFlatOverhead; } // Enforce that kMaxFlatSize maps to a well-known exact tag value. -static_assert(TagToAllocatedSize(224) == kMaxFlatSize, "Bad tag logic"); +static_assert(TagToAllocatedSize(MAX_FLAT_TAG) == kMaxLargeFlatSize, + "Bad tag logic"); struct CordRepFlat : public CordRep { + // Tag for explicit 'large flat' allocation + struct Large {}; + // Creates a new flat node. - static CordRepFlat* New(size_t len) { + template <size_t max_flat_size, typename... Args> + static CordRepFlat* NewImpl(size_t len, Args... args ABSL_ATTRIBUTE_UNUSED) { if (len <= kMinFlatLength) { len = kMinFlatLength; - } else if (len > kMaxFlatLength) { - len = kMaxFlatLength; + } else if (len > max_flat_size - kFlatOverhead) { + len = max_flat_size - kFlatOverhead; } // Round size up so it matches a size we can exactly express in a tag. @@ -101,6 +125,12 @@ struct CordRepFlat : public CordRep { return rep; } + static CordRepFlat* New(size_t len) { return NewImpl<kMaxFlatSize>(len); } + + static CordRepFlat* New(Large, size_t len) { + return NewImpl<kMaxLargeFlatSize>(len); + } + // Deletes a CordRepFlat instance created previously through a call to New(). // Flat CordReps are allocated and constructed with raw ::operator new and // placement new, and must be destructed and deallocated accordingly. @@ -117,9 +147,20 @@ struct CordRepFlat : public CordRep { #endif } + // Create a CordRepFlat containing `data`, with an optional additional + // extra capacity of up to `extra` bytes. Requires that `data.size()` + // is less than kMaxFlatLength. + static CordRepFlat* Create(absl::string_view data, size_t extra = 0) { + assert(data.size() <= kMaxFlatLength); + CordRepFlat* flat = New(data.size() + (std::min)(extra, kMaxFlatLength)); + memcpy(flat->Data(), data.data(), data.size()); + flat->length = data.size(); + return flat; + } + // Returns a pointer to the data inside this flat rep. - char* Data() { return storage; } - const char* Data() const { return storage; } + char* Data() { return reinterpret_cast<char*>(storage); } + const char* Data() const { return reinterpret_cast<const char*>(storage); } // Returns the maximum capacity (payload size) of this instance. size_t Capacity() const { return TagToLength(tag); } |