Instead of using bit rotate for CRC hashing, use multiplication by 3. Motivation: we realized that using bit rotation results in the two CRCs being dependent on each other because CRC and bit rotation are both linear in GF(2). Multiplication is nonlinear in GF(2) because it has carry bits. Multiplication by 3 can be done in 1 cycle on x86 (using lea) and ARM (using add with shifted operand). Note that lea can also do multiplication by 5 or 9, but 3 did slightly better in probe benchmarks. Probe benchmarks regress significantly in this change, but they are not significantly worse than they were before CRC-hashing with rotation was introduced. Loadtests are neutral or leaning positive. PiperOrigin-RevId: 874740746 Change-Id: I5d88e6d8cc57c9f22a55432bce7c4de5ac07341b
The repository contains the Abseil C++ library code. Abseil is an open-source collection of C++ code (compliant to C++17) designed to augment the C++ standard library.
Abseil is an open-source collection of C++ library code designed to augment the C++ standard library. The Abseil library code is collected from Google's own C++ code base, has been extensively tested and used in production, and is the same code we depend on in our daily coding lives.
In some cases, Abseil provides pieces missing from the C++ standard; in others, Abseil provides alternatives to the standard for special needs we've found through usage in the Google code base. We denote those cases clearly within the library code we provide you.
Abseil is not meant to be a competitor to the standard library; we've just found that many of these utilities serve a purpose within our code base, and we now want to provide those resources to the C++ community as a whole.
If you want to just get started, make sure you at least run through the Abseil Quickstart. The Quickstart contains information about setting up your development environment, downloading the Abseil code, running tests, and getting a simple binary working.
Bazel and CMake are the official build systems for Abseil. See the quickstart for more information on building Abseil using the Bazel build system. If you require CMake support, please check the CMake build instructions and CMake Quickstart.
Abseil follows Google's Foundational C++ Support Policy. See this table for a list of currently supported versions compilers, platforms, and build tools.
Abseil contains the following C++ library components:
base base library contains initialization code and other code which all other Abseil code depends on. Code within base may not depend on any other code (other than the C++ standard library).algorithm algorithm library contains additions to the C++ <algorithm> library and container-based versions of such algorithms.cleanup cleanup library contains the control-flow-construct-like type absl::Cleanup which is used for executing a callback on scope exit.container container library contains additional STL-style containers, including Abseil's unordered “Swiss table” containers.crc The crc library contains code for computing error-detecting cyclic redundancy checks on data.debugging debugging library contains code useful for enabling leak checks, and stacktrace and symbolization utilities.flags flags library contains code for handling command line flags for libraries and binaries built with Abseil.hash hash library contains the hashing framework and default hash functor implementations for hashable types in Abseil.log log library contains LOG and CHECK macros and facilities for writing logged messages out to disk, stderr, or user-extensible destinations.memory memory library contains memory management facilities that augment C++'s <memory> library.meta meta library contains type checks similar to those available in the C++ <type_traits> library.numeric numeric library contains 128-bit integer types as well as implementations of C++20's bitwise math functions.profiling profiling library contains utility code for profiling C++ entities. It is currently a private dependency of other Abseil libraries.random random library contains functions for generating pseudorandom values.status status library contains abstractions for error handling, specifically absl::Status and absl::StatusOr<T>.strings strings library contains a variety of strings routines and utilities.synchronization synchronization library contains concurrency primitives (Abseil's absl::Mutex class, an alternative to std::mutex) and a variety of synchronization abstractions.time time library contains abstractions for computing with absolute points in time, durations of time, and formatting and parsing time within time zones.types types library contains non-container utility types.utility utility library contains utility and helper code.Abseil recommends users “live-at-head” (update to the latest commit from the master branch as often as possible). However, we realize this philosophy doesn't work for every project, so we also provide Long Term Support Releases to which we backport fixes for severe bugs. See our release management document for more details.
The Abseil C++ library is licensed under the terms of the Apache license. See LICENSE for more information.
For more information about Abseil: