57abc0ee3f
Optimize multiply() (renamed to MultiplyWithExtraX33()) to eliminate
several instructions that were present only to avoid introducing an
extra factor of x^33 into the multiplication. It's actually fine to
introduce the extra factor of x^33 as long as it's canceled out with an
extra factor of x^-33 in all the kCRC32CPowers[] entries.
To make this work, the number of bits dropped by ComputeZeroConstant()
had to be increased from 2 to at least 3, since 2^(i + 3 +
kNumDroppedBits) - 33 must be >= 0 for all i including i=0; otherwise
kCRC32CPowers[0] would need a negative power of x. However, this is
fine since it's more efficient to utilize CRC32_u32() and CRC32_u64()
for bits 2 and 3 anyway. So, increase kNumDroppedBits to 4.
Add a Python script that generates the updated kCRC32CPowers[]. It
isn't wired up to the build system, but rather is just added so that
kCRC32CPowers[] can be reproduced.
Also add a test which tests ExtendCrc32cByZeroes() with all the length
bits, thus testing all the entries of kCRC32CPowers[].
Note that the kCRC32CPowers[] generation script and new test case are
things we should have had anyway, regardless of the x^33 optimization.
This change slightly improves the performance of Extend() for lengths
greater than or equal to 2048 bytes, and also the performance of
ExtendByZeroes(). It also slightly reduces the binary code size.
Before:
BM_Calculate/2048 84.3 ns 84.3 ns 8307735
BM_Calculate/10000 376 ns 375 ns 1865976
BM_Calculate/500000 18538 ns 18531 ns 37813
BM_ExtendByZeroes/1 3.55 ns 3.55 ns 197111095
BM_ExtendByZeroes/10 3.90 ns 3.89 ns 179773877
BM_ExtendByZeroes/100 6.06 ns 6.06 ns 115242160
BM_ExtendByZeroes/1000 12.0 ns 12.0 ns 58078004
BM_ExtendByZeroes/10000 9.97 ns 9.97 ns 70335772
BM_ExtendByZeroes/100000 12.1 ns 12.1 ns 58157829
BM_ExtendByZeroes/1000000 14.4 ns 14.4 ns 48527365
After:
BM_Calculate/2048 82.8 ns 82.7 ns 8478296
BM_Calculate/10000 375 ns 375 ns 1869663
BM_Calculate/500000 18547 ns 18538 ns 37846
BM_ExtendByZeroes/1 2.96 ns 2.96 ns 236772500
BM_ExtendByZeroes/10 3.85 ns 3.85 ns 182059238
BM_ExtendByZeroes/100 5.42 ns 5.42 ns 129077546
BM_ExtendByZeroes/1000 9.43 ns 9.42 ns 74232457
BM_ExtendByZeroes/10000 8.14 ns 8.14 ns 86244218
BM_ExtendByZeroes/100000 10.7 ns 10.7 ns 65467391
BM_ExtendByZeroes/1000000 11.0 ns 11.0 ns 63575936
PiperOrigin-RevId: 786828855
Change-Id: I6208625fd1c35c2c137e756cf5fadc1adccfdd5d
Abseil - C++ Common Libraries
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.
Table of Contents
About Abseil
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.
Quickstart
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.
Building Abseil
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.
Support
Abseil follows Google's Foundational C++ Support Policy. See this table for a list of currently supported versions compilers, platforms, and build tools.
Codemap
Abseil contains the following C++ library components:
base
Thebaselibrary contains initialization code and other code which all other Abseil code depends on. Code withinbasemay not depend on any other code (other than the C++ standard library).algorithm
Thealgorithmlibrary contains additions to the C++<algorithm>library and container-based versions of such algorithms.cleanup
Thecleanuplibrary contains the control-flow-construct-like typeabsl::Cleanupwhich is used for executing a callback on scope exit.container
Thecontainerlibrary contains additional STL-style containers, including Abseil's unordered "Swiss table" containers.crcThecrclibrary contains code for computing error-detecting cyclic redundancy checks on data.debugging
Thedebugginglibrary contains code useful for enabling leak checks, and stacktrace and symbolization utilities.flags
Theflagslibrary contains code for handling command line flags for libraries and binaries built with Abseil.hash
Thehashlibrary contains the hashing framework and default hash functor implementations for hashable types in Abseil.log
Theloglibrary containsLOGandCHECKmacros and facilities for writing logged messages out to disk,stderr, or user-extensible destinations.memory
Thememorylibrary contains memory management facilities that augment C++'s<memory>library.meta
Themetalibrary contains type checks similar to those available in the C++<type_traits>library.numeric
Thenumericlibrary contains 128-bit integer types as well as implementations of C++20's bitwise math functions.profiling
Theprofilinglibrary contains utility code for profiling C++ entities. It is currently a private dependency of other Abseil libraries.random
Therandomlibrary contains functions for generating pseudorandom values.status
Thestatuslibrary contains abstractions for error handling, specificallyabsl::Statusandabsl::StatusOr<T>.strings
Thestringslibrary contains a variety of strings routines and utilities.synchronization
Thesynchronizationlibrary contains concurrency primitives (Abseil'sabsl::Mutexclass, an alternative tostd::mutex) and a variety of synchronization abstractions.time
Thetimelibrary contains abstractions for computing with absolute points in time, durations of time, and formatting and parsing time within time zones.types
Thetypeslibrary contains non-container utility types.utility
Theutilitylibrary contains utility and helper code.
Releases
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.
License
The Abseil C++ library is licensed under the terms of the Apache license. See LICENSE for more information.
Links
For more information about Abseil:
- Consult our Abseil Introduction
- Read Why Adopt Abseil to understand our design philosophy.
- Peruse our Abseil Compatibility Guarantees to understand both what we promise to you, and what we expect of you in return.