Optimize CRC32 Extend for large inputs on Arm This is a temporary workaround for an apparent compiler bug with pmull(2) instructions. The current hot loop looks like this: mov w14, #0xef02, lsl x15, x15, #6, mov x13, xzr, movk w14, #0x740e, lsl #16, sub x15, x15, #0x40, ldr q4, [x16, #0x4e0], _LOOP_START: add x16, x9, x13, add x17, x12, x13, fmov d19, x14, <--------- This is Loop invariant and expensive add x13, x13, #0x40, cmp x15, x13, prfm pldl1keep, [x16, #0x140], prfm pldl1keep, [x17, #0x140], ldp x18, x0, [x16, #0x40], crc32cx w10, w10, x18, ldp x2, x18, [x16, #0x50], crc32cx w10, w10, x0, crc32cx w10, w10, x2, ldp x0, x2, [x16, #0x60], crc32cx w10, w10, x18, ldp x18, x16, [x16, #0x70], pmull2 v5.1q, v1.2d, v4.2d, pmull2 v6.1q, v0.2d, v4.2d, pmull2 v7.1q, v2.2d, v4.2d, pmull2 v16.1q, v3.2d, v4.2d, ldp q17, q18, [x17, #0x40], crc32cx w10, w10, x0, pmull v1.1q, v1.1d, v19.1d, crc32cx w10, w10, x2, pmull v0.1q, v0.1d, v19.1d, crc32cx w10, w10, x18, pmull v2.1q, v2.1d, v19.1d, crc32cx w10, w10, x16, pmull v3.1q, v3.1d, v19.1d, ldp q20, q21, [x17, #0x60], eor v1.16b, v17.16b, v1.16b, eor v0.16b, v18.16b, v0.16b, eor v1.16b, v1.16b, v5.16b, eor v2.16b, v20.16b, v2.16b, eor v0.16b, v0.16b, v6.16b, eor v3.16b, v21.16b, v3.16b, eor v2.16b, v2.16b, v7.16b, eor v3.16b, v3.16b, v16.16b, b.ne _LOOP_START There is a redundant fmov that moves the same constant into a Neon register every loop iteration to be used in the PMULL instructions. The PMULL2 instructions already have this constant loaded into Neon registers. After this change, both the PMULL and PMULL2 instructions use the values in q4, and they are not reloaded every iteration. This fmov was expensive because it contends for execution units with crc32cx instructions. This is up to 20% faster for large inputs. PiperOrigin-RevId: 567391972 Change-Id: I4c8e49750cfa5cc5730c3bb713bd9fd67657804a
The repository contains the Abseil C++ library code. Abseil is an open-source collection of C++ code (compliant to C++14) 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 compatible versions of type checks available within C++14 and C++17 versions of 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 psuedorandom 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, including a C++14-compatible version of the C++17 std::string_view type.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, like a C++14-compatible version of the C++17 std::optional type.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: