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This variant uses the CASP instruction available on armv8.1-a CPU cores, which is detected when __ARM_FEATURE_ATOMICS is set (gcc-linaro >= 7, mainline >= 9). This one was tested on cortex-A55 (S905D3) and on AWS' Graviton2 CPUs. The instruction performs way better on high thread counts since it guarantees some forward progress when facing extreme contention while the original LL/SC approach is light on low-thread counts but doesn't guarantee progress. The implementation is not the most optimal possible. In particular since the instruction requires to work on register pairs and there doesn't seem to be a way to force gcc to emit register pairs, we have to decide to force to use the pair (x0,x1) to store the old value, and (x2,x3) to store the new one, and this necessarily involves some extra moves. But at least it does improve the situation with 16 threads and more. See issue #958 for more context. Note, a first implementation of this function was making use of an input/output constraint passed using "+Q"(*(void**)target), which was resulting in smaller overall code than passing "target" as an input register only. It turned out that the cause was directly related to whether the function was inlined or not, hence the "forceinline" attribute. Any changes to this code should still pay attention to this important factor.
The HAProxy documentation has been split into a number of different files for ease of use. Please refer to the following files depending on what you're looking for : - INSTALL for instructions on how to build and install HAProxy - BRANCHES to understand the project's life cycle and what version to use - LICENSE for the project's license - CONTRIBUTING for the process to follow to submit contributions The more detailed documentation is located into the doc/ directory : - doc/intro.txt for a quick introduction on HAProxy - doc/configuration.txt for the configuration's reference manual - doc/lua.txt for the Lua's reference manual - doc/SPOE.txt for how to use the SPOE engine - doc/network-namespaces.txt for how to use network namespaces under Linux - doc/management.txt for the management guide - doc/regression-testing.txt for how to use the regression testing suite - doc/peers.txt for the peers protocol reference - doc/coding-style.txt for how to adopt HAProxy's coding style - doc/internals for developer-specific documentation (not all up to date)