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.. contents::
.. sectnum::
==========================
Clang implementation notes
==========================
This document provides more details specific to the Clang/LLVM implementation of the eBPF instruction set.
Versions
========
Clang defined "CPU" versions, where a CPU version of 3 corresponds to the current eBPF ISA.
Clang can select the eBPF ISA version using ``-mcpu=v3`` for example to select version 3.
Arithmetic instructions
=======================
For CPU versions prior to 3, Clang v7.0 and later can enable ``BPF_ALU`` support with
``-Xclang -target-feature -Xclang +alu32``. In CPU version 3, support is automatically included.
Atomic operations
=================
Clang can generate atomic instructions by default when ``-mcpu=v3`` is
enabled. If a lower version for ``-mcpu`` is set, the only atomic instruction
Clang can generate is ``BPF_ADD`` *without* ``BPF_FETCH``. If you need to enable
the atomics features, while keeping a lower ``-mcpu`` version, you can use
``-Xclang -target-feature -Xclang +alu32``.
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