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1 | .. contents:: |
2 | .. sectnum:: | |
3 | ||
4 | ========================== | |
5 | Clang implementation notes | |
6 | ========================== | |
7 | ||
8 | This document provides more details specific to the Clang/LLVM implementation of the eBPF instruction set. | |
9 | ||
10 | Versions | |
11 | ======== | |
12 | ||
13 | Clang defined "CPU" versions, where a CPU version of 3 corresponds to the current eBPF ISA. | |
14 | ||
15 | Clang can select the eBPF ISA version using ``-mcpu=v3`` for example to select version 3. | |
16 | ||
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17 | Arithmetic instructions |
18 | ======================= | |
19 | ||
20 | For CPU versions prior to 3, Clang v7.0 and later can enable ``BPF_ALU`` support with | |
21 | ``-Xclang -target-feature -Xclang +alu32``. In CPU version 3, support is automatically included. | |
22 | ||
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23 | Atomic operations |
24 | ================= | |
25 | ||
26 | Clang can generate atomic instructions by default when ``-mcpu=v3`` is | |
27 | enabled. If a lower version for ``-mcpu`` is set, the only atomic instruction | |
28 | Clang can generate is ``BPF_ADD`` *without* ``BPF_FETCH``. If you need to enable | |
29 | the atomics features, while keeping a lower ``-mcpu`` version, you can use | |
30 | ``-Xclang -target-feature -Xclang +alu32``. |