| .. 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. |
| |
| Jump instructions |
| ================= |
| |
| If ``-O0`` is used, Clang will generate the ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d) |
| instruction, which is not supported by the Linux kernel verifier. |
| |
| 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``. |
