# Run-time CPU feature detection
This module has run-time CPU feature detection implementations.
Here is the table of targets that support run-time CPU feature detection and the instruction or API used:
| target_arch | target_os/target_env | instruction/API | features | note |
| aarch64 | linux (gnu/ohos/uclibc) | getauxval | all | Enabled by default (dlsym is used by default if needed for compatibility with older versions) |
| aarch64 | linux (musl) | getauxval | all | Only enabled by default when dynamic linking or `std` feature enabled (both disabled by default, see [auxv.rs](auxv.rs)) |
| aarch64 | android | getauxval | all | Enabled by default |
| aarch64 | freebsd | elf_aux_info | all | Enabled by default |
| aarch64 | netbsd | sysctlbyname | all | Enabled by default |
| aarch64 | openbsd | sysctl | all | Enabled by default |
| aarch64 | macos/ios/tvos/watchos/visionos | sysctlbyname | all | Currently only used in tests (see [aarch64_apple.rs](aarch64_apple.rs)) |
| aarch64 | illumos | getisax | lse, lse2 | Disabled by default (see [aarch64_illumos.rs](aarch64_illumos.rs)) |
| aarch64/arm64ec | windows | IsProcessorFeaturePresent | lse | Enabled by default |
| aarch64 | fuchsia | zx_system_get_features | lse | Enabled by default |
| riscv32/riscv64 | linux/android | riscv_hwprobe | all | Enabled by default |
| powerpc64 | linux (gnu/ohos/uclibc) | getauxval | all | Enabled by default (dlsym is used by default if needed for compatibility with older versions) |
| powerpc64 | linux (musl) | getauxval | all | Only enabled by default when dynamic linking or `std` feature enabled (both disabled by default, see [auxv.rs](auxv.rs)) |
| powerpc64 | freebsd | elf_aux_info | all | Enabled by default (dlsym is used by default for compatibility with older versions) |
| powerpc64 | openbsd | elf_aux_info | all | Enabled by default (dlsym is used by default for compatibility with older versions) |
| powerpc64 | aix | getsystemcfg | all | Requires LLVM 20+. Disabled by default (see [powerpc64_aix.rs](powerpc64_aix.rs)) |
Run-time detection is enabled by default on most targets and can be disabled with `--cfg portable_atomic_no_outline_atomics`.
On some targets, run-time detection is disabled by default mainly for compatibility with incomplete build environments or support for it is experimental, and can be enabled by `--cfg portable_atomic_outline_atomics`. (When both cfg are enabled, `*_no_*` cfg is preferred.)
`dlsym` usually not working with static linking, so detection using implementations that use `dlsym` for compatibility will be disabled if static linking is enabled.
You can use `--cfg portable_atomic_outline_atomics` to force the use of non-`dlsym` implementations and enable run-time detection in such an environment.
For targets not included in the above table, run-time detection is always disabled and works the same as when `--cfg portable_atomic_no_outline_atomics` is set.
See [auxv.rs](auxv.rs) module-level comments for more details on Linux/Android/FreeBSD/OpenBSD.
See also [docs about `portable_atomic_no_outline_atomics` cfg](https://github.com/taiki-e/portable-atomic/blob/HEAD/README.md#optional-cfg-no-outline-atomics) in the top-level readme.