Initial vendor packages

Signed-off-by: Valentin Popov <valentin@popov.link>

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+# Implementation of 128-bit atomics
+
+## 128-bit atomics instructions
+
+Here is the table of targets that support 128-bit atomics and the instructions used:
+
+| target_arch | load | store | CAS | RMW | note |
+| ----------- | ---- | ----- | --- | --- | ---- |
+| x86_64 | cmpxchg16b or vmovdqa | cmpxchg16b or vmovdqa | cmpxchg16b | cmpxchg16b | cmpxchg16b target feature required. vmovdqa requires Intel or AMD CPU with AVX. <br> Both compile-time and run-time detection are supported for cmpxchg16b. vmovdqa is currently run-time detection only. <br> Requires rustc 1.59+ when cmpxchg16b target feature is enabled at compile-time, otherwise requires rustc 1.69+ |
+| aarch64 | ldxp/stxp or casp or ldp/ldiapp | ldxp/stxp or casp or stp/stilp/swpp | ldxp/stxp or casp | ldxp/stxp or casp/swpp/ldclrp/ldsetp | casp requires lse target feature, ldp/stp requires lse2 target feature, ldiapp/stilp requires lse2 and rcpc3 target features, swpp/ldclrp/ldsetp requires lse128 target feature. <br> Both compile-time and run-time detection are supported for lse and lse2. Others are currently compile-time detection only. <br> Requires rustc 1.59+ |
+| powerpc64 | lq | stq | lqarx/stqcx. | lqarx/stqcx. | Requires target-cpu pwr8+ (powerpc64le is pwr8 by default). Both compile-time and run-time detection are supported (run-time detection is currently disabled by default). <br> Requires nightly |
+| s390x | lpq | stpq | cdsg | cdsg | Requires nightly |
+
+On compiler versions or platforms where these are not supported, the fallback implementation is used.
+
+See [aarch64.rs](aarch64.rs) module-level comments for more details on the instructions used on aarch64.
+
+## Comparison with core::intrinsics::atomic_\* (core::sync::atomic::Atomic{I,U}128)
+
+This directory has target-specific implementations with inline assembly ([aarch64.rs](aarch64.rs), [x86_64.rs](x86_64.rs), [powerpc64.rs](powerpc64.rs), [s390x.rs](s390x.rs)) and an implementation without inline assembly ([intrinsics.rs](intrinsics.rs)). The latter currently always needs nightly compilers and is only used for Miri and ThreadSanitizer, which do not support inline assembly.
+
+Implementations with inline assembly generate assemblies almost equivalent to the `core::intrinsics::atomic_*` (used in `core::sync::atomic::Atomic{I,U}128`) for many operations, but some operations may or may not generate more efficient code. For example:
+
+- On x86_64, implementation with inline assembly contains additional optimizations (e.g., [#16](https://github.com/taiki-e/portable-atomic/pull/16)) and is much faster for some operations.
+- On aarch64, implementation with inline assembly supports outline-atomics on more operating systems, and may be faster in environments where outline-atomics can improve performance.
+- On powerpc64 and s390x, LLVM does not support generating some 128-bit atomic operations (see [intrinsics.rs](intrinsics.rs) module-level comments), and we use CAS loop to implement them, so implementation with inline assembly may be faster for those operations.
+- In implementations without inline assembly, the compiler may reuse condition flags that have changed as a result of the operation, or use immediate values instead of registers, depending on the situation.
+
+As 128-bit atomics-related APIs stabilize in the standard library, implementations with inline assembly are planned to be updated to get the benefits of both.
+
+## Run-time feature detection
+
+[detect](detect) module has run-time feature detection implementations.
+
+Here is the table of targets that support run-time feature detection and the instruction or API used:
+
+| target_arch | target_os/target_env | instruction/API | features | note |
+| ----------- | -------------------- | --------------- | -------- | ---- |
+| x86_64      | all (except for sgx) | cpuid           | all      | Enabled by default |
+| aarch64     | linux                | getauxval       | all      | Only enabled by default on `*-linux-gnu*`, and `*-linux-musl*"` (default is static linking)/`*-linux-ohos*` (default is dynamic linking) with dynamic linking enabled. |
+| aarch64     | android              | getauxval       | all      | Enabled by default |
+| aarch64     | freebsd              | elf_aux_info    | lse, lse2 | Enabled by default |
+| aarch64     | netbsd               | sysctl          | all      | Enabled by default |
+| aarch64     | openbsd              | sysctl          | lse      | Enabled by default |
+| aarch64     | macos                | sysctl          | all      | Currently only used in tests because FEAT_LSE and FEAT_LSE2 are always available at compile-time. |
+| aarch64     | windows              | IsProcessorFeaturePresent | lse | Enabled by default |
+| aarch64     | fuchsia              | zx_system_get_features | lse | Enabled by default |
+| powerpc64   | linux                | getauxval       | all      | Disabled by default |
+| powerpc64   | freebsd              | elf_aux_info    | all      | Disabled by default |
+
+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 older versions of operating systems or incomplete build environments, and can be enabled by `--cfg portable_atomic_outline_atomics`. (When both cfg are enabled, `*_no_*` cfg is preferred.)
+
+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 [detect/auxv.rs](detect/auxv.rs) module-level comments for more details on Linux/Android/FreeBSD.
+
+See also [docs on `portable_atomic_no_outline_atomics`](https://github.com/taiki-e/portable-atomic/blob/HEAD/README.md#optional-cfg-no-outline-atomics) in the top-level readme.