std/sys/
configure_builtins.rs

1/// Enable LSE atomic operations at startup, if supported.
2///
3/// Linker sections are based on what [`ctor`] does, with priorities to run slightly before user
4/// code:
5///
6/// - Apple uses the section `__mod_init_func`, `mod_init_funcs` is needed to set
7///   `S_MOD_INIT_FUNC_POINTERS`. There doesn't seem to be a way to indicate priorities.
8/// - Windows uses `.CRT$XCT`, which is run before user constructors (these should use `.CRT$XCU`).
9/// - ELF uses `.init_array` with a priority of 90, which runs before our `ARGV_INIT_ARRAY`
10///   initializer (priority 99). Both are within the 0-100 implementation-reserved range, per docs
11///   for the [`prio-ctor-dtor`] warning, and this matches compiler-rt's `CONSTRUCTOR_PRIORITY`.
12///
13/// To save startup time, the initializer is only run if outline atomic routines from
14/// compiler-builtins may be used. If LSE is known to be available then the calls are never
15/// emitted, and if we build the C intrinsics then it has its own initializer using the symbol
16/// `__aarch64_have_lse_atomics`.
17///
18/// Initialization is done in a global constructor to so we get the same behavior regardless of
19/// whether Rust's `init` is used, or if we are in a `dylib` or `no_main` situation (as opposed
20/// to doing it as part of pre-main startup). This also matches C implementations.
21///
22/// Ideally `core` would have something similar, but detecting the CPU features requires the
23/// auxiliary vector from the OS. We do the initialization in `std` rather than as part of
24/// `compiler-builtins` because a builtins->std dependency isn't possible, and inlining parts of
25/// `std-detect` would be much messier.
26///
27/// [`ctor`]: https://github.com/mmastrac/rust-ctor/blob/63382b833ddcbfb8b064f4e86bfa1ed4026ff356/shared/src/macros/mod.rs#L522-L534
28/// [`prio-ctor-dtor`]: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
29#[cfg(all(
30    target_arch = "aarch64",
31    target_feature = "outline-atomics",
32    not(target_feature = "lse"),
33    not(feature = "compiler-builtins-c"),
34))]
35#[used]
36#[cfg_attr(target_vendor = "apple", unsafe(link_section = "__DATA,__mod_init_func,mod_init_funcs"))]
37#[cfg_attr(target_os = "windows", unsafe(link_section = ".CRT$XCT"))]
38#[cfg_attr(
39    not(any(target_vendor = "apple", target_os = "windows")),
40    unsafe(link_section = ".init_array.90")
41)]
42static RUST_LSE_INIT: extern "C" fn() = {
43    extern "C" fn init_lse() {
44        use crate::arch;
45
46        // This is provided by compiler-builtins::aarch64_outline_atomics.
47        unsafe extern "C" {
48            fn __rust_enable_lse();
49        }
50
51        if arch::is_aarch64_feature_detected!("lse") {
52            unsafe {
53                __rust_enable_lse();
54            }
55        }
56    }
57    init_lse
58};
std/sys/configure_builtins.rs

std/sys/
configure_builtins.rs
