63 lines
2.8 KiB
Rust
63 lines
2.8 KiB
Rust
//! The configure builtins provides runtime support compiler-builtin features
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//! which require dynamic initialization to work as expected, e.g. aarch64
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//! outline-atomics.
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/// Enable LSE atomic operations at startup, if supported.
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///
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/// Linker sections are based on what [`ctor`] does, with priorities to run slightly before user
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/// code:
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///
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/// - Apple uses the section `__mod_init_func`, `mod_init_funcs` is needed to set
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/// `S_MOD_INIT_FUNC_POINTERS`. There doesn't seem to be a way to indicate priorities.
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/// - Windows uses `.CRT$XCT`, which is run before user constructors (these should use `.CRT$XCU`).
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/// - ELF uses `.init_array` with a priority of 90, which runs before our `ARGV_INIT_ARRAY`
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/// initializer (priority 99). Both are within the 0-100 implementation-reserved range, per docs
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/// for the [`prio-ctor-dtor`] warning, and this matches compiler-rt's `CONSTRUCTOR_PRIORITY`.
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///
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/// To save startup time, the initializer is only run if outline atomic routines from
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/// compiler-builtins may be used. If LSE is known to be available then the calls are never
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/// emitted, and if we build the C intrinsics then it has its own initializer using the symbol
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/// `__aarch64_have_lse_atomics`.
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///
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/// Initialization is done in a global constructor to so we get the same behavior regardless of
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/// whether Rust's `init` is used, or if we are in a `dylib` or `no_main` situation (as opposed
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/// to doing it as part of pre-main startup). This also matches C implementations.
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///
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/// Ideally `core` would have something similar, but detecting the CPU features requires the
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/// auxiliary vector from the OS. We do the initialization in `std` rather than as part of
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/// `compiler-builtins` because a builtins->std dependency isn't possible, and inlining parts of
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/// `std-detect` would be much messier.
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///
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/// [`ctor`]: https://github.com/mmastrac/rust-ctor/blob/63382b833ddcbfb8b064f4e86bfa1ed4026ff356/shared/src/macros/mod.rs#L522-L534
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/// [`prio-ctor-dtor`]: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
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#[cfg(all(
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target_arch = "aarch64",
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target_feature = "outline-atomics",
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not(target_feature = "lse"),
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not(feature = "compiler-builtins-c"),
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))]
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#[used]
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#[cfg_attr(target_vendor = "apple", unsafe(link_section = "__DATA,__mod_init_func,mod_init_funcs"))]
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#[cfg_attr(target_os = "windows", unsafe(link_section = ".CRT$XCT"))]
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#[cfg_attr(
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not(any(target_vendor = "apple", target_os = "windows")),
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unsafe(link_section = ".init_array.90")
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)]
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static RUST_LSE_INIT: extern "C" fn() = {
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extern "C" fn init_lse() {
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use crate::arch;
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// This is provided by compiler-builtins::aarch64_outline_atomics.
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unsafe extern "C" {
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fn __rust_enable_lse();
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}
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if arch::is_aarch64_feature_detected!("lse") {
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unsafe {
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__rust_enable_lse();
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}
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}
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}
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init_lse
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};
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