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    // Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    #![cfg(target_thread_local)]
    #![unstable(feature = "thread_local_internals", issue = "0")]

    // Since what appears to be glibc 2.18 this symbol has been shipped which
    // GCC and clang both use to invoke destructors in thread_local globals, so
    // let's do the same!
    //
    // Note, however, that we run on lots older linuxes, as well as cross
    // compiling from a newer linux to an older linux, so we also have a
    // fallback implementation to use as well.
    //
    // Due to rust-lang/rust#18804, make sure this is not generic!
    #[cfg(target_os = "linux")]
    pub unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
        use libc;
        use mem;
        use sys_common::thread_local::register_dtor_fallback;

        extern {
            #[linkage = "extern_weak"]
            static __dso_handle: *mut u8;
            #[linkage = "extern_weak"]
            static __cxa_thread_atexit_impl: *const libc::c_void;
        }
        if !__cxa_thread_atexit_impl.is_null() {
            type F = unsafe extern fn(dtor: unsafe extern fn(*mut u8),
                                      arg: *mut u8,
                                      dso_handle: *mut u8) -> libc::c_int;
            mem::transmute::<*const libc::c_void, F>(__cxa_thread_atexit_impl)
                (dtor, t, &__dso_handle as *const _ as *mut _);
            return
        }
        register_dtor_fallback(t, dtor);
    }

    // macOS's analog of the above linux function is this _tlv_atexit function.
    // The disassembly of thread_local globals in C++ (at least produced by
    // clang) will have this show up in the output.
    #[cfg(target_os = "macos")]
    pub unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
        extern {
            fn _tlv_atexit(dtor: unsafe extern fn(*mut u8),
                           arg: *mut u8);
        }
        _tlv_atexit(dtor, t);
    }

    // Just use the thread_local fallback implementation, at least until there's
    // a more direct implementation.
    #[cfg(target_os = "fuchsia")]
    pub use sys_common::thread_local::register_dtor_fallback as register_dtor;

    pub fn requires_move_before_drop() -> bool {
        // The macOS implementation of TLS apparently had an odd aspect to it
        // where the pointer we have may be overwritten while this destructor
        // is running. Specifically if a TLS destructor re-accesses TLS it may
        // trigger a re-initialization of all TLS variables, paving over at
        // least some destroyed ones with initial values.
        //
        // This means that if we drop a TLS value in place on macOS that we could
        // revert the value to its original state halfway through the
        // destructor, which would be bad!
        //
        // Hence, we use `ptr::read` on macOS (to move to a "safe" location)
        // instead of drop_in_place.
        cfg!(target_os = "macos")
    }

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