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Work around potential merging/duplication issues in sys/windows/compat.

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This commit is contained in:
Mara Bos 2020-10-01 16:49:55 +02:00
parent 09cbaf4367
commit 63b6007d5b
1 changed files with 19 additions and 3 deletions
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22
library/std/src/sys/windows/compat.rs
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@ -38,11 +38,28 @@ macro_rules! compat_fn {
use crate::sync::atomic::{AtomicUsize, Ordering};
use crate::mem;
type F = unsafe extern "system" fn($($argtype),*) -> $rettype;
static PTR: AtomicUsize = AtomicUsize::new(0);
#[allow(unused_variables)]
unsafe extern "system" fn fallback($($argname: $argtype),*) -> $rettype $body
/// This address is stored in `PTR` to incidate an unavailable API.
///
/// This way, call() will end up calling fallback() if it is unavailable.
///
/// This is a `static` to avoid rustc duplicating `fn fallback()`
/// into both load() and is_available(), which would break
/// is_available()'s comparison. By using the same static variable
/// in both places, they'll refer to the same (copy of the)
/// function.
///
/// LLVM merging the address of fallback with other functions
/// (because of unnamed_addr) is fine, since it's only compared to
/// an address from GetProcAddress from an external dll.
static FALLBACK: F = fallback;
#[cold]
fn load() -> usize {
// There is no locking here. It's okay if this is executed by multiple threads in
@ -51,7 +68,7 @@ macro_rules! compat_fn {
// about memory ordering, as this involves just a single atomic variable which is
// not used to protect or order anything else.
let addr = crate::sys::compat::lookup($module, stringify!($symbol))
.unwrap_or(fallback as usize);
.unwrap_or(FALLBACK as usize);
PTR.store(addr, Ordering::Relaxed);
addr
}
@ -65,11 +82,10 @@ macro_rules! compat_fn {
#[allow(dead_code)]
pub fn is_available() -> bool {
addr() != fallback as usize
addr() != FALLBACK as usize
}
pub unsafe fn call($($argname: $argtype),*) -> $rettype {
type F = unsafe extern "system" fn($($argtype),*) -> $rettype;
mem::transmute::<usize, F>(addr())($($argname),*)
}
}