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Rollup merge of #76979 - fusion-engineering-forks:windows-fallback-check, r=dtolnay

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Improve std::sys::windows::compat

Improves the compat_fn macro in sys::windows, which is used for conditionally loading APIs that might not be available.

- The module (dll) name can now be any string, not just an ident. (Not all Windows api modules are valid Rust identifiers. E.g. `WaitOnAddress` comes from `API-MS-Win-Core-Synch-l1-2-0.dll`.)
- Adds `FuncName::is_available()` for checking if a function is really available without having to do a duplicate lookup.
- Add comment explaining the lack of locking.
- Use `$_:block` to simplify the macro_rules.
- Apply `allow(unused_variables)` only to the fallback instead of everything.

---

The second point (`is_available()`) simplifies code that needs to pick an implementation depening on what is available, like `sys/windows/mutex.rs`. Before this change, it'd do its own lookup and keep its own `AtomicUsize` to track the result. Now it can just use `c::AcquireSRWLockExclusive::is_available()` directly.

This will also be useful when park/unpark/CondVar/etc. get improved implementations (e.g. from parking_lot or something else), as the best APIs for those are not available before Windows 8.
This commit is contained in:
Yuki Okushi 2020-10-02 08:25:11 +09:00 committed by GitHub
commit 00b3450bbc
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3 changed files with 54 additions and 46 deletions
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2
library/std/src/sys/windows/c.rs
View File

@ -1032,7 +1032,7 @@ extern "system" {
// Functions that aren't available on every version of Windows that we support,
// but we still use them and just provide some form of a fallback implementation.
compat_fn! {
kernel32:
"kernel32":
pub fn CreateSymbolicLinkW(_lpSymlinkFileName: LPCWSTR,
_lpTargetFileName: LPCWSTR,

77
library/std/src/sys/windows/compat.rs
View File

@ -12,7 +12,6 @@
//! function is available but afterwards it's just a load and a jump.
use crate::ffi::CString;
use crate::sync::atomic::{AtomicUsize, Ordering};
use crate::sys::c;
pub fn lookup(module: &str, symbol: &str) -> Option<usize> {
@ -28,45 +27,69 @@ pub fn lookup(module: &str, symbol: &str) -> Option<usize> {
}
}
pub fn store_func(ptr: &AtomicUsize, module: &str, symbol: &str, fallback: usize) -> usize {
let value = lookup(module, symbol).unwrap_or(fallback);
ptr.store(value, Ordering::SeqCst);
value
}
macro_rules! compat_fn {
($module:ident: $(
($module:literal: $(
$(#[$meta:meta])*
pub fn $symbol:ident($($argname:ident: $argtype:ty),*)
-> $rettype:ty {
$($body:expr);*
}
pub fn $symbol:ident($($argname:ident: $argtype:ty),*) -> $rettype:ty $body:block
)*) => ($(
#[allow(unused_variables)]
$(#[$meta])*
pub unsafe fn $symbol($($argname: $argtype),*) -> $rettype {
pub mod $symbol {
use super::*;
use crate::sync::atomic::{AtomicUsize, Ordering};
use crate::mem;
type F = unsafe extern "system" fn($($argtype),*) -> $rettype;
static PTR: AtomicUsize = AtomicUsize::new(0);
#[allow(unused_variables)]
unsafe extern "system" fn fallback($($argname: $argtype),*) -> $rettype $body
/// This address is stored in `PTR` to incidate an unavailable API.
///
/// This way, call() will end up calling fallback() if it is unavailable.
///
/// This is a `static` to avoid rustc duplicating `fn fallback()`
/// into both load() and is_available(), which would break
/// is_available()'s comparison. By using the same static variable
/// in both places, they'll refer to the same (copy of the)
/// function.
///
/// LLVM merging the address of fallback with other functions
/// (because of unnamed_addr) is fine, since it's only compared to
/// an address from GetProcAddress from an external dll.
static FALLBACK: F = fallback;
#[cold]
fn load() -> usize {
crate::sys::compat::store_func(&PTR,
stringify!($module),
stringify!($symbol),
fallback as usize)
}
unsafe extern "system" fn fallback($($argname: $argtype),*)
-> $rettype {
$($body);*
// There is no locking here. It's okay if this is executed by multiple threads in
// parallel. `lookup` will result in the same value, and it's okay if they overwrite
// eachothers result as long as they do so atomically. We don't need any guarantees
// 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);
PTR.store(addr, Ordering::Relaxed);
addr
}
let addr = match PTR.load(Ordering::SeqCst) {
0 => load(),
n => n,
};
mem::transmute::<usize, F>(addr)($($argname),*)
fn addr() -> usize {
match PTR.load(Ordering::Relaxed) {
0 => load(),
addr => addr,
}
}
#[allow(dead_code)]
pub fn is_available() -> bool {
addr() != FALLBACK as usize
}
pub unsafe fn call($($argname: $argtype),*) -> $rettype {
mem::transmute::<usize, F>(addr())($($argname),*)
}
}
pub use $symbol::call as $symbol;
)*)
}

21
library/std/src/sys/windows/mutex.rs
View File

@ -23,7 +23,6 @@ use crate::cell::{Cell, UnsafeCell};
use crate::mem::{self, MaybeUninit};
use crate::sync::atomic::{AtomicUsize, Ordering};
use crate::sys::c;
use crate::sys::compat;
pub struct Mutex {
// This is either directly an SRWLOCK (if supported), or a Box<Inner> otherwise.
@ -40,8 +39,8 @@ struct Inner {
#[derive(Clone, Copy)]
enum Kind {
SRWLock = 1,
CriticalSection = 2,
SRWLock,
CriticalSection,
}
#[inline]
@ -130,21 +129,7 @@ impl Mutex {
}
fn kind() -> Kind {
static KIND: AtomicUsize = AtomicUsize::new(0);
let val = KIND.load(Ordering::SeqCst);
if val == Kind::SRWLock as usize {
return Kind::SRWLock;
} else if val == Kind::CriticalSection as usize {
return Kind::CriticalSection;
}
let ret = match compat::lookup("kernel32", "AcquireSRWLockExclusive") {
None => Kind::CriticalSection,
Some(..) => Kind::SRWLock,
};
KIND.store(ret as usize, Ordering::SeqCst);
ret
if c::AcquireSRWLockExclusive::is_available() { Kind::SRWLock } else { Kind::CriticalSection }
}
pub struct ReentrantMutex {