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// Copyright 2016 Amanieu d'Antras
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// copied, modified, or distributed except according to those terms.
#[cfg(all(feature = "hardware-lock-elision", any(target_arch = "x86", target_arch = "x86_64")))]
use std::arch::asm;
use std::sync::atomic::AtomicUsize;
// Extension trait to add lock elision primitives to atomic types
pub trait AtomicElisionExt {
type IntType;
// Perform a compare_exchange and start a transaction
fn elision_compare_exchange_acquire(
&self,
current: Self::IntType,
new: Self::IntType,
) -> Result<Self::IntType, Self::IntType>;
// Perform a fetch_sub and end a transaction
fn elision_fetch_sub_release(&self, val: Self::IntType) -> Self::IntType;
}
// Indicates whether the target architecture supports lock elision
#[inline]
pub fn have_elision() -> bool {
cfg!(all(
feature = "hardware-lock-elision",
any(target_arch = "x86", target_arch = "x86_64"),
))
}
// This implementation is never actually called because it is guarded by
// have_elision().
#[cfg(not(all(feature = "hardware-lock-elision", any(target_arch = "x86", target_arch = "x86_64"))))]
impl AtomicElisionExt for AtomicUsize {
type IntType = usize;
#[inline]
fn elision_compare_exchange_acquire(&self, _: usize, _: usize) -> Result<usize, usize> {
unreachable!();
}
#[inline]
fn elision_fetch_sub_release(&self, _: usize) -> usize {
unreachable!();
}
}
#[cfg(all(feature = "hardware-lock-elision", any(target_arch = "x86", target_arch = "x86_64")))]
impl AtomicElisionExt for AtomicUsize {
type IntType = usize;
#[inline]
fn elision_compare_exchange_acquire(&self, current: usize, new: usize) -> Result<usize, usize> {
unsafe {
use core::arch::asm;
let prev: usize;
#[cfg(target_pointer_width = "32")]
asm!(
"xacquire",
"lock",
"cmpxchg [{:e}], {:e}",
in(reg) self,
in(reg) new,
inout("eax") current => prev,
);
#[cfg(target_pointer_width = "64")]
asm!(
"xacquire",
"lock",
"cmpxchg [{}], {}",
in(reg) self,
in(reg) new,
inout("rax") current => prev,
);
if prev == current {
Ok(prev)
} else {
Err(prev)
}
}
}
#[inline]
fn elision_fetch_sub_release(&self, val: usize) -> usize {
unsafe {
use core::arch::asm;
let prev: usize;
#[cfg(target_pointer_width = "32")]
asm!(
"xrelease",
"lock",
"xadd [{:e}], {:e}",
in(reg) self,
inout(reg) val.wrapping_neg() => prev,
);
#[cfg(target_pointer_width = "64")]
asm!(
"xrelease",
"lock",
"xadd [{}], {}",
in(reg) self,
inout(reg) val.wrapping_neg() => prev,
);
prev
}
}
}