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// Copyright 2016 Amanieu d'Antras
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// http://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.
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 = "nightly",
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 = "nightly", 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 = "nightly", any(target_arch = "x86", target_arch = "x86_64")))]
impl AtomicElisionExt for AtomicUsize {
type IntType = usize;
#[cfg(target_pointer_width = "32")]
#[inline]
fn elision_compare_exchange_acquire(&self, current: usize, new: usize) -> Result<usize, usize> {
unsafe {
let prev: usize;
llvm_asm!("xacquire; lock; cmpxchgl $2, $1"
: "={eax}" (prev), "+*m" (self)
: "r" (new), "{eax}" (current)
: "memory"
: "volatile");
if prev == current {
Ok(prev)
} else {
Err(prev)
}
}
}
#[cfg(target_pointer_width = "64")]
#[inline]
fn elision_compare_exchange_acquire(&self, current: usize, new: usize) -> Result<usize, usize> {
unsafe {
let prev: usize;
llvm_asm!("xacquire; lock; cmpxchgq $2, $1"
: "={rax}" (prev), "+*m" (self)
: "r" (new), "{rax}" (current)
: "memory"
: "volatile");
if prev == current {
Ok(prev)
} else {
Err(prev)
}
}
}
#[cfg(target_pointer_width = "32")]
#[inline]
fn elision_fetch_sub_release(&self, val: usize) -> usize {
unsafe {
let prev: usize;
llvm_asm!("xrelease; lock; xaddl $2, $1"
: "=r" (prev), "+*m" (self)
: "0" (val.wrapping_neg())
: "memory"
: "volatile");
prev
}
}
#[cfg(target_pointer_width = "64")]
#[inline]
fn elision_fetch_sub_release(&self, val: usize) -> usize {
unsafe {
let prev: usize;
llvm_asm!("xrelease; lock; xaddq $2, $1"
: "=r" (prev), "+*m" (self)
: "0" (val.wrapping_neg())
: "memory"
: "volatile");
prev
}
}
}