vendor/parking_lot-0.9.0/src/elision.rs

   1 // Copyright 2016 Amanieu d'Antras
   2 //
   3 // Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
   4 // http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
   5 // http://opensource.org/licenses/MIT>, at your option. This file may not be
   6 // copied, modified, or distributed except according to those terms.
   7
   8 use std::sync::atomic::AtomicUsize;
   9
  10 // Extension trait to add lock elision primitives to atomic types
  11 pub trait AtomicElisionExt {
  12     type IntType;
  13
  14     // Perform a compare_exchange and start a transaction
  15     fn elision_compare_exchange_acquire(
  16         &self,
  17         current: Self::IntType,
  18         new: Self::IntType,
  19     ) -> Result<Self::IntType, Self::IntType>;
  20
  21     // Perform a fetch_sub and end a transaction
  22     fn elision_fetch_sub_release(&self, val: Self::IntType) -> Self::IntType;
  23 }
  24
  25 // Indicates whether the target architecture supports lock elision
  26 #[inline]
  27 pub fn have_elision() -> bool {
  28     cfg!(all(feature = "nightly", any(target_arch = "x86", target_arch = "x86_64"),))
  29 }
  30
  31 // This implementation is never actually called because it is guarded by
  32 // have_elision().
  33 #[cfg(not(all(feature = "nightly", any(target_arch = "x86", target_arch = "x86_64"))))]
  34 impl AtomicElisionExt for AtomicUsize {
  35     type IntType = usize;
  36
  37     #[inline]
  38     fn elision_compare_exchange_acquire(&self, _: usize, _: usize) -> Result<usize, usize> {
  39         unreachable!();
  40     }
  41
  42     #[inline]
  43     fn elision_fetch_sub_release(&self, _: usize) -> usize {
  44         unreachable!();
  45     }
  46 }
  47
  48 #[cfg(all(feature = "nightly", any(target_arch = "x86", target_arch = "x86_64")))]
  49 impl AtomicElisionExt for AtomicUsize {
  50     type IntType = usize;
  51
  52     #[cfg(target_pointer_width = "32")]
  53     #[inline]
  54     fn elision_compare_exchange_acquire(&self, current: usize, new: usize) -> Result<usize, usize> {
  55         unsafe {
  56             let prev: usize;
  57             asm!("xacquire; lock; cmpxchgl $2, $1"
  58                  : "={eax}" (prev), "+*m" (self)
  59                  : "r" (new), "{eax}" (current)
  60                  : "memory"
  61                  : "volatile");
  62             if prev == current { Ok(prev) } else { Err(prev) }
  63         }
  64     }
  65     #[cfg(target_pointer_width = "64")]
  66     #[inline]
  67     fn elision_compare_exchange_acquire(&self, current: usize, new: usize) -> Result<usize, usize> {
  68         unsafe {
  69             let prev: usize;
  70             asm!("xacquire; lock; cmpxchgq $2, $1"
  71                  : "={rax}" (prev), "+*m" (self)
  72                  : "r" (new), "{rax}" (current)
  73                  : "memory"
  74                  : "volatile");
  75             if prev == current { Ok(prev) } else { Err(prev) }
  76         }
  77     }
  78
  79     #[cfg(target_pointer_width = "32")]
  80     #[inline]
  81     fn elision_fetch_sub_release(&self, val: usize) -> usize {
  82         unsafe {
  83             let prev: usize;
  84             asm!("xrelease; lock; xaddl $2, $1"
  85                  : "=r" (prev), "+*m" (self)
  86                  : "0" (val.wrapping_neg())
  87                  : "memory"
  88                  : "volatile");
  89             prev
  90         }
  91     }
  92     #[cfg(target_pointer_width = "64")]
  93     #[inline]
  94     fn elision_fetch_sub_release(&self, val: usize) -> usize {
  95         unsafe {
  96             let prev: usize;
  97             asm!("xrelease; lock; xaddq $2, $1"
  98                  : "=r" (prev), "+*m" (self)
  99                  : "0" (val.wrapping_neg())
 100                  : "memory"
 101                  : "volatile");
 102             prev
 103         }
 104     }
 105 }