Auto merge of #30969 - Amanieu:extended_atomic_cmpxchg, r=alexcrichton

This is an implementation of rust-lang/rfcs#1443.

10 files changed, 577 insertions, 49 deletions

diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index 2e2292d63f4..47f5d68f311 100644
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -58,6 +58,33 @@ extern "rust-intrinsic" {
     pub fn atomic_cxchg_rel<T>(dst: *mut T, old: T, src: T) -> T;
     pub fn atomic_cxchg_acqrel<T>(dst: *mut T, old: T, src: T) -> T;
     pub fn atomic_cxchg_relaxed<T>(dst: *mut T, old: T, src: T) -> T;
+    #[cfg(not(stage0))]
+    pub fn atomic_cxchg_failrelaxed<T>(dst: *mut T, old: T, src: T) -> T;
+    #[cfg(not(stage0))]
+    pub fn atomic_cxchg_failacq<T>(dst: *mut T, old: T, src: T) -> T;
+    #[cfg(not(stage0))]
+    pub fn atomic_cxchg_acq_failrelaxed<T>(dst: *mut T, old: T, src: T) -> T;
+    #[cfg(not(stage0))]
+    pub fn atomic_cxchg_acqrel_failrelaxed<T>(dst: *mut T, old: T, src: T) -> T;
+
+    #[cfg(not(stage0))]
+    pub fn atomic_cxchgweak<T>(dst: *mut T, old: T, src: T) -> (T, bool);
+    #[cfg(not(stage0))]
+    pub fn atomic_cxchgweak_acq<T>(dst: *mut T, old: T, src: T) -> (T, bool);
+    #[cfg(not(stage0))]
+    pub fn atomic_cxchgweak_rel<T>(dst: *mut T, old: T, src: T) -> (T, bool);
+    #[cfg(not(stage0))]
+    pub fn atomic_cxchgweak_acqrel<T>(dst: *mut T, old: T, src: T) -> (T, bool);
+    #[cfg(not(stage0))]
+    pub fn atomic_cxchgweak_relaxed<T>(dst: *mut T, old: T, src: T) -> (T, bool);
+    #[cfg(not(stage0))]
+    pub fn atomic_cxchgweak_failrelaxed<T>(dst: *mut T, old: T, src: T) -> (T, bool);
+    #[cfg(not(stage0))]
+    pub fn atomic_cxchgweak_failacq<T>(dst: *mut T, old: T, src: T) -> (T, bool);
+    #[cfg(not(stage0))]
+    pub fn atomic_cxchgweak_acq_failrelaxed<T>(dst: *mut T, old: T, src: T) -> (T, bool);
+    #[cfg(not(stage0))]
+    pub fn atomic_cxchgweak_acqrel_failrelaxed<T>(dst: *mut T, old: T, src: T) -> (T, bool);
 
     pub fn atomic_load<T>(src: *const T) -> T;
     pub fn atomic_load_acq<T>(src: *const T) -> T;
diff --git a/src/libcore/sync/atomic.rs b/src/libcore/sync/atomic.rs
index 700a577e20c..0c831bff763 100644
--- a/src/libcore/sync/atomic.rs
+++ b/src/libcore/sync/atomic.rs
@@ -311,10 +311,101 @@ impl AtomicBool {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn compare_and_swap(&self, current: bool, new: bool, order: Ordering) -> bool {
+        self.compare_exchange(current, new, order, strongest_failure_ordering(order))
+    }
+
+    /// Stores a value into the `bool` if the current value is the same as the `current` value.
+    ///
+    /// The return value is always the previous value. If it is equal to `current`, then the value
+    /// was updated.
+    ///
+    /// `compare_exchange` takes two `Ordering` arguments to describe the memory ordering of this
+    /// operation. The first describes the required ordering if the operation succeeds while the
+    /// second describes the required ordering when the operation fails. The failure ordering can't
+    /// be `Acquire` or `AcqRel` and must be equivalent or weaker than the success ordering.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(extended_compare_and_swap)]
+    /// use std::sync::atomic::{AtomicBool, Ordering};
+    ///
+    /// let some_bool = AtomicBool::new(true);
+    ///
+    /// assert_eq!(some_bool.compare_exchange(true,
+    ///                                       false,
+    ///                                       Ordering::Acquire,
+    ///                                       Ordering::Relaxed),
+    ///            true);
+    /// assert_eq!(some_bool.load(Ordering::Relaxed), false);
+    ///
+    /// assert_eq!(some_bool.compare_exchange(true, true,
+    ///                                       Ordering::SeqCst,
+    ///                                       Ordering::Acquire),
+    ///            false);
+    /// assert_eq!(some_bool.load(Ordering::Relaxed), false);
+    /// ```
+    #[inline]
+    #[unstable(feature = "extended_compare_and_swap", reason = "recently added", issue = "31767")]
+    pub fn compare_exchange(&self,
+                            current: bool,
+                            new: bool,
+                            success: Ordering,
+                            failure: Ordering) -> bool {
         let current = if current { UINT_TRUE } else { 0 };
         let new = if new { UINT_TRUE } else { 0 };
 
-        unsafe { atomic_compare_and_swap(self.v.get(), current, new, order) > 0 }
+        unsafe { atomic_compare_exchange(self.v.get(), current, new, success, failure) > 0 }
+    }
+
+    /// Stores a value into the `bool` if the current value is the same as the `current` value.
+    ///
+    /// Unlike `compare_exchange`, this function is allowed to spuriously fail even when the
+    /// comparison succeeds, which can result in more efficient code on some platforms. The
+    /// returned value is a tuple of the existing value and a flag indicating whether the
+    /// new value was written.
+    ///
+    /// `compare_exchange_weak` takes two `Ordering` arguments to describe the memory
+    /// ordering of this operation. The first describes the required ordering if the operation
+    /// succeeds while the second describes the required ordering when the operation fails. The
+    /// failure ordering can't be `Acquire` or `AcqRel` and must be equivalent or weaker than the
+    /// success ordering.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(extended_compare_and_swap)]
+    /// use std::sync::atomic::{AtomicBool, Ordering};
+    ///
+    /// let val = AtomicBool::new(false);
+    ///
+    /// let new = true;
+    /// let mut old = val.load(Ordering::Relaxed);
+    /// loop {
+    ///     let result = val.compare_exchange_weak(old, new,
+    ///                                            Ordering::SeqCst,
+    ///                                            Ordering::Relaxed);
+    ///     if result.1 {
+    ///         break;
+    ///     } else {
+    ///         old = result.0;
+    ///     }
+    /// }
+    /// ```
+    #[inline]
+    #[unstable(feature = "extended_compare_and_swap", reason = "recently added", issue = "31767")]
+    pub fn compare_exchange_weak(&self,
+                                 current: bool,
+                                 new: bool,
+                                 success: Ordering,
+                                 failure: Ordering) -> (bool, bool) {
+        let current = if current { UINT_TRUE } else { 0 };
+        let new = if new { UINT_TRUE } else { 0 };
+
+        let result = unsafe {
+            atomic_compare_exchange_weak(self.v.get(), current, new, success, failure)
+        };
+        (result.0 > 0, result.1)
     }
 
     /// Logical "and" with a boolean value.
@@ -553,7 +644,91 @@ impl AtomicIsize {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn compare_and_swap(&self, current: isize, new: isize, order: Ordering) -> isize {
-        unsafe { atomic_compare_and_swap(self.v.get(), current, new, order) }
+        self.compare_exchange(current, new, order, strongest_failure_ordering(order))
+    }
+
+    /// Stores a value into the `isize` if the current value is the same as the `current` value.
+    ///
+    /// The return value is always the previous value. If it is equal to `current`, then the value
+    /// was updated.
+    ///
+    /// `compare_exchange` takes two `Ordering` arguments to describe the memory ordering of this
+    /// operation. The first describes the required ordering if the operation succeeds while the
+    /// second describes the required ordering when the operation fails. The failure ordering can't
+    /// be `Acquire` or `AcqRel` and must be equivalent or weaker than the success ordering.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(extended_compare_and_swap)]
+    /// use std::sync::atomic::{AtomicIsize, Ordering};
+    ///
+    /// let some_isize = AtomicIsize::new(5);
+    ///
+    /// assert_eq!(some_isize.compare_exchange(5, 10,
+    ///                                        Ordering::Acquire,
+    ///                                        Ordering::Relaxed),
+    ///            5);
+    /// assert_eq!(some_isize.load(Ordering::Relaxed), 10);
+    ///
+    /// assert_eq!(some_isize.compare_exchange(6, 12,
+    ///                                        Ordering::SeqCst,
+    ///                                        Ordering::Acquire),
+    ///            10);
+    /// assert_eq!(some_isize.load(Ordering::Relaxed), 10);
+    /// ```
+    #[inline]
+    #[unstable(feature = "extended_compare_and_swap", reason = "recently added", issue = "31767")]
+    pub fn compare_exchange(&self,
+                            current: isize,
+                            new: isize,
+                            success: Ordering,
+                            failure: Ordering) -> isize {
+        unsafe { atomic_compare_exchange(self.v.get(), current, new, success, failure) }
+    }
+
+    /// Stores a value into the `isize if the current value is the same as the `current` value.
+    ///
+    /// Unlike `compare_exchange`, this function is allowed to spuriously fail even when the
+    /// comparison succeeds, which can result in more efficient code on some platforms. The
+    /// returned value is a tuple of the existing value and a flag indicating whether the
+    /// new value was written.
+    ///
+    /// `compare_exchange_weak` takes two `Ordering` arguments to describe the memory
+    /// ordering of this operation. The first describes the required ordering if the operation
+    /// succeeds while the second describes the required ordering when the operation fails. The
+    /// failure ordering can't be `Acquire` or `AcqRel` and must be equivalent or weaker than the
+    /// success ordering.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(extended_compare_and_swap)]
+    /// use std::sync::atomic::{AtomicIsize, Ordering};
+    ///
+    /// let val = AtomicIsize::new(4);
+    ///
+    /// let mut old = val.load(Ordering::Relaxed);
+    /// loop {
+    ///     let new = old * 2;
+    ///     let result = val.compare_exchange_weak(old, new,
+    ///                                            Ordering::SeqCst,
+    ///                                            Ordering::Relaxed);
+    ///     if result.1 {
+    ///         break;
+    ///     } else {
+    ///         old = result.0;
+    ///     }
+    /// }
+    /// ```
+    #[inline]
+    #[unstable(feature = "extended_compare_and_swap", reason = "recently added", issue = "31767")]
+    pub fn compare_exchange_weak(&self,
+                                 current: isize,
+                                 new: isize,
+                                 success: Ordering,
+                                 failure: Ordering) -> (isize, bool) {
+        unsafe { atomic_compare_exchange_weak(self.v.get(), current, new, success, failure) }
     }
 
     /// Add an isize to the current value, returning the previous value.
@@ -746,7 +921,91 @@ impl AtomicUsize {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn compare_and_swap(&self, current: usize, new: usize, order: Ordering) -> usize {
-        unsafe { atomic_compare_and_swap(self.v.get(), current, new, order) }
+        self.compare_exchange(current, new, order, strongest_failure_ordering(order))
+    }
+
+    /// Stores a value into the `usize` if the current value is the same as the `current` value.
+    ///
+    /// The return value is always the previous value. If it is equal to `current`, then the value
+    /// was updated.
+    ///
+    /// `compare_exchange` takes two `Ordering` arguments to describe the memory ordering of this
+    /// operation. The first describes the required ordering if the operation succeeds while the
+    /// second describes the required ordering when the operation fails. The failure ordering can't
+    /// be `Acquire` or `AcqRel` and must be equivalent or weaker than the success ordering.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(extended_compare_and_swap)]
+    /// use std::sync::atomic::{AtomicUsize, Ordering};
+    ///
+    /// let some_isize = AtomicUsize::new(5);
+    ///
+    /// assert_eq!(some_isize.compare_exchange(5, 10,
+    ///                                        Ordering::Acquire,
+    ///                                        Ordering::Relaxed),
+    ///            5);
+    /// assert_eq!(some_isize.load(Ordering::Relaxed), 10);
+    ///
+    /// assert_eq!(some_isize.compare_exchange(6, 12,
+    ///                                        Ordering::SeqCst,
+    ///                                        Ordering::Acquire),
+    ///            10);
+    /// assert_eq!(some_isize.load(Ordering::Relaxed), 10);
+    /// ```
+    #[inline]
+    #[unstable(feature = "extended_compare_and_swap", reason = "recently added", issue = "31767")]
+    pub fn compare_exchange(&self,
+                            current: usize,
+                            new: usize,
+                            success: Ordering,
+                            failure: Ordering) -> usize {
+        unsafe { atomic_compare_exchange(self.v.get(), current, new, success, failure) }
+    }
+
+    /// Stores a value into the `usize` if the current value is the same as the `current` value.
+    ///
+    /// Unlike `compare_exchange`, this function is allowed to spuriously fail even when the
+    /// comparison succeeds, which can result in more efficient code on some platforms. The
+    /// returned value is a tuple of the existing value and a flag indicating whether the
+    /// new value was written.
+    ///
+    /// `compare_exchange_weak` takes two `Ordering` arguments to describe the memory
+    /// ordering of this operation. The first describes the required ordering if the operation
+    /// succeeds while the second describes the required ordering when the operation fails. The
+    /// failure ordering can't be `Acquire` or `AcqRel` and must be equivalent or weaker than the
+    /// success ordering.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(extended_compare_and_swap)]
+    /// use std::sync::atomic::{AtomicUsize, Ordering};
+    ///
+    /// let val = AtomicUsize::new(4);
+    ///
+    /// let mut old = val.load(Ordering::Relaxed);
+    /// loop {
+    ///     let new = old * 2;
+    ///     let result = val.compare_exchange_weak(old, new,
+    ///                                            Ordering::SeqCst,
+    ///                                            Ordering::Relaxed);
+    ///     if result.1 {
+    ///         break;
+    ///     } else {
+    ///         old = result.0;
+    ///     }
+    /// }
+    /// ```
+    #[inline]
+    #[unstable(feature = "extended_compare_and_swap", reason = "recently added", issue = "31767")]
+    pub fn compare_exchange_weak(&self,
+                                 current: usize,
+                                 new: usize,
+                                 success: Ordering,
+                                 failure: Ordering) -> (usize, bool) {
+        unsafe { atomic_compare_exchange_weak(self.v.get(), current, new, success, failure) }
     }
 
     /// Add to the current usize, returning the previous value.
@@ -947,15 +1206,109 @@ impl<T> AtomicPtr<T> {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn compare_and_swap(&self, current: *mut T, new: *mut T, order: Ordering) -> *mut T {
+        self.compare_exchange(current, new, order, strongest_failure_ordering(order))
+    }
+
+    /// Stores a value into the pointer if the current value is the same as the `current` value.
+    ///
+    /// The return value is always the previous value. If it is equal to `current`, then the value
+    /// was updated.
+    ///
+    /// `compare_exchange` takes two `Ordering` arguments to describe the memory ordering of this
+    /// operation. The first describes the required ordering if the operation succeeds while the
+    /// second describes the required ordering when the operation fails. The failure ordering can't
+    /// be `Acquire` or `AcqRel` and must be equivalent or weaker than the success ordering.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(extended_compare_and_swap)]
+    /// use std::sync::atomic::{AtomicPtr, Ordering};
+    ///
+    /// let ptr = &mut 5;
+    /// let some_ptr  = AtomicPtr::new(ptr);
+    ///
+    /// let other_ptr   = &mut 10;
+    /// let another_ptr = &mut 10;
+    ///
+    /// let value = some_ptr.compare_exchange(other_ptr, another_ptr,
+    ///                                       Ordering::SeqCst, Ordering::Relaxed);
+    /// ```
+    #[inline]
+    #[unstable(feature = "extended_compare_and_swap", reason = "recently added", issue = "31767")]
+    pub fn compare_exchange(&self,
+                            current: *mut T,
+                            new: *mut T,
+                            success: Ordering,
+                            failure: Ordering) -> *mut T {
         unsafe {
-            atomic_compare_and_swap(self.p.get() as *mut usize, current as usize,
-                                    new as usize, order) as *mut T
+            atomic_compare_exchange(self.p.get() as *mut usize, current as usize,
+                                    new as usize, success, failure) as *mut T
         }
     }
+
+    /// Stores a value into the pointer if the current value is the same as the `current` value.
+    ///
+    /// Unlike `compare_exchange`, this function is allowed to spuriously fail even when the
+    /// comparison succeeds, which can result in more efficient code on some platforms. The
+    /// returned value is a tuple of the existing value and a flag indicating whether the
+    /// new value was written.
+    ///
+    /// `compare_exchange_weak` takes two `Ordering` arguments to describe the memory
+    /// ordering of this operation. The first describes the required ordering if the operation
+    /// succeeds while the second describes the required ordering when the operation fails. The
+    /// failure ordering can't be `Acquire` or `AcqRel` and must be equivalent or weaker than the
+    /// success ordering.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #![feature(extended_compare_and_swap)]
+    /// use std::sync::atomic::{AtomicPtr, Ordering};
+    ///
+    /// let some_ptr = AtomicPtr::new(&mut 5);
+    ///
+    /// let new = &mut 10;
+    /// let mut old = some_ptr.load(Ordering::Relaxed);
+    /// loop {
+    ///     let result = some_ptr.compare_exchange_weak(old, new,
+    ///                                                 Ordering::SeqCst,
+    ///                                                 Ordering::Relaxed);
+    ///     if result.1 {
+    ///         break;
+    ///     } else {
+    ///         old = result.0;
+    ///     }
+    /// }
+    /// ```
+    #[inline]
+    #[unstable(feature = "extended_compare_and_swap", reason = "recently added", issue = "31767")]
+    pub fn compare_exchange_weak(&self,
+                                 current: *mut T,
+                                 new: *mut T,
+                                 success: Ordering,
+                                 failure: Ordering) -> (*mut T, bool) {
+        let result = unsafe {
+            atomic_compare_exchange_weak(self.p.get() as *mut usize, current as usize,
+                                         new as usize, success, failure)
+        };
+        (result.0 as *mut T, result.1)
+    }
 }
 
 #[inline]
-unsafe fn atomic_store<T>(dst: *mut T, val: T, order:Ordering) {
+fn strongest_failure_ordering(order: Ordering) -> Ordering {
+    match order {
+        Release => Relaxed,
+        Relaxed => Relaxed,
+        SeqCst  => SeqCst,
+        Acquire => Acquire,
+        AcqRel  => Acquire,
+    }
+}
+
+#[inline]
+unsafe fn atomic_store<T>(dst: *mut T, val: T, order: Ordering) {
     match order {
         Release => intrinsics::atomic_store_rel(dst, val),
         Relaxed => intrinsics::atomic_store_relaxed(dst, val),
@@ -966,7 +1319,7 @@ unsafe fn atomic_store<T>(dst: *mut T, val: T, order:Ordering) {
 }
 
 #[inline]
-unsafe fn atomic_load<T>(dst: *const T, order:Ordering) -> T {
+unsafe fn atomic_load<T>(dst: *const T, order: Ordering) -> T {
     match order {
         Acquire => intrinsics::atomic_load_acq(dst),
         Relaxed => intrinsics::atomic_load_relaxed(dst),
@@ -1012,8 +1365,36 @@ unsafe fn atomic_sub<T>(dst: *mut T, val: T, order: Ordering) -> T {
 }
 
 #[inline]
-unsafe fn atomic_compare_and_swap<T>(dst: *mut T, old:T, new:T, order: Ordering) -> T {
-    match order {
+#[cfg(not(stage0))]
+unsafe fn atomic_compare_exchange<T>(dst: *mut T,
+                                     old: T,
+                                     new: T,
+                                     success: Ordering,
+                                     failure: Ordering) -> T {
+    match (success, failure) {
+        (Acquire, Acquire) => intrinsics::atomic_cxchg_acq(dst, old, new),
+        (Release, Relaxed) => intrinsics::atomic_cxchg_rel(dst, old, new),
+        (AcqRel, Acquire)  => intrinsics::atomic_cxchg_acqrel(dst, old, new),
+        (Relaxed, Relaxed) => intrinsics::atomic_cxchg_relaxed(dst, old, new),
+        (SeqCst, SeqCst)   => intrinsics::atomic_cxchg(dst, old, new),
+        (Acquire, Relaxed) => intrinsics::atomic_cxchg_acq_failrelaxed(dst, old, new),
+        (AcqRel, Relaxed)  => intrinsics::atomic_cxchg_acqrel_failrelaxed(dst, old, new),
+        (SeqCst, Relaxed)  => intrinsics::atomic_cxchg_failrelaxed(dst, old, new),
+        (SeqCst, Acquire)  => intrinsics::atomic_cxchg_failacq(dst, old, new),
+        (_, Release) => panic!("there is no such thing as an acquire/release failure ordering"),
+        (_, AcqRel) => panic!("there is no such thing as a release failure ordering"),
+        _ => panic!("a failure ordering can't be stronger than a success ordering"),
+    }
+}
+
+#[inline]
+#[cfg(stage0)]
+unsafe fn atomic_compare_exchange<T>(dst: *mut T,
+                                     old: T,
+                                     new: T,
+                                     success: Ordering,
+                                     _: Ordering) -> T {
+    match success {
         Acquire => intrinsics::atomic_cxchg_acq(dst, old, new),
         Release => intrinsics::atomic_cxchg_rel(dst, old, new),
         AcqRel  => intrinsics::atomic_cxchg_acqrel(dst, old, new),
@@ -1023,6 +1404,42 @@ unsafe fn atomic_compare_and_swap<T>(dst: *mut T, old:T, new:T, order: Ordering)
 }
 
 #[inline]
+#[cfg(not(stage0))]
+unsafe fn atomic_compare_exchange_weak<T>(dst: *mut T,
+                                          old: T,
+                                          new: T,
+                                          success: Ordering,
+                                          failure: Ordering) -> (T, bool) {
+    match (success, failure) {
+        (Acquire, Acquire) => intrinsics::atomic_cxchgweak_acq(dst, old, new),
+        (Release, Relaxed) => intrinsics::atomic_cxchgweak_rel(dst, old, new),
+        (AcqRel, Acquire)  => intrinsics::atomic_cxchgweak_acqrel(dst, old, new),
+        (Relaxed, Relaxed) => intrinsics::atomic_cxchgweak_relaxed(dst, old, new),
+        (SeqCst, SeqCst)   => intrinsics::atomic_cxchgweak(dst, old, new),
+        (Acquire, Relaxed) => intrinsics::atomic_cxchgweak_acq_failrelaxed(dst, old, new),
+        (AcqRel, Relaxed)  => intrinsics::atomic_cxchgweak_acqrel_failrelaxed(dst, old, new),
+        (SeqCst, Relaxed)  => intrinsics::atomic_cxchgweak_failrelaxed(dst, old, new),
+        (SeqCst, Acquire)  => intrinsics::atomic_cxchgweak_failacq(dst, old, new),
+        (_, Release) => panic!("there is no such thing as an acquire/release failure ordering"),
+        (_, AcqRel) => panic!("there is no such thing as a release failure ordering"),
+        _ => panic!("a failure ordering can't be stronger than a success ordering"),
+    }
+}
+
+#[inline]
+#[cfg(stage0)]
+unsafe fn atomic_compare_exchange_weak<T>(dst: *mut T,
+                                          old: T,
+                                          new: T,
+                                          success: Ordering,
+                                          failure: Ordering) -> (T, bool)
+    where T: ::cmp::Eq + ::marker::Copy
+{
+    let result = atomic_compare_exchange(dst, old, new, success, failure);
+    (result, result == old)
+}
+
+#[inline]
 unsafe fn atomic_and<T>(dst: *mut T, val: T, order: Ordering) -> T {
     match order {
         Acquire => intrinsics::atomic_and_acq(dst, val),
diff --git a/src/librustc_llvm/lib.rs b/src/librustc_llvm/lib.rs
index dfaf3970237..13acf79a0f1 100644
--- a/src/librustc_llvm/lib.rs
+++ b/src/librustc_llvm/lib.rs
@@ -1584,7 +1584,8 @@ extern {
                                   CMP: ValueRef,
                                   RHS: ValueRef,
                                   Order: AtomicOrdering,
-                                  FailureOrder: AtomicOrdering)
+                                  FailureOrder: AtomicOrdering,
+                                  Weak: Bool)
                                   -> ValueRef;
     pub fn LLVMBuildAtomicRMW(B: BuilderRef,
                               Op: AtomicBinOp,
diff --git a/src/librustc_trans/trans/build.rs b/src/librustc_trans/trans/build.rs
index ce541c8d411..2b4fcf436cb 100644
--- a/src/librustc_trans/trans/build.rs
+++ b/src/librustc_trans/trans/build.rs
@@ -1067,8 +1067,9 @@ pub fn Resume(cx: Block, exn: ValueRef) -> ValueRef {
 pub fn AtomicCmpXchg(cx: Block, dst: ValueRef,
                      cmp: ValueRef, src: ValueRef,
                      order: AtomicOrdering,
-                     failure_order: AtomicOrdering) -> ValueRef {
-    B(cx).atomic_cmpxchg(dst, cmp, src, order, failure_order)
+                     failure_order: AtomicOrdering,
+                     weak: llvm::Bool) -> ValueRef {
+    B(cx).atomic_cmpxchg(dst, cmp, src, order, failure_order, weak)
 }
 pub fn AtomicRMW(cx: Block, op: AtomicBinOp,
                  dst: ValueRef, src: ValueRef,
diff --git a/src/librustc_trans/trans/builder.rs b/src/librustc_trans/trans/builder.rs
index 878d01f46b6..434fca41688 100644
--- a/src/librustc_trans/trans/builder.rs
+++ b/src/librustc_trans/trans/builder.rs
@@ -1077,10 +1077,11 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     pub fn atomic_cmpxchg(&self, dst: ValueRef,
                          cmp: ValueRef, src: ValueRef,
                          order: AtomicOrdering,
-                         failure_order: AtomicOrdering) -> ValueRef {
+                         failure_order: AtomicOrdering,
+                         weak: llvm::Bool) -> ValueRef {
         unsafe {
             llvm::LLVMBuildAtomicCmpXchg(self.llbuilder, dst, cmp, src,
-                                         order, failure_order)
+                                         order, failure_order, weak)
         }
     }
     pub fn atomic_rmw(&self, op: AtomicBinOp,
diff --git a/src/librustc_trans/trans/intrinsic.rs b/src/librustc_trans/trans/intrinsic.rs
index 188fb7de9dd..b7b520f6c82 100644
--- a/src/librustc_trans/trans/intrinsic.rs
+++ b/src/librustc_trans/trans/intrinsic.rs
@@ -678,49 +678,54 @@ pub fn trans_intrinsic_call<'a, 'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
         // "atomic_<operation>[_<ordering>]", and no ordering means SeqCst
         (_, name) if name.starts_with("atomic_") => {
             let split: Vec<&str> = name.split('_').collect();
-            assert!(split.len() >= 2, "Atomic intrinsic not correct format");
 
-            let order = if split.len() == 2 {
-                llvm::SequentiallyConsistent
-            } else {
-                match split[2] {
-                    "unordered" => llvm::Unordered,
-                    "relaxed" => llvm::Monotonic,
-                    "acq"     => llvm::Acquire,
-                    "rel"     => llvm::Release,
-                    "acqrel"  => llvm::AcquireRelease,
+            let (order, failorder) = match split.len() {
+                2 => (llvm::SequentiallyConsistent, llvm::SequentiallyConsistent),
+                3 => match split[2] {
+                    "unordered" => (llvm::Unordered, llvm::Unordered),
+                    "relaxed" => (llvm::Monotonic, llvm::Monotonic),
+                    "acq"     => (llvm::Acquire, llvm::Acquire),
+                    "rel"     => (llvm::Release, llvm::Monotonic),
+                    "acqrel"  => (llvm::AcquireRelease, llvm::Acquire),
+                    "failrelaxed" if split[1] == "cxchg" || split[1] == "cxchgweak" =>
+                        (llvm::SequentiallyConsistent, llvm::Monotonic),
+                    "failacq" if split[1] == "cxchg" || split[1] == "cxchgweak" =>
+                        (llvm::SequentiallyConsistent, llvm::Acquire),
                     _ => ccx.sess().fatal("unknown ordering in atomic intrinsic")
-                }
+                },
+                4 => match (split[2], split[3]) {
+                    ("acq", "failrelaxed") if split[1] == "cxchg" || split[1] == "cxchgweak" =>
+                        (llvm::Acquire, llvm::Monotonic),
+                    ("acqrel", "failrelaxed") if split[1] == "cxchg" || split[1] == "cxchgweak" =>
+                        (llvm::AcquireRelease, llvm::Monotonic),
+                    _ => ccx.sess().fatal("unknown ordering in atomic intrinsic")
+                },
+                _ => ccx.sess().fatal("Atomic intrinsic not in correct format"),
             };
 
             match split[1] {
                 "cxchg" => {
-                    // See include/llvm/IR/Instructions.h for their implementation
-                    // of this, I assume that it's good enough for us to use for
-                    // now.
-                    let strongest_failure_ordering = match order {
-                        llvm::NotAtomic | llvm::Unordered =>
-                            ccx.sess().fatal("cmpxchg must be atomic"),
-
-                        llvm::Monotonic | llvm::Release =>
-                            llvm::Monotonic,
-
-                        llvm::Acquire | llvm::AcquireRelease =>
-                            llvm::Acquire,
-
-                        llvm::SequentiallyConsistent =>
-                            llvm::SequentiallyConsistent
-                    };
-
                     let tp_ty = *substs.types.get(FnSpace, 0);
                     let ptr = to_arg_ty_ptr(bcx, llargs[0], tp_ty);
                     let cmp = from_arg_ty(bcx, llargs[1], tp_ty);
                     let src = from_arg_ty(bcx, llargs[2], tp_ty);
-                    let res = AtomicCmpXchg(bcx, ptr, cmp, src, order,
-                                            strongest_failure_ordering);
+                    let res = AtomicCmpXchg(bcx, ptr, cmp, src, order, failorder, llvm::False);
                     ExtractValue(bcx, res, 0)
                 }
 
+                "cxchgweak" => {
+                    let tp_ty = *substs.types.get(FnSpace, 0);
+                    let ptr = to_arg_ty_ptr(bcx, llargs[0], tp_ty);
+                    let cmp = from_arg_ty(bcx, llargs[1], tp_ty);
+                    let src = from_arg_ty(bcx, llargs[2], tp_ty);
+                    let val = AtomicCmpXchg(bcx, ptr, cmp, src, order, failorder, llvm::True);
+                    let result = ExtractValue(bcx, val, 0);
+                    let success = ZExt(bcx, ExtractValue(bcx, val, 1), Type::bool(bcx.ccx()));
+                    Store(bcx, result, StructGEP(bcx, llresult, 0));
+                    Store(bcx, success, StructGEP(bcx, llresult, 1));
+                    C_nil(ccx)
+                }
+
                 "load" => {
                     let tp_ty = *substs.types.get(FnSpace, 0);
                     let ptr = to_arg_ty_ptr(bcx, llargs[0], tp_ty);
diff --git a/src/librustc_typeck/check/intrinsic.rs b/src/librustc_typeck/check/intrinsic.rs
index ba6fa9aed3d..5e1dc35870b 100644
--- a/src/librustc_typeck/check/intrinsic.rs
+++ b/src/librustc_typeck/check/intrinsic.rs
@@ -83,6 +83,10 @@ pub fn check_intrinsic_type(ccx: &CrateCtxt, it: &hir::ForeignItem) {
                                 param(ccx, 0),
                                 param(ccx, 0)),
                         param(ccx, 0)),
+            "cxchgweak" => (1, vec!(tcx.mk_mut_ptr(param(ccx, 0)),
+                                param(ccx, 0),
+                                param(ccx, 0)),
+                            tcx.mk_tup(vec!(param(ccx, 0), tcx.types.bool))),
             "load" => (1, vec!(tcx.mk_imm_ptr(param(ccx, 0))),
                        param(ccx, 0)),
             "store" => (1, vec!(tcx.mk_mut_ptr(param(ccx, 0)), param(ccx, 0)),
diff --git a/src/rustllvm/RustWrapper.cpp b/src/rustllvm/RustWrapper.cpp
index ecf9146a1d0..4ebe49512d7 100644
--- a/src/rustllvm/RustWrapper.cpp
+++ b/src/rustllvm/RustWrapper.cpp
@@ -191,11 +191,15 @@ extern "C" LLVMValueRef LLVMBuildAtomicCmpXchg(LLVMBuilderRef B,
                                                LLVMValueRef old,
                                                LLVMValueRef source,
                                                AtomicOrdering order,
-                                               AtomicOrdering failure_order) {
-    return wrap(unwrap(B)->CreateAtomicCmpXchg(unwrap(target), unwrap(old),
-                                               unwrap(source), order,
-                                               failure_order
-                                               ));
+                                               AtomicOrdering failure_order,
+                                               LLVMBool weak) {
+    AtomicCmpXchgInst* acxi = unwrap(B)->CreateAtomicCmpXchg(unwrap(target),
+                                                             unwrap(old),
+                                                             unwrap(source),
+                                                             order,
+                                                             failure_order);
+    acxi->setWeak(weak);
+    return wrap(acxi);
 }
 extern "C" LLVMValueRef LLVMBuildAtomicFence(LLVMBuilderRef B,
                                              AtomicOrdering order,
diff --git a/src/test/run-pass/atomic-compare_exchange.rs b/src/test/run-pass/atomic-compare_exchange.rs
new file mode 100644
index 00000000000..7946704709c
--- /dev/null
+++ b/src/test/run-pass/atomic-compare_exchange.rs
@@ -0,0 +1,37 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.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.
+
+#![feature(extended_compare_and_swap)]
+use std::sync::atomic::{AtomicIsize, ATOMIC_ISIZE_INIT};
+use std::sync::atomic::Ordering::*;
+
+static ATOMIC: AtomicIsize = ATOMIC_ISIZE_INIT;
+
+fn main() {
+    // Make sure trans can emit all the intrinsics correctly
+    ATOMIC.compare_exchange(0, 1, Relaxed, Relaxed);
+    ATOMIC.compare_exchange(0, 1, Acquire, Relaxed);
+    ATOMIC.compare_exchange(0, 1, Release, Relaxed);
+    ATOMIC.compare_exchange(0, 1, AcqRel, Relaxed);
+    ATOMIC.compare_exchange(0, 1, SeqCst, Relaxed);
+    ATOMIC.compare_exchange(0, 1, Acquire, Acquire);
+    ATOMIC.compare_exchange(0, 1, AcqRel, Acquire);
+    ATOMIC.compare_exchange(0, 1, SeqCst, Acquire);
+    ATOMIC.compare_exchange(0, 1, SeqCst, SeqCst);
+    ATOMIC.compare_exchange_weak(0, 1, Relaxed, Relaxed);
+    ATOMIC.compare_exchange_weak(0, 1, Acquire, Relaxed);
+    ATOMIC.compare_exchange_weak(0, 1, Release, Relaxed);
+    ATOMIC.compare_exchange_weak(0, 1, AcqRel, Relaxed);
+    ATOMIC.compare_exchange_weak(0, 1, SeqCst, Relaxed);
+    ATOMIC.compare_exchange_weak(0, 1, Acquire, Acquire);
+    ATOMIC.compare_exchange_weak(0, 1, AcqRel, Acquire);
+    ATOMIC.compare_exchange_weak(0, 1, SeqCst, Acquire);
+    ATOMIC.compare_exchange_weak(0, 1, SeqCst, SeqCst);
+}
diff --git a/src/test/run-pass/intrinsic-atomics.rs b/src/test/run-pass/intrinsic-atomics.rs
index 4ccab55e943..3cc125e9513 100644
--- a/src/test/run-pass/intrinsic-atomics.rs
+++ b/src/test/run-pass/intrinsic-atomics.rs
@@ -19,6 +19,10 @@ mod rusti {
         pub fn atomic_cxchg_acq<T>(dst: *mut T, old: T, src: T) -> T;
         pub fn atomic_cxchg_rel<T>(dst: *mut T, old: T, src: T) -> T;
 
+        pub fn atomic_cxchgweak<T>(dst: *mut T, old: T, src: T) -> (T, bool);
+        pub fn atomic_cxchgweak_acq<T>(dst: *mut T, old: T, src: T) -> (T, bool);
+        pub fn atomic_cxchgweak_rel<T>(dst: *mut T, old: T, src: T) -> (T, bool);
+
         pub fn atomic_load<T>(src: *const T) -> T;
         pub fn atomic_load_acq<T>(src: *const T) -> T;
 
@@ -79,5 +83,32 @@ pub fn main() {
         assert_eq!(rusti::atomic_xsub_acq(&mut *x, 1), 2);
         assert_eq!(rusti::atomic_xsub_rel(&mut *x, 1), 1);
         assert_eq!(*x, 0);
+
+        loop {
+            let res = rusti::atomic_cxchgweak(&mut *x, 0, 1);
+            assert_eq!(res.0, 0);
+            if res.1 {
+                break;
+            }
+        }
+        assert_eq!(*x, 1);
+
+        loop {
+            let res = rusti::atomic_cxchgweak_acq(&mut *x, 1, 2);
+            assert_eq!(res.0, 1);
+            if res.1 {
+                break;
+            }
+        }
+        assert_eq!(*x, 2);
+
+        loop {
+            let res = rusti::atomic_cxchgweak_rel(&mut *x, 2, 3);
+            assert_eq!(res.0, 2);
+            if res.1 {
+                break;
+            }
+        }
+        assert_eq!(*x, 3);
     }
 }