intrinsics.rs.html -- source
Description: Source to the Rust file `src/libcore/intrinsics.rs`.
Keywords: rust, rustlang, rust-lang

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    // Copyright 2013 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.

    //! rustc compiler intrinsics.
    //!
    //! The corresponding definitions are in librustc_trans/intrinsic.rs.
    //!
    //! # Volatiles
    //!
    //! The volatile intrinsics provide operations intended to act on I/O
    //! memory, which are guaranteed to not be reordered by the compiler
    //! across other volatile intrinsics. See the LLVM documentation on
    //! [[volatile]].
    //!
    //! [volatile]: http://llvm.org/docs/LangRef.html#volatile-memory-accesses
    //!
    //! # Atomics
    //!
    //! The atomic intrinsics provide common atomic operations on machine
    //! words, with multiple possible memory orderings. They obey the same
    //! semantics as C++11. See the LLVM documentation on [[atomics]].
    //!
    //! [atomics]: http://llvm.org/docs/Atomics.html
    //!
    //! A quick refresher on memory ordering:
    //!
    //! * Acquire - a barrier for acquiring a lock. Subsequent reads and writes
    //!   take place after the barrier.
    //! * Release - a barrier for releasing a lock. Preceding reads and writes
    //!   take place before the barrier.
    //! * Sequentially consistent - sequentially consistent operations are
    //!   guaranteed to happen in order. This is the standard mode for working
    //!   with atomic types and is equivalent to Java's `volatile`.

    #![unstable(feature = "core_intrinsics",
                reason = "intrinsics are unlikely to ever be stabilized, instead \
                          they should be used through stabilized interfaces \
                          in the rest of the standard library",
                issue = "0")]
    #![allow(missing_docs)]

    extern "rust-intrinsic" {

        // NB: These intrinsics take raw pointers because they mutate aliased
        // memory, which is not valid for either `&` or `&mut`.

        pub fn atomic_cxchg<T>(dst: *mut T, old: T, src: T) -> (T, bool);
        pub fn atomic_cxchg_acq<T>(dst: *mut T, old: T, src: T) -> (T, bool);
        pub fn atomic_cxchg_rel<T>(dst: *mut T, old: T, src: T) -> (T, bool);
        pub fn atomic_cxchg_acqrel<T>(dst: *mut T, old: T, src: T) -> (T, bool);
        pub fn atomic_cxchg_relaxed<T>(dst: *mut T, old: T, src: T) -> (T, bool);
        pub fn atomic_cxchg_failrelaxed<T>(dst: *mut T, old: T, src: T) -> (T, bool);
        pub fn atomic_cxchg_failacq<T>(dst: *mut T, old: T, src: T) -> (T, bool);
        pub fn atomic_cxchg_acq_failrelaxed<T>(dst: *mut T, old: T, src: T) -> (T, bool);
        pub fn atomic_cxchg_acqrel_failrelaxed<T>(dst: *mut T, old: T, src: T) -> (T, bool);

        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_cxchgweak_acqrel<T>(dst: *mut T, old: T, src: T) -> (T, bool);
        pub fn atomic_cxchgweak_relaxed<T>(dst: *mut T, old: T, src: T) -> (T, bool);
        pub fn atomic_cxchgweak_failrelaxed<T>(dst: *mut T, old: T, src: T) -> (T, bool);
        pub fn atomic_cxchgweak_failacq<T>(dst: *mut T, old: T, src: T) -> (T, bool);
        pub fn atomic_cxchgweak_acq_failrelaxed<T>(dst: *mut T, old: T, src: T) -> (T, bool);
        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;
        pub fn atomic_load_relaxed<T>(src: *const T) -> T;
        pub fn atomic_load_unordered<T>(src: *const T) -> T;

        pub fn atomic_store<T>(dst: *mut T, val: T);
        pub fn atomic_store_rel<T>(dst: *mut T, val: T);
        pub fn atomic_store_relaxed<T>(dst: *mut T, val: T);
        pub fn atomic_store_unordered<T>(dst: *mut T, val: T);

        pub fn atomic_xchg<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xchg_acq<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xchg_rel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xchg_acqrel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xchg_relaxed<T>(dst: *mut T, src: T) -> T;

        pub fn atomic_xadd<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xadd_acq<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xadd_rel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xadd_acqrel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xadd_relaxed<T>(dst: *mut T, src: T) -> T;

        pub fn atomic_xsub<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xsub_acq<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xsub_rel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xsub_acqrel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xsub_relaxed<T>(dst: *mut T, src: T) -> T;

        pub fn atomic_and<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_and_acq<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_and_rel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_and_acqrel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_and_relaxed<T>(dst: *mut T, src: T) -> T;

        pub fn atomic_nand<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_nand_acq<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_nand_rel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_nand_acqrel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_nand_relaxed<T>(dst: *mut T, src: T) -> T;

        pub fn atomic_or<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_or_acq<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_or_rel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_or_acqrel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_or_relaxed<T>(dst: *mut T, src: T) -> T;

        pub fn atomic_xor<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xor_acq<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xor_rel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xor_acqrel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_xor_relaxed<T>(dst: *mut T, src: T) -> T;

        pub fn atomic_max<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_max_acq<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_max_rel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_max_acqrel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_max_relaxed<T>(dst: *mut T, src: T) -> T;

        pub fn atomic_min<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_min_acq<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_min_rel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_min_acqrel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_min_relaxed<T>(dst: *mut T, src: T) -> T;

        pub fn atomic_umin<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_umin_acq<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_umin_rel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_umin_acqrel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_umin_relaxed<T>(dst: *mut T, src: T) -> T;

        pub fn atomic_umax<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_umax_acq<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_umax_rel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_umax_acqrel<T>(dst: *mut T, src: T) -> T;
        pub fn atomic_umax_relaxed<T>(dst: *mut T, src: T) -> T;
    }

    extern "rust-intrinsic" {

        pub fn atomic_fence();
        pub fn atomic_fence_acq();
        pub fn atomic_fence_rel();
        pub fn atomic_fence_acqrel();

        /// A compiler-only memory barrier.
        ///
        /// Memory accesses will never be reordered across this barrier by the
        /// compiler, but no instructions will be emitted for it. This is
        /// appropriate for operations on the same thread that may be preempted,
        /// such as when interacting with signal handlers.
        pub fn atomic_singlethreadfence();
        pub fn atomic_singlethreadfence_acq();
        pub fn atomic_singlethreadfence_rel();
        pub fn atomic_singlethreadfence_acqrel();

        /// Magic intrinsic that derives its meaning from attributes
        /// attached to the function.
        ///
        /// For example, dataflow uses this to inject static assertions so
        /// that `rustc_peek(potentially_uninitialized)` would actually
        /// double-check that dataflow did indeed compute that it is
        /// uninitialized at that point in the control flow.
        pub fn rustc_peek<T>(_: T) -> T;

        /// Aborts the execution of the process.
        pub fn abort() -> !;

        /// Tells LLVM that this point in the code is not reachable,
        /// enabling further optimizations.
        ///
        /// NB: This is very different from the `unreachable!()` macro!
        pub fn unreachable() -> !;

        /// Informs the optimizer that a condition is always true.
        /// If the condition is false, the behavior is undefined.
        ///
        /// No code is generated for this intrinsic, but the optimizer will try
        /// to preserve it (and its condition) between passes, which may interfere
        /// with optimization of surrounding code and reduce performance. It should
        /// not be used if the invariant can be discovered by the optimizer on its
        /// own, or if it does not enable any significant optimizations.
        pub fn assume(b: bool);

        /// Hints to the compiler that branch condition is likely to be true.
        /// Returns the value passed to it.
        ///
        /// Any use other than with `if` statements will probably not have an effect.
        pub fn likely(b: bool) -> bool;

        /// Hints to the compiler that branch condition is likely to be false.
        /// Returns the value passed to it.
        ///
        /// Any use other than with `if` statements will probably not have an effect.
        pub fn unlikely(b: bool) -> bool;

        /// Executes a breakpoint trap, for inspection by a debugger.
        pub fn breakpoint();

        /// The size of a type in bytes.
        ///
        /// More specifically, this is the offset in bytes between successive
        /// items of the same type, including alignment padding.
        pub fn size_of<T>() -> usize;

        /// Moves a value to an uninitialized memory location.
        ///
        /// Drop glue is not run on the destination.
        pub fn move_val_init<T>(dst: *mut T, src: T);

        pub fn min_align_of<T>() -> usize;
        pub fn pref_align_of<T>() -> usize;

        pub fn size_of_val<T: ?Sized>(_: &T) -> usize;
        pub fn min_align_of_val<T: ?Sized>(_: &T) -> usize;

        /// Executes the destructor (if any) of the pointed-to value.
        ///
        /// This has two use cases:
        ///
        /// * It is *required* to use `drop_in_place` to drop unsized types like
        ///   trait objects, because they can't be read out onto the stack and
        ///   dropped normally.
        ///
        /// * It is friendlier to the optimizer to do this over `ptr::read` when
        ///   dropping manually allocated memory (e.g. when writing Box/Rc/Vec),
        ///   as the compiler doesn't need to prove that it's sound to elide the
        ///   copy.
        ///
        /// # Undefined Behavior
        ///
        /// This has all the same safety problems as `ptr::read` with respect to
        /// invalid pointers, types, and double drops.
        #[stable(feature = "drop_in_place", since = "1.8.0")]
        pub fn drop_in_place<T: ?Sized>(to_drop: *mut T);

        /// Gets a static string slice containing the name of a type.
        pub fn type_name<T: ?Sized>() -> &'static str;

        /// Gets an identifier which is globally unique to the specified type. This
        /// function will return the same value for a type regardless of whichever
        /// crate it is invoked in.
        pub fn type_id<T: ?Sized + 'static>() -> u64;

        /// Creates a value initialized to zero.
        ///
        /// `init` is unsafe because it returns a zeroed-out datum,
        /// which is unsafe unless T is `Copy`.  Also, even if T is
        /// `Copy`, an all-zero value may not correspond to any legitimate
        /// state for the type in question.
        pub fn init<T>() -> T;

        /// Creates an uninitialized value.
        ///
        /// `uninit` is unsafe because there is no guarantee of what its
        /// contents are. In particular its drop-flag may be set to any
        /// state, which means it may claim either dropped or
        /// undropped. In the general case one must use `ptr::write` to
        /// initialize memory previous set to the result of `uninit`.
        pub fn uninit<T>() -> T;

        /// Moves a value out of scope without running drop glue.
        pub fn forget<T>(_: T) -> ();

        /// Reinterprets the bits of a value of one type as another type.
        ///
        /// Both types must have the same size. Neither the original, nor the result,
        /// may be an [invalid value](../../nomicon/meet-safe-and-unsafe.html).
        ///
        /// `transmute` is semantically equivalent to a bitwise move of one type
        /// into another. It copies the bits from the source value into the
        /// destination value, then forgets the original. It's equivalent to C's
        /// `memcpy` under the hood, just like `transmute_copy`.
        ///
        /// `transmute` is **incredibly** unsafe. There are a vast number of ways to
        /// cause [undefined behavior][ub] with this function. `transmute` should be
        /// the absolute last resort.
        ///
        /// The [nomicon](../../nomicon/transmutes.html) has additional
        /// documentation.
        ///
        /// [ub]: ../../reference.html#behavior-considered-undefined
        ///
        /// # Examples
        ///
        /// There are a few things that `transmute` is really useful for.
        ///
        /// Getting the bitpattern of a floating point type (or, more generally,
        /// type punning, when `T` and `U` aren't pointers):
        ///
        /// ```
        /// let bitpattern = unsafe {
        ///     std::mem::transmute::<f32, u32>(1.0)
        /// };
        /// assert_eq!(bitpattern, 0x3F800000);
        /// ```
        ///
        /// Turning a pointer into a function pointer. This is *not* portable to
        /// machines where function pointers and data pointers have different sizes.
        ///
        /// ```
        /// fn foo() -> i32 {
        ///     0
        /// }
        /// let pointer = foo as *const ();
        /// let function = unsafe {
        ///     std::mem::transmute::<*const (), fn() -> i32>(pointer)
        /// };
        /// assert_eq!(function(), 0);
        /// ```
        ///
        /// Extending a lifetime, or shortening an invariant lifetime. This is
        /// advanced, very unsafe Rust!
        ///
        /// ```
        /// struct R<'a>(&'a i32);
        /// unsafe fn extend_lifetime<'b>(r: R<'b>) -> R<'static> {
        ///     std::mem::transmute::<R<'b>, R<'static>>(r)
        /// }
        ///
        /// unsafe fn shorten_invariant_lifetime<'b, 'c>(r: &'b mut R<'static>)
        ///                                              -> &'b mut R<'c> {
        ///     std::mem::transmute::<&'b mut R<'static>, &'b mut R<'c>>(r)
        /// }
        /// ```
        ///
        /// # Alternatives
        ///
        /// Don't despair: many uses of `transmute` can be achieved through other means.
        /// Below are common applications of `transmute` which can be replaced with safer
        /// constructs.
        ///
        /// Turning a pointer into a `usize`:
        ///
        /// ```
        /// let ptr = &0;
        /// let ptr_num_transmute = unsafe {
        ///     std::mem::transmute::<&i32, usize>(ptr)
        /// };
        ///
        /// // Use an `as` cast instead
        /// let ptr_num_cast = ptr as *const i32 as usize;
        /// ```
        ///
        /// Turning a `*mut T` into an `&mut T`:
        ///
        /// ```
        /// let ptr: *mut i32 = &mut 0;
        /// let ref_transmuted = unsafe {
        ///     std::mem::transmute::<*mut i32, &mut i32>(ptr)
        /// };
        ///
        /// // Use a reborrow instead
        /// let ref_casted = unsafe { &mut *ptr };
        /// ```
        ///
        /// Turning an `&mut T` into an `&mut U`:
        ///
        /// ```
        /// let ptr = &mut 0;
        /// let val_transmuted = unsafe {
        ///     std::mem::transmute::<&mut i32, &mut u32>(ptr)
        /// };
        ///
        /// // Now, put together `as` and reborrowing - note the chaining of `as`
        /// // `as` is not transitive
        /// let val_casts = unsafe { &mut *(ptr as *mut i32 as *mut u32) };
        /// ```
        ///
        /// Turning an `&str` into an `&[u8]`:
        ///
        /// ```
        /// // this is not a good way to do this.
        /// let slice = unsafe { std::mem::transmute::<&str, &[u8]>("Rust") };
        /// assert_eq!(slice, &[82, 117, 115, 116]);
        ///
        /// // You could use `str::as_bytes`
        /// let slice = "Rust".as_bytes();
        /// assert_eq!(slice, &[82, 117, 115, 116]);
        ///
        /// // Or, just use a byte string, if you have control over the string
        /// // literal
        /// assert_eq!(b"Rust", &[82, 117, 115, 116]);
        /// ```
        ///
        /// Turning a `Vec<&T>` into a `Vec<Option<&T>>`:
        ///
        /// ```
        /// let store = [0, 1, 2, 3];
        /// let mut v_orig = store.iter().collect::<Vec<&i32>>();
        ///
        /// // Using transmute: this is Undefined Behavior, and a bad idea.
        /// // However, it is no-copy.
        /// let v_transmuted = unsafe {
        ///     std::mem::transmute::<Vec<&i32>, Vec<Option<&i32>>>(
        ///         v_orig.clone())
        /// };
        ///
        /// // This is the suggested, safe way.
        /// // It does copy the entire vector, though, into a new array.
        /// let v_collected = v_orig.clone()
        ///                         .into_iter()
        ///                         .map(|r| Some(r))
        ///                         .collect::<Vec<Option<&i32>>>();
        ///
        /// // The no-copy, unsafe way, still using transmute, but not UB.
        /// // This is equivalent to the original, but safer, and reuses the
        /// // same Vec internals. Therefore the new inner type must have the
        /// // exact same size, and the same or lesser alignment, as the old
        /// // type. The same caveats exist for this method as transmute, for
        /// // the original inner type (`&i32`) to the converted inner type
        /// // (`Option<&i32>`), so read the nomicon pages linked above.
        /// let v_from_raw = unsafe {
        ///     Vec::from_raw_parts(v_orig.as_mut_ptr(),
        ///                         v_orig.len(),
        ///                         v_orig.capacity())
        /// };
        /// std::mem::forget(v_orig);
        /// ```
        ///
        /// Implementing `split_at_mut`:
        ///
        /// ```
        /// use std::{slice, mem};
        ///
        /// // There are multiple ways to do this; and there are multiple problems
        /// // with the following, transmute, way.
        /// fn split_at_mut_transmute<T>(slice: &mut [T], mid: usize)
        ///                              -> (&mut [T], &mut [T]) {
        ///     let len = slice.len();
        ///     assert!(mid <= len);
        ///     unsafe {
        ///         let slice2 = mem::transmute::<&mut [T], &mut [T]>(slice);
        ///         // first: transmute is not typesafe; all it checks is that T and
        ///         // U are of the same size. Second, right here, you have two
        ///         // mutable references pointing to the same memory.
        ///         (&mut slice[0..mid], &mut slice2[mid..len])
        ///     }
        /// }
        ///
        /// // This gets rid of the typesafety problems; `&mut *` will *only* give
        /// // you an `&mut T` from an `&mut T` or `*mut T`.
        /// fn split_at_mut_casts<T>(slice: &mut [T], mid: usize)
        ///                          -> (&mut [T], &mut [T]) {
        ///     let len = slice.len();
        ///     assert!(mid <= len);
        ///     unsafe {
        ///         let slice2 = &mut *(slice as *mut [T]);
        ///         // however, you still have two mutable references pointing to
        ///         // the same memory.
        ///         (&mut slice[0..mid], &mut slice2[mid..len])
        ///     }
        /// }
        ///
        /// // This is how the standard library does it. This is the best method, if
        /// // you need to do something like this
        /// fn split_at_stdlib<T>(slice: &mut [T], mid: usize)
        ///                       -> (&mut [T], &mut [T]) {
        ///     let len = slice.len();
        ///     assert!(mid <= len);
        ///     unsafe {
        ///         let ptr = slice.as_mut_ptr();
        ///         // This now has three mutable references pointing at the same
        ///         // memory. `slice`, the rvalue ret.0, and the rvalue ret.1.
        ///         // `slice` is never used after `let ptr = ...`, and so one can
        ///         // treat it as "dead", and therefore, you only have two real
        ///         // mutable slices.
        ///         (slice::from_raw_parts_mut(ptr, mid),
        ///          slice::from_raw_parts_mut(ptr.offset(mid as isize), len - mid))
        ///     }
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn transmute<T, U>(e: T) -> U;

        /// Returns `true` if the actual type given as `T` requires drop
        /// glue; returns `false` if the actual type provided for `T`
        /// implements `Copy`.
        ///
        /// If the actual type neither requires drop glue nor implements
        /// `Copy`, then may return `true` or `false`.
        pub fn needs_drop<T>() -> bool;

        /// Calculates the offset from a pointer.
        ///
        /// This is implemented as an intrinsic to avoid converting to and from an
        /// integer, since the conversion would throw away aliasing information.
        ///
        /// # Safety
        ///
        /// Both the starting and resulting pointer must be either in bounds or one
        /// byte past the end of an allocated object. If either pointer is out of
        /// bounds or arithmetic overflow occurs then any further use of the
        /// returned value will result in undefined behavior.
        pub fn offset<T>(dst: *const T, offset: isize) -> *const T;

        /// Calculates the offset from a pointer, potentially wrapping.
        ///
        /// This is implemented as an intrinsic to avoid converting to and from an
        /// integer, since the conversion inhibits certain optimizations.
        ///
        /// # Safety
        ///
        /// Unlike the `offset` intrinsic, this intrinsic does not restrict the
        /// resulting pointer to point into or one byte past the end of an allocated
        /// object, and it wraps with two's complement arithmetic. The resulting
        /// value is not necessarily valid to be used to actually access memory.
        pub fn arith_offset<T>(dst: *const T, offset: isize) -> *const T;

        /// Copies `count * size_of<T>` bytes from `src` to `dst`. The source
        /// and destination may *not* overlap.
        ///
        /// `copy_nonoverlapping` is semantically equivalent to C's `memcpy`.
        ///
        /// # Safety
        ///
        /// Beyond requiring that the program must be allowed to access both regions
        /// of memory, it is Undefined Behavior for source and destination to
        /// overlap. Care must also be taken with the ownership of `src` and
        /// `dst`. This method semantically moves the values of `src` into `dst`.
        /// However it does not drop the contents of `dst`, or prevent the contents
        /// of `src` from being dropped or used.
        ///
        /// # Examples
        ///
        /// A safe swap function:
        ///
        /// ```
        /// use std::mem;
        /// use std::ptr;
        ///
        /// # #[allow(dead_code)]
        /// fn swap<T>(x: &mut T, y: &mut T) {
        ///     unsafe {
        ///         // Give ourselves some scratch space to work with
        ///         let mut t: T = mem::uninitialized();
        ///
        ///         // Perform the swap, `&mut` pointers never alias
        ///         ptr::copy_nonoverlapping(x, &mut t, 1);
        ///         ptr::copy_nonoverlapping(y, x, 1);
        ///         ptr::copy_nonoverlapping(&t, y, 1);
        ///
        ///         // y and t now point to the same thing, but we need to completely forget `tmp`
        ///         // because it's no longer relevant.
        ///         mem::forget(t);
        ///     }
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize);

        /// Copies `count * size_of<T>` bytes from `src` to `dst`. The source
        /// and destination may overlap.
        ///
        /// `copy` is semantically equivalent to C's `memmove`.
        ///
        /// # Safety
        ///
        /// Care must be taken with the ownership of `src` and `dst`.
        /// This method semantically moves the values of `src` into `dst`.
        /// However it does not drop the contents of `dst`, or prevent the contents of `src`
        /// from being dropped or used.
        ///
        /// # Examples
        ///
        /// Efficiently create a Rust vector from an unsafe buffer:
        ///
        /// ```
        /// use std::ptr;
        ///
        /// # #[allow(dead_code)]
        /// unsafe fn from_buf_raw<T>(ptr: *const T, elts: usize) -> Vec<T> {
        ///     let mut dst = Vec::with_capacity(elts);
        ///     dst.set_len(elts);
        ///     ptr::copy(ptr, dst.as_mut_ptr(), elts);
        ///     dst
        /// }
        /// ```
        ///
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn copy<T>(src: *const T, dst: *mut T, count: usize);

        /// Invokes memset on the specified pointer, setting `count * size_of::<T>()`
        /// bytes of memory starting at `dst` to `val`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::ptr;
        ///
        /// let mut vec = vec![0; 4];
        /// unsafe {
        ///     let vec_ptr = vec.as_mut_ptr();
        ///     ptr::write_bytes(vec_ptr, b'a', 2);
        /// }
        /// assert_eq!(vec, [b'a', b'a', 0, 0]);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn write_bytes<T>(dst: *mut T, val: u8, count: usize);

        /// Equivalent to the appropriate `llvm.memcpy.p0i8.0i8.*` intrinsic, with
        /// a size of `count` * `size_of::<T>()` and an alignment of
        /// `min_align_of::<T>()`
        ///
        /// The volatile parameter is set to `true`, so it will not be optimized out.
        pub fn volatile_copy_nonoverlapping_memory<T>(dst: *mut T, src: *const T,
                                                      count: usize);
        /// Equivalent to the appropriate `llvm.memmove.p0i8.0i8.*` intrinsic, with
        /// a size of `count` * `size_of::<T>()` and an alignment of
        /// `min_align_of::<T>()`
        ///
        /// The volatile parameter is set to `true`, so it will not be optimized out.
        pub fn volatile_copy_memory<T>(dst: *mut T, src: *const T, count: usize);
        /// Equivalent to the appropriate `llvm.memset.p0i8.*` intrinsic, with a
        /// size of `count` * `size_of::<T>()` and an alignment of
        /// `min_align_of::<T>()`.
        ///
        /// The volatile parameter is set to `true`, so it will not be optimized out.
        pub fn volatile_set_memory<T>(dst: *mut T, val: u8, count: usize);

        /// Perform a volatile load from the `src` pointer.
        pub fn volatile_load<T>(src: *const T) -> T;
        /// Perform a volatile store to the `dst` pointer.
        pub fn volatile_store<T>(dst: *mut T, val: T);

        /// Returns the square root of an `f32`
        pub fn sqrtf32(x: f32) -> f32;
        /// Returns the square root of an `f64`
        pub fn sqrtf64(x: f64) -> f64;

        /// Raises an `f32` to an integer power.
        pub fn powif32(a: f32, x: i32) -> f32;
        /// Raises an `f64` to an integer power.
        pub fn powif64(a: f64, x: i32) -> f64;

        /// Returns the sine of an `f32`.
        pub fn sinf32(x: f32) -> f32;
        /// Returns the sine of an `f64`.
        pub fn sinf64(x: f64) -> f64;

        /// Returns the cosine of an `f32`.
        pub fn cosf32(x: f32) -> f32;
        /// Returns the cosine of an `f64`.
        pub fn cosf64(x: f64) -> f64;

        /// Raises an `f32` to an `f32` power.
        pub fn powf32(a: f32, x: f32) -> f32;
        /// Raises an `f64` to an `f64` power.
        pub fn powf64(a: f64, x: f64) -> f64;

        /// Returns the exponential of an `f32`.
        pub fn expf32(x: f32) -> f32;
        /// Returns the exponential of an `f64`.
        pub fn expf64(x: f64) -> f64;

        /// Returns 2 raised to the power of an `f32`.
        pub fn exp2f32(x: f32) -> f32;
        /// Returns 2 raised to the power of an `f64`.
        pub fn exp2f64(x: f64) -> f64;

        /// Returns the natural logarithm of an `f32`.
        pub fn logf32(x: f32) -> f32;
        /// Returns the natural logarithm of an `f64`.
        pub fn logf64(x: f64) -> f64;

        /// Returns the base 10 logarithm of an `f32`.
        pub fn log10f32(x: f32) -> f32;
        /// Returns the base 10 logarithm of an `f64`.
        pub fn log10f64(x: f64) -> f64;

        /// Returns the base 2 logarithm of an `f32`.
        pub fn log2f32(x: f32) -> f32;
        /// Returns the base 2 logarithm of an `f64`.
        pub fn log2f64(x: f64) -> f64;

        /// Returns `a * b + c` for `f32` values.
        pub fn fmaf32(a: f32, b: f32, c: f32) -> f32;
        /// Returns `a * b + c` for `f64` values.
        pub fn fmaf64(a: f64, b: f64, c: f64) -> f64;

        /// Returns the absolute value of an `f32`.
        pub fn fabsf32(x: f32) -> f32;
        /// Returns the absolute value of an `f64`.
        pub fn fabsf64(x: f64) -> f64;

        /// Copies the sign from `y` to `x` for `f32` values.
        pub fn copysignf32(x: f32, y: f32) -> f32;
        /// Copies the sign from `y` to `x` for `f64` values.
        pub fn copysignf64(x: f64, y: f64) -> f64;

        /// Returns the largest integer less than or equal to an `f32`.
        pub fn floorf32(x: f32) -> f32;
        /// Returns the largest integer less than or equal to an `f64`.
        pub fn floorf64(x: f64) -> f64;

        /// Returns the smallest integer greater than or equal to an `f32`.
        pub fn ceilf32(x: f32) -> f32;
        /// Returns the smallest integer greater than or equal to an `f64`.
        pub fn ceilf64(x: f64) -> f64;

        /// Returns the integer part of an `f32`.
        pub fn truncf32(x: f32) -> f32;
        /// Returns the integer part of an `f64`.
        pub fn truncf64(x: f64) -> f64;

        /// Returns the nearest integer to an `f32`. May raise an inexact floating-point exception
        /// if the argument is not an integer.
        pub fn rintf32(x: f32) -> f32;
        /// Returns the nearest integer to an `f64`. May raise an inexact floating-point exception
        /// if the argument is not an integer.
        pub fn rintf64(x: f64) -> f64;

        /// Returns the nearest integer to an `f32`.
        pub fn nearbyintf32(x: f32) -> f32;
        /// Returns the nearest integer to an `f64`.
        pub fn nearbyintf64(x: f64) -> f64;

        /// Returns the nearest integer to an `f32`. Rounds half-way cases away from zero.
        pub fn roundf32(x: f32) -> f32;
        /// Returns the nearest integer to an `f64`. Rounds half-way cases away from zero.
        pub fn roundf64(x: f64) -> f64;

        /// Float addition that allows optimizations based on algebraic rules.
        /// May assume inputs are finite.
        pub fn fadd_fast<T>(a: T, b: T) -> T;

        /// Float subtraction that allows optimizations based on algebraic rules.
        /// May assume inputs are finite.
        pub fn fsub_fast<T>(a: T, b: T) -> T;

        /// Float multiplication that allows optimizations based on algebraic rules.
        /// May assume inputs are finite.
        pub fn fmul_fast<T>(a: T, b: T) -> T;

        /// Float division that allows optimizations based on algebraic rules.
        /// May assume inputs are finite.
        pub fn fdiv_fast<T>(a: T, b: T) -> T;

        /// Float remainder that allows optimizations based on algebraic rules.
        /// May assume inputs are finite.
        pub fn frem_fast<T>(a: T, b: T) -> T;

        /// Returns the number of bits set in an integer type `T`
        pub fn ctpop<T>(x: T) -> T;

        /// Returns the number of leading bits unset in an integer type `T`
        pub fn ctlz<T>(x: T) -> T;

        /// Returns the number of trailing bits unset in an integer type `T`
        pub fn cttz<T>(x: T) -> T;

        /// Reverses the bytes in an integer type `T`.
        pub fn bswap<T>(x: T) -> T;

        /// Performs checked integer addition.
        pub fn add_with_overflow<T>(x: T, y: T) -> (T, bool);

        /// Performs checked integer subtraction
        pub fn sub_with_overflow<T>(x: T, y: T) -> (T, bool);

        /// Performs checked integer multiplication
        pub fn mul_with_overflow<T>(x: T, y: T) -> (T, bool);

        /// Performs an unchecked division, resulting in undefined behavior
        /// where y = 0 or x = `T::min_value()` and y = -1
        pub fn unchecked_div<T>(x: T, y: T) -> T;
        /// Returns the remainder of an unchecked division, resulting in
        /// undefined behavior where y = 0 or x = `T::min_value()` and y = -1
        pub fn unchecked_rem<T>(x: T, y: T) -> T;

        /// Returns (a + b) mod 2^N, where N is the width of T in bits.
        pub fn overflowing_add<T>(a: T, b: T) -> T;
        /// Returns (a - b) mod 2^N, where N is the width of T in bits.
        pub fn overflowing_sub<T>(a: T, b: T) -> T;
        /// Returns (a * b) mod 2^N, where N is the width of T in bits.
        pub fn overflowing_mul<T>(a: T, b: T) -> T;

        /// Returns the value of the discriminant for the variant in 'v',
        /// cast to a `u64`; if `T` has no discriminant, returns 0.
        pub fn discriminant_value<T>(v: &T) -> u64;

        /// Rust's "try catch" construct which invokes the function pointer `f` with
        /// the data pointer `data`.
        ///
        /// The third pointer is a target-specific data pointer which is filled in
        /// with the specifics of the exception that occurred. For examples on Unix
        /// platforms this is a `*mut *mut T` which is filled in by the compiler and
        /// on MSVC it's `*mut [usize; 2]`. For more information see the compiler's
        /// source as well as std's catch implementation.
        pub fn try(f: fn(*mut u8), data: *mut u8, local_ptr: *mut u8) -> i32;
    }

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