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    // Copyright 2014-2015 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.

    #![unstable(feature = "heap_api",
                reason = "the precise API and guarantees it provides may be tweaked \
                          slightly, especially to possibly take into account the \
                          types being stored to make room for a future \
                          tracing garbage collector",
                issue = "27700")]

    use core::{isize, usize};
    use core::intrinsics::{min_align_of_val, size_of_val};

    #[allow(improper_ctypes)]
    extern "C" {
        #[allocator]
        fn __rust_allocate(size: usize, align: usize) -> *mut u8;
        fn __rust_allocate_zeroed(size: usize, align: usize) -> *mut u8;
        fn __rust_deallocate(ptr: *mut u8, old_size: usize, align: usize);
        fn __rust_reallocate(ptr: *mut u8, old_size: usize, size: usize, align: usize) -> *mut u8;
        fn __rust_reallocate_inplace(ptr: *mut u8,
                                     old_size: usize,
                                     size: usize,
                                     align: usize)
                                     -> usize;
        fn __rust_usable_size(size: usize, align: usize) -> usize;
    }

    #[inline(always)]
    fn check_size_and_alignment(size: usize, align: usize) {
        debug_assert!(size != 0);
        debug_assert!(size <= isize::MAX as usize,
                      "Tried to allocate too much: {} bytes",
                      size);
        debug_assert!(usize::is_power_of_two(align),
                      "Invalid alignment of allocation: {}",
                      align);
    }

    // FIXME: #13996: mark the `allocate` and `reallocate` return value as `noalias`

    /// Return a pointer to `size` bytes of memory aligned to `align`.
    ///
    /// On failure, return a null pointer.
    ///
    /// Behavior is undefined if the requested size is 0 or the alignment is not a
    /// power of 2. The alignment must be no larger than the largest supported page
    /// size on the platform.
    #[inline]
    pub unsafe fn allocate(size: usize, align: usize) -> *mut u8 {
        check_size_and_alignment(size, align);
        __rust_allocate(size, align)
    }

    /// Return a pointer to `size` bytes of memory aligned to `align` and
    /// initialized to zeroes.
    ///
    /// On failure, return a null pointer.
    ///
    /// Behavior is undefined if the requested size is 0 or the alignment is not a
    /// power of 2. The alignment must be no larger than the largest supported page
    /// size on the platform.
    #[inline]
    pub unsafe fn allocate_zeroed(size: usize, align: usize) -> *mut u8 {
        check_size_and_alignment(size, align);
        __rust_allocate_zeroed(size, align)
    }

    /// Resize the allocation referenced by `ptr` to `size` bytes.
    ///
    /// On failure, return a null pointer and leave the original allocation intact.
    ///
    /// If the allocation was relocated, the memory at the passed-in pointer is
    /// undefined after the call.
    ///
    /// Behavior is undefined if the requested size is 0 or the alignment is not a
    /// power of 2. The alignment must be no larger than the largest supported page
    /// size on the platform.
    ///
    /// The `old_size` and `align` parameters are the parameters that were used to
    /// create the allocation referenced by `ptr`. The `old_size` parameter may be
    /// any value in range_inclusive(requested_size, usable_size).
    #[inline]
    pub unsafe fn reallocate(ptr: *mut u8, old_size: usize, size: usize, align: usize) -> *mut u8 {
        check_size_and_alignment(size, align);
        __rust_reallocate(ptr, old_size, size, align)
    }

    /// Resize the allocation referenced by `ptr` to `size` bytes.
    ///
    /// If the operation succeeds, it returns `usable_size(size, align)` and if it
    /// fails (or is a no-op) it returns `usable_size(old_size, align)`.
    ///
    /// Behavior is undefined if the requested size is 0 or the alignment is not a
    /// power of 2. The alignment must be no larger than the largest supported page
    /// size on the platform.
    ///
    /// The `old_size` and `align` parameters are the parameters that were used to
    /// create the allocation referenced by `ptr`. The `old_size` parameter may be
    /// any value in range_inclusive(requested_size, usable_size).
    #[inline]
    pub unsafe fn reallocate_inplace(ptr: *mut u8,
                                     old_size: usize,
                                     size: usize,
                                     align: usize)
                                     -> usize {
        check_size_and_alignment(size, align);
        __rust_reallocate_inplace(ptr, old_size, size, align)
    }

    /// Deallocates the memory referenced by `ptr`.
    ///
    /// The `ptr` parameter must not be null.
    ///
    /// The `old_size` and `align` parameters are the parameters that were used to
    /// create the allocation referenced by `ptr`. The `old_size` parameter may be
    /// any value in range_inclusive(requested_size, usable_size).
    #[inline]
    pub unsafe fn deallocate(ptr: *mut u8, old_size: usize, align: usize) {
        __rust_deallocate(ptr, old_size, align)
    }

    /// Returns the usable size of an allocation created with the specified the
    /// `size` and `align`.
    #[inline]
    pub fn usable_size(size: usize, align: usize) -> usize {
        unsafe { __rust_usable_size(size, align) }
    }

    /// An arbitrary non-null address to represent zero-size allocations.
    ///
    /// This preserves the non-null invariant for types like `Box<T>`. The address
    /// may overlap with non-zero-size memory allocations.
    pub const EMPTY: *mut () = 0x1 as *mut ();

    /// The allocator for unique pointers.
    // This function must not unwind. If it does, MIR trans will fail.
    #[cfg(not(test))]
    #[lang = "exchange_malloc"]
    #[inline]
    unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
        if size == 0 {
            EMPTY as *mut u8
        } else {
            let ptr = allocate(size, align);
            if ptr.is_null() {
                ::oom()
            }
            ptr
        }
    }

    #[cfg_attr(not(test), lang = "box_free")]
    #[inline]
    pub(crate) unsafe fn box_free<T: ?Sized>(ptr: *mut T) {
        let size = size_of_val(&*ptr);
        let align = min_align_of_val(&*ptr);
        // We do not allocate for Box<T> when T is ZST, so deallocation is also not necessary.
        if size != 0 {
            deallocate(ptr as *mut u8, size, align);
        }
    }

    #[cfg(test)]
    mod tests {
        extern crate test;
        use self::test::Bencher;
        use boxed::Box;
        use heap;

        #[test]
        fn allocate_zeroed() {
            unsafe {
                let size = 1024;
                let ptr = heap::allocate_zeroed(size, 1);
                if ptr.is_null() {
                    ::oom()
                }

                let end = ptr.offset(size as isize);
                let mut i = ptr;
                while i < end {
                    assert_eq!(*i, 0);
                    i = i.offset(1);
                }
                heap::deallocate(ptr, size, 1);
            }
        }

        #[test]
        fn basic_reallocate_inplace_noop() {
            unsafe {
                let size = 4000;
                let ptr = heap::allocate(size, 8);
                if ptr.is_null() {
                    ::oom()
                }
                let ret = heap::reallocate_inplace(ptr, size, size, 8);
                heap::deallocate(ptr, size, 8);
                assert_eq!(ret, heap::usable_size(size, 8));
            }
        }

        #[bench]
        fn alloc_owned_small(b: &mut Bencher) {
            b.iter(|| {
                let _: Box<_> = box 10;
            })
        }
    }

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