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    use crate::ptr;
    use crate::sys_common::alloc::{MIN_ALIGN, realloc_fallback};
    use crate::alloc::{GlobalAlloc, Layout, System};

    #[stable(feature = "alloc_system_type", since = "1.28.0")]
    unsafe impl GlobalAlloc for System {
        #[inline]
        unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
            // jemalloc provides alignment less than MIN_ALIGN for small allocations.
            // So only rely on MIN_ALIGN if size >= align.
            // Also see <https://github.com/rust-lang/rust/issues/45955> and
            // <https://github.com/rust-lang/rust/issues/62251#issuecomment-507580914>.
            if layout.align() <= MIN_ALIGN && layout.align() <= layout.size() {
                libc::malloc(layout.size()) as *mut u8
            } else {
                #[cfg(target_os = "macos")]
                {
                    if layout.align() > (1 << 31) {
                        return ptr::null_mut()
                    }
                }
                aligned_malloc(&layout)
            }
        }

        #[inline]
        unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
            // See the comment above in `alloc` for why this check looks the way it does.
            if layout.align() <= MIN_ALIGN && layout.align() <= layout.size() {
                libc::calloc(layout.size(), 1) as *mut u8
            } else {
                let ptr = self.alloc(layout);
                if !ptr.is_null() {
                    ptr::write_bytes(ptr, 0, layout.size());
                }
                ptr
            }
        }

        #[inline]
        unsafe fn dealloc(&self, ptr: *mut u8, _layout: Layout) {
            libc::free(ptr as *mut libc::c_void)
        }

        #[inline]
        unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
            if layout.align() <= MIN_ALIGN && layout.align() <= new_size {
                libc::realloc(ptr as *mut libc::c_void, new_size) as *mut u8
            } else {
                realloc_fallback(self, ptr, layout, new_size)
            }
        }
    }

    #[cfg(any(target_os = "android",
              target_os = "hermit",
              target_os = "redox",
              target_os = "solaris"))]
    #[inline]
    unsafe fn aligned_malloc(layout: &Layout) -> *mut u8 {
        // On android we currently target API level 9 which unfortunately
        // doesn't have the `posix_memalign` API used below. Instead we use
        // `memalign`, but this unfortunately has the property on some systems
        // where the memory returned cannot be deallocated by `free`!
        //
        // Upon closer inspection, however, this appears to work just fine with
        // Android, so for this platform we should be fine to call `memalign`
        // (which is present in API level 9). Some helpful references could
        // possibly be chromium using memalign [1], attempts at documenting that
        // memalign + free is ok [2] [3], or the current source of chromium
        // which still uses memalign on android [4].
        //
        // [1]: https://codereview.chromium.org/10796020/
        // [2]: https://code.google.com/p/android/issues/detail?id=35391
        // [3]: https://bugs.chromium.org/p/chromium/issues/detail?id=138579
        // [4]: https://chromium.googlesource.com/chromium/src/base/+/master/
        //                                       /memory/aligned_memory.cc
        libc::memalign(layout.align(), layout.size()) as *mut u8
    }

    #[cfg(not(any(target_os = "android",
                  target_os = "hermit",
                  target_os = "redox",
                  target_os = "solaris")))]
    #[inline]
    unsafe fn aligned_malloc(layout: &Layout) -> *mut u8 {
        let mut out = ptr::null_mut();
        // posix_memalign requires that the alignment be a multiple of `sizeof(void*)`.
        // Since these are all powers of 2, we can just use max.
        let align = layout.align().max(crate::mem::size_of::<usize>());
        let ret = libc::posix_memalign(&mut out, align, layout.size());
        if ret != 0 {
            ptr::null_mut()
        } else {
            out as *mut u8
        }
    }

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