1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188

use core::ptr::NonNull;

#[doc(inline)]
pub use alloc_crate::alloc::{alloc, alloc_zeroed, dealloc, handle_alloc_error, realloc};

use crate::stable::{assume, invalid_mut};

use super::{AllocError, Allocator, Layout};

/// The global memory allocator.
///
/// This type implements the [`Allocator`] trait by forwarding calls
/// to the allocator registered with the `#[global_allocator]` attribute
/// if there is one, or the `std` crate’s default.
///
/// Note: while this type is unstable, the functionality it provides can be
/// accessed through the [free functions in `alloc`](crate#functions).
#[derive(Copy, Clone, Default, Debug)]
pub struct Global;

impl Global {
    #[inline(always)]
    fn alloc_impl(&self, layout: Layout, zeroed: bool) -> Result<NonNull<[u8]>, AllocError> {
        match layout.size() {
            0 => Ok(unsafe {
                NonNull::new_unchecked(core::ptr::slice_from_raw_parts_mut(
                    invalid_mut(layout.align()),
                    0,
                ))
            }),
            // SAFETY: `layout` is non-zero in size,
            size => unsafe {
                let raw_ptr = if zeroed {
                    alloc_zeroed(layout)
                } else {
                    alloc(layout)
                };
                let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
                Ok(NonNull::new_unchecked(core::ptr::slice_from_raw_parts_mut(
                    ptr.as_ptr(),
                    size,
                )))
            },
        }
    }

    // SAFETY: Same as `Allocator::grow`
    #[inline(always)]
    unsafe fn grow_impl(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
        zeroed: bool,
    ) -> Result<NonNull<[u8]>, AllocError> {
        debug_assert!(
            new_layout.size() >= old_layout.size(),
            "`new_layout.size()` must be greater than or equal to `old_layout.size()`"
        );

        match old_layout.size() {
            0 => self.alloc_impl(new_layout, zeroed),

            // SAFETY: `new_size` is non-zero as `old_size` is greater than or equal to `new_size`
            // as required by safety conditions. Other conditions must be upheld by the caller
            old_size if old_layout.align() == new_layout.align() => unsafe {
                let new_size = new_layout.size();

                // `realloc` probably checks for `new_size >= old_layout.size()` or something similar.
                assume(new_size >= old_layout.size());

                let raw_ptr = realloc(ptr.as_ptr(), old_layout, new_size);
                let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
                if zeroed {
                    raw_ptr.add(old_size).write_bytes(0, new_size - old_size);
                }
                Ok(NonNull::new_unchecked(core::ptr::slice_from_raw_parts_mut(
                    ptr.as_ptr(),
                    new_size,
                )))
            },

            // SAFETY: because `new_layout.size()` must be greater than or equal to `old_size`,
            // both the old and new memory allocation are valid for reads and writes for `old_size`
            // bytes. Also, because the old allocation wasn't yet deallocated, it cannot overlap
            // `new_ptr`. Thus, the call to `copy_nonoverlapping` is safe. The safety contract
            // for `dealloc` must be upheld by the caller.
            old_size => unsafe {
                let new_ptr = self.alloc_impl(new_layout, zeroed)?;
                core::ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_ptr().cast(), old_size);
                self.deallocate(ptr, old_layout);
                Ok(new_ptr)
            },
        }
    }
}

unsafe impl Allocator for Global {
    #[inline(always)]
    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
        self.alloc_impl(layout, false)
    }

    #[inline(always)]
    fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
        self.alloc_impl(layout, true)
    }

    #[inline(always)]
    unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
        if layout.size() != 0 {
            // SAFETY: `layout` is non-zero in size,
            // other conditions must be upheld by the caller
            unsafe { dealloc(ptr.as_ptr(), layout) }
        }
    }

    #[inline(always)]
    unsafe fn grow(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        // SAFETY: all conditions must be upheld by the caller
        unsafe { self.grow_impl(ptr, old_layout, new_layout, false) }
    }

    #[inline(always)]
    unsafe fn grow_zeroed(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        // SAFETY: all conditions must be upheld by the caller
        unsafe { self.grow_impl(ptr, old_layout, new_layout, true) }
    }

    #[inline(always)]
    unsafe fn shrink(
        &self,
        ptr: NonNull<u8>,
        old_layout: Layout,
        new_layout: Layout,
    ) -> Result<NonNull<[u8]>, AllocError> {
        debug_assert!(
            new_layout.size() <= old_layout.size(),
            "`new_layout.size()` must be smaller than or equal to `old_layout.size()`"
        );

        match new_layout.size() {
            // SAFETY: conditions must be upheld by the caller
            0 => unsafe {
                self.deallocate(ptr, old_layout);
                Ok(NonNull::new_unchecked(core::ptr::slice_from_raw_parts_mut(
                    invalid_mut(new_layout.align()),
                    0,
                )))
            },

            // SAFETY: `new_size` is non-zero. Other conditions must be upheld by the caller
            new_size if old_layout.align() == new_layout.align() => unsafe {
                // `realloc` probably checks for `new_size <= old_layout.size()` or something similar.
                assume(new_size <= old_layout.size());

                let raw_ptr = realloc(ptr.as_ptr(), old_layout, new_size);
                let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
                Ok(NonNull::new_unchecked(core::ptr::slice_from_raw_parts_mut(
                    ptr.as_ptr(),
                    new_size,
                )))
            },

            // SAFETY: because `new_size` must be smaller than or equal to `old_layout.size()`,
            // both the old and new memory allocation are valid for reads and writes for `new_size`
            // bytes. Also, because the old allocation wasn't yet deallocated, it cannot overlap
            // `new_ptr`. Thus, the call to `copy_nonoverlapping` is safe. The safety contract
            // for `dealloc` must be upheld by the caller.
            new_size => unsafe {
                let new_ptr = self.allocate(new_layout)?;
                core::ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_ptr().cast(), new_size);
                self.deallocate(ptr, old_layout);
                Ok(new_ptr)
            },
        }
    }
}