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
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
use super::{Pointer, Tag};
use crate::stable_hasher::{HashStable, StableHasher};
use std::fmt;
use std::hash::{Hash, Hasher};
use std::marker::PhantomData;
use std::mem::ManuallyDrop;
use std::num::NonZero;
use std::ops::{Deref, DerefMut};
use std::ptr::NonNull;

/// A [`Copy`] tagged pointer.
///
/// This is essentially `{ pointer: P, tag: T }` packed in a single pointer.
///
/// You should use this instead of the [`TaggedPtr`] type in all cases where
/// `P` implements [`Copy`].
///
/// If `COMPARE_PACKED` is true, then the pointers will be compared and hashed without
/// unpacking. Otherwise we don't implement [`PartialEq`], [`Eq`] and [`Hash`];
/// if you want that, wrap the [`CopyTaggedPtr`].
///
/// [`TaggedPtr`]: crate::tagged_ptr::TaggedPtr
pub struct CopyTaggedPtr<P, T, const COMPARE_PACKED: bool>
where
    P: Pointer,
    T: Tag,
{
    /// This is semantically a pair of `pointer: P` and `tag: T` fields,
    /// however we pack them in a single pointer, to save space.
    ///
    /// We pack the tag into the **most**-significant bits of the pointer to
    /// ease retrieval of the value. A left shift is a multiplication and
    /// those are embeddable in instruction encoding, for example:
    ///
    /// ```asm
    /// // (<https://godbolt.org/z/jqcYPWEr3>)
    /// example::shift_read3:
    ///     mov     eax, dword ptr [8*rdi]
    ///     ret
    ///
    /// example::mask_read3:
    ///     and     rdi, -8
    ///     mov     eax, dword ptr [rdi]
    ///     ret
    /// ```
    ///
    /// This is ASM outputted by rustc for reads of values behind tagged
    /// pointers for different approaches of tagging:
    /// - `shift_read3` uses `<< 3` (the tag is in the most-significant bits)
    /// - `mask_read3` uses `& !0b111` (the tag is in the least-significant bits)
    ///
    /// The shift approach thus produces less instructions and is likely faster
    /// (see <https://godbolt.org/z/Y913sMdWb>).
    ///
    /// Encoding diagram:
    /// ```text
    /// [ packed.addr                     ]
    /// [ tag ] [ pointer.addr >> T::BITS ] <-- usize::BITS - T::BITS bits
    ///    ^
    ///    |
    /// T::BITS bits
    /// ```
    ///
    /// The tag can be retrieved by `packed.addr() >> T::BITS` and the pointer
    /// can be retrieved by `packed.map_addr(|addr| addr << T::BITS)`.
    packed: NonNull<P::Target>,
    tag_ghost: PhantomData<T>,
}

// Note that even though `CopyTaggedPtr` is only really expected to work with
// `P: Copy`, can't add `P: Copy` bound, because `CopyTaggedPtr` is used in the
// `TaggedPtr`'s implementation.
impl<P, T, const CP: bool> CopyTaggedPtr<P, T, CP>
where
    P: Pointer,
    T: Tag,
{
    /// Tags `pointer` with `tag`.
    ///
    /// Note that this leaks `pointer`: it won't be dropped when
    /// `CopyTaggedPtr` is dropped. If you have a pointer with a significant
    /// drop, use [`TaggedPtr`] instead.
    ///
    /// [`TaggedPtr`]: crate::tagged_ptr::TaggedPtr
    #[inline]
    pub fn new(pointer: P, tag: T) -> Self {
        Self { packed: Self::pack(P::into_ptr(pointer), tag), tag_ghost: PhantomData }
    }

    /// Retrieves the pointer.
    #[inline]
    pub fn pointer(self) -> P
    where
        P: Copy,
    {
        // SAFETY: pointer_raw returns the original pointer
        //
        // Note that this isn't going to double-drop or anything because we have
        // P: Copy
        unsafe { P::from_ptr(self.pointer_raw()) }
    }

    /// Retrieves the tag.
    #[inline]
    pub fn tag(&self) -> T {
        // Unpack the tag, according to the `self.packed` encoding scheme
        let tag = self.packed.addr().get() >> Self::TAG_BIT_SHIFT;

        // Safety:
        // The shift retrieves the original value from `T::into_usize`,
        // satisfying `T::from_usize`'s preconditions.
        unsafe { T::from_usize(tag) }
    }

    /// Sets the tag to a new value.
    #[inline]
    pub fn set_tag(&mut self, tag: T) {
        self.packed = Self::pack(self.pointer_raw(), tag);
    }

    const TAG_BIT_SHIFT: u32 = usize::BITS - T::BITS;
    const ASSERTION: () = { assert!(T::BITS <= P::BITS) };

    /// Pack pointer `ptr` that comes from [`P::into_ptr`] with a `tag`,
    /// according to `self.packed` encoding scheme.
    ///
    /// [`P::into_ptr`]: Pointer::into_ptr
    #[inline]
    fn pack(ptr: NonNull<P::Target>, tag: T) -> NonNull<P::Target> {
        // Trigger assert!
        let () = Self::ASSERTION;

        let packed_tag = tag.into_usize() << Self::TAG_BIT_SHIFT;

        ptr.map_addr(|addr| {
            // Safety:
            // - The pointer is `NonNull` => it's address is `NonZero<usize>`
            // - `P::BITS` least significant bits are always zero (`Pointer` contract)
            // - `T::BITS <= P::BITS` (from `Self::ASSERTION`)
            //
            // Thus `addr >> T::BITS` is guaranteed to be non-zero.
            //
            // `{non_zero} | packed_tag` can't make the value zero.

            let packed = (addr.get() >> T::BITS) | packed_tag;
            unsafe { NonZero::new_unchecked(packed) }
        })
    }

    /// Retrieves the original raw pointer from `self.packed`.
    #[inline]
    pub(super) fn pointer_raw(&self) -> NonNull<P::Target> {
        self.packed.map_addr(|addr| unsafe { NonZero::new_unchecked(addr.get() << T::BITS) })
    }

    /// This provides a reference to the `P` pointer itself, rather than the
    /// `Deref::Target`. It is used for cases where we want to call methods
    /// that may be implement differently for the Pointer than the Pointee
    /// (e.g., `Rc::clone` vs cloning the inner value).
    pub(super) fn with_pointer_ref<R>(&self, f: impl FnOnce(&P) -> R) -> R {
        // Safety:
        // - `self.raw.pointer_raw()` is originally returned from `P::into_ptr`
        //   and as such is valid for `P::from_ptr`.
        //   - This also allows us to not care whatever `f` panics or not.
        // - Even though we create a copy of the pointer, we store it inside
        //   `ManuallyDrop` and only access it by-ref, so we don't double-drop.
        //
        // Semantically this is just `f(&self.pointer)` (where `self.pointer`
        // is non-packed original pointer).
        //
        // Note that even though `CopyTaggedPtr` is only really expected to
        // work with `P: Copy`, we have to assume `P: ?Copy`, because
        // `CopyTaggedPtr` is used in the `TaggedPtr`'s implementation.
        let ptr = unsafe { ManuallyDrop::new(P::from_ptr(self.pointer_raw())) };
        f(&ptr)
    }
}

impl<P, T, const CP: bool> Copy for CopyTaggedPtr<P, T, CP>
where
    P: Pointer + Copy,
    T: Tag,
{
}

impl<P, T, const CP: bool> Clone for CopyTaggedPtr<P, T, CP>
where
    P: Pointer + Copy,
    T: Tag,
{
    #[inline]
    fn clone(&self) -> Self {
        *self
    }
}

impl<P, T, const CP: bool> Deref for CopyTaggedPtr<P, T, CP>
where
    P: Pointer,
    T: Tag,
{
    type Target = P::Target;

    #[inline]
    fn deref(&self) -> &Self::Target {
        // Safety:
        // `pointer_raw` returns the original pointer from `P::into_ptr` which,
        // by the `Pointer`'s contract, must be valid.
        unsafe { self.pointer_raw().as_ref() }
    }
}

impl<P, T, const CP: bool> DerefMut for CopyTaggedPtr<P, T, CP>
where
    P: Pointer + DerefMut,
    T: Tag,
{
    #[inline]
    fn deref_mut(&mut self) -> &mut Self::Target {
        // Safety:
        // `pointer_raw` returns the original pointer from `P::into_ptr` which,
        // by the `Pointer`'s contract, must be valid for writes if
        // `P: DerefMut`.
        unsafe { self.pointer_raw().as_mut() }
    }
}

impl<P, T, const CP: bool> fmt::Debug for CopyTaggedPtr<P, T, CP>
where
    P: Pointer + fmt::Debug,
    T: Tag + fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.with_pointer_ref(|ptr| {
            f.debug_struct("CopyTaggedPtr").field("pointer", ptr).field("tag", &self.tag()).finish()
        })
    }
}

impl<P, T> PartialEq for CopyTaggedPtr<P, T, true>
where
    P: Pointer,
    T: Tag,
{
    #[inline]
    fn eq(&self, other: &Self) -> bool {
        self.packed == other.packed
    }
}

impl<P, T> Eq for CopyTaggedPtr<P, T, true>
where
    P: Pointer,
    T: Tag,
{
}

impl<P, T> Hash for CopyTaggedPtr<P, T, true>
where
    P: Pointer,
    T: Tag,
{
    #[inline]
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.packed.hash(state);
    }
}

impl<P, T, HCX, const CP: bool> HashStable<HCX> for CopyTaggedPtr<P, T, CP>
where
    P: Pointer + HashStable<HCX>,
    T: Tag + HashStable<HCX>,
{
    fn hash_stable(&self, hcx: &mut HCX, hasher: &mut StableHasher) {
        self.with_pointer_ref(|ptr| ptr.hash_stable(hcx, hasher));
        self.tag().hash_stable(hcx, hasher);
    }
}

// Safety:
// `CopyTaggedPtr<P, T, ..>` is semantically just `{ ptr: P, tag: T }`, as such
// it's ok to implement `Sync` as long as `P: Sync, T: Sync`
unsafe impl<P, T, const CP: bool> Sync for CopyTaggedPtr<P, T, CP>
where
    P: Sync + Pointer,
    T: Sync + Tag,
{
}

// Safety:
// `CopyTaggedPtr<P, T, ..>` is semantically just `{ ptr: P, tag: T }`, as such
// it's ok to implement `Send` as long as `P: Send, T: Send`
unsafe impl<P, T, const CP: bool> Send for CopyTaggedPtr<P, T, CP>
where
    P: Send + Pointer,
    T: Send + Tag,
{
}

/// Test that `new` does not compile if there is not enough alignment for the
/// tag in the pointer.
///
/// ```compile_fail,E0080
/// use rustc_data_structures::tagged_ptr::{CopyTaggedPtr, Tag};
///
/// #[derive(Copy, Clone, Debug, PartialEq, Eq)]
/// enum Tag2 { B00 = 0b00, B01 = 0b01, B10 = 0b10, B11 = 0b11 };
///
/// unsafe impl Tag for Tag2 {
///     const BITS: u32 = 2;
///
///     fn into_usize(self) -> usize { todo!() }
///     unsafe fn from_usize(tag: usize) -> Self { todo!() }
/// }
///
/// let value = 12u16;
/// let reference = &value;
/// let tag = Tag2::B01;
///
/// let _ptr = CopyTaggedPtr::<_, _, true>::new(reference, tag);
/// ```
// For some reason miri does not get the compile error
// probably it `check`s instead of `build`ing?
#[cfg(not(miri))]
const _: () = ();

#[cfg(test)]
mod tests;