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
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
//! The virtual memory representation of the MIR interpreter.

mod init_mask;
mod provenance_map;

use std::borrow::Cow;
use std::fmt;
use std::hash;
use std::hash::Hash;
use std::ops::{Deref, DerefMut, Range};
use std::ptr;

use either::{Left, Right};

use rustc_ast::Mutability;
use rustc_data_structures::intern::Interned;
use rustc_target::abi::{Align, HasDataLayout, Size};

use super::{
    read_target_uint, write_target_uint, AllocId, BadBytesAccess, CtfeProvenance, InterpError,
    InterpResult, Pointer, PointerArithmetic, Provenance, ResourceExhaustionInfo, Scalar,
    ScalarSizeMismatch, UndefinedBehaviorInfo, UnsupportedOpInfo,
};
use crate::ty;
use init_mask::*;
use provenance_map::*;

pub use init_mask::{InitChunk, InitChunkIter};

/// Functionality required for the bytes of an `Allocation`.
pub trait AllocBytes:
    Clone + fmt::Debug + Eq + PartialEq + Hash + Deref<Target = [u8]> + DerefMut<Target = [u8]>
{
    /// Create an `AllocBytes` from a slice of `u8`.
    fn from_bytes<'a>(slice: impl Into<Cow<'a, [u8]>>, _align: Align) -> Self;

    /// Create a zeroed `AllocBytes` of the specified size and alignment.
    /// Returns `None` if we ran out of memory on the host.
    fn zeroed(size: Size, _align: Align) -> Option<Self>;

    /// Gives direct access to the raw underlying storage.
    ///
    /// Crucially this pointer is compatible with:
    /// - other pointers retunred by this method, and
    /// - references returned from `deref()`, as long as there was no write.
    fn as_mut_ptr(&mut self) -> *mut u8;
}

/// Default `bytes` for `Allocation` is a `Box<u8>`.
impl AllocBytes for Box<[u8]> {
    fn from_bytes<'a>(slice: impl Into<Cow<'a, [u8]>>, _align: Align) -> Self {
        Box::<[u8]>::from(slice.into())
    }

    fn zeroed(size: Size, _align: Align) -> Option<Self> {
        let bytes = Box::<[u8]>::try_new_zeroed_slice(size.bytes_usize()).ok()?;
        // SAFETY: the box was zero-allocated, which is a valid initial value for Box<[u8]>
        let bytes = unsafe { bytes.assume_init() };
        Some(bytes)
    }

    fn as_mut_ptr(&mut self) -> *mut u8 {
        // Carefully avoiding any intermediate references.
        ptr::addr_of_mut!(**self).cast()
    }
}

/// This type represents an Allocation in the Miri/CTFE core engine.
///
/// Its public API is rather low-level, working directly with allocation offsets and a custom error
/// type to account for the lack of an AllocId on this level. The Miri/CTFE core engine `memory`
/// module provides higher-level access.
// Note: for performance reasons when interning, some of the `Allocation` fields can be partially
// hashed. (see the `Hash` impl below for more details), so the impl is not derived.
#[derive(Clone, Eq, PartialEq, TyEncodable, TyDecodable)]
#[derive(HashStable)]
pub struct Allocation<Prov: Provenance = CtfeProvenance, Extra = (), Bytes = Box<[u8]>> {
    /// The actual bytes of the allocation.
    /// Note that the bytes of a pointer represent the offset of the pointer.
    bytes: Bytes,
    /// Maps from byte addresses to extra provenance data for each pointer.
    /// Only the first byte of a pointer is inserted into the map; i.e.,
    /// every entry in this map applies to `pointer_size` consecutive bytes starting
    /// at the given offset.
    provenance: ProvenanceMap<Prov>,
    /// Denotes which part of this allocation is initialized.
    init_mask: InitMask,
    /// The alignment of the allocation to detect unaligned reads.
    /// (`Align` guarantees that this is a power of two.)
    pub align: Align,
    /// `true` if the allocation is mutable.
    /// Also used by codegen to determine if a static should be put into mutable memory,
    /// which happens for `static mut` and `static` with interior mutability.
    pub mutability: Mutability,
    /// Extra state for the machine.
    pub extra: Extra,
}

/// This is the maximum size we will hash at a time, when interning an `Allocation` and its
/// `InitMask`. Note, we hash that amount of bytes twice: at the start, and at the end of a buffer.
/// Used when these two structures are large: we only partially hash the larger fields in that
/// situation. See the comment at the top of their respective `Hash` impl for more details.
const MAX_BYTES_TO_HASH: usize = 64;

/// This is the maximum size (in bytes) for which a buffer will be fully hashed, when interning.
/// Otherwise, it will be partially hashed in 2 slices, requiring at least 2 `MAX_BYTES_TO_HASH`
/// bytes.
const MAX_HASHED_BUFFER_LEN: usize = 2 * MAX_BYTES_TO_HASH;

// Const allocations are only hashed for interning. However, they can be large, making the hashing
// expensive especially since it uses `FxHash`: it's better suited to short keys, not potentially
// big buffers like the actual bytes of allocation. We can partially hash some fields when they're
// large.
impl hash::Hash for Allocation {
    fn hash<H: hash::Hasher>(&self, state: &mut H) {
        let Self {
            bytes,
            provenance,
            init_mask,
            align,
            mutability,
            extra: (), // don't bother hashing ()
        } = self;

        // Partially hash the `bytes` buffer when it is large. To limit collisions with common
        // prefixes and suffixes, we hash the length and some slices of the buffer.
        let byte_count = bytes.len();
        if byte_count > MAX_HASHED_BUFFER_LEN {
            // Hash the buffer's length.
            byte_count.hash(state);

            // And its head and tail.
            bytes[..MAX_BYTES_TO_HASH].hash(state);
            bytes[byte_count - MAX_BYTES_TO_HASH..].hash(state);
        } else {
            bytes.hash(state);
        }

        // Hash the other fields as usual.
        provenance.hash(state);
        init_mask.hash(state);
        align.hash(state);
        mutability.hash(state);
    }
}

/// Interned types generally have an `Outer` type and an `Inner` type, where
/// `Outer` is a newtype around `Interned<Inner>`, and all the operations are
/// done on `Outer`, because all occurrences are interned. E.g. `Ty` is an
/// outer type and `TyKind` is its inner type.
///
/// Here things are different because only const allocations are interned. This
/// means that both the inner type (`Allocation`) and the outer type
/// (`ConstAllocation`) are used quite a bit.
#[derive(Copy, Clone, PartialEq, Eq, Hash, HashStable)]
#[rustc_pass_by_value]
pub struct ConstAllocation<'tcx>(pub Interned<'tcx, Allocation>);

impl<'tcx> fmt::Debug for ConstAllocation<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        // The debug representation of this is very verbose and basically useless,
        // so don't print it.
        write!(f, "ConstAllocation {{ .. }}")
    }
}

impl<'tcx> ConstAllocation<'tcx> {
    pub fn inner(self) -> &'tcx Allocation {
        self.0.0
    }
}

/// We have our own error type that does not know about the `AllocId`; that information
/// is added when converting to `InterpError`.
#[derive(Debug)]
pub enum AllocError {
    /// A scalar had the wrong size.
    ScalarSizeMismatch(ScalarSizeMismatch),
    /// Encountered a pointer where we needed raw bytes.
    ReadPointerAsInt(Option<BadBytesAccess>),
    /// Partially overwriting a pointer.
    OverwritePartialPointer(Size),
    /// Partially copying a pointer.
    ReadPartialPointer(Size),
    /// Using uninitialized data where it is not allowed.
    InvalidUninitBytes(Option<BadBytesAccess>),
}
pub type AllocResult<T = ()> = Result<T, AllocError>;

impl From<ScalarSizeMismatch> for AllocError {
    fn from(s: ScalarSizeMismatch) -> Self {
        AllocError::ScalarSizeMismatch(s)
    }
}

impl AllocError {
    pub fn to_interp_error<'tcx>(self, alloc_id: AllocId) -> InterpError<'tcx> {
        use AllocError::*;
        match self {
            ScalarSizeMismatch(s) => {
                InterpError::UndefinedBehavior(UndefinedBehaviorInfo::ScalarSizeMismatch(s))
            }
            ReadPointerAsInt(info) => InterpError::Unsupported(
                UnsupportedOpInfo::ReadPointerAsInt(info.map(|b| (alloc_id, b))),
            ),
            OverwritePartialPointer(offset) => InterpError::Unsupported(
                UnsupportedOpInfo::OverwritePartialPointer(Pointer::new(alloc_id, offset)),
            ),
            ReadPartialPointer(offset) => InterpError::Unsupported(
                UnsupportedOpInfo::ReadPartialPointer(Pointer::new(alloc_id, offset)),
            ),
            InvalidUninitBytes(info) => InterpError::UndefinedBehavior(
                UndefinedBehaviorInfo::InvalidUninitBytes(info.map(|b| (alloc_id, b))),
            ),
        }
    }
}

/// The information that makes up a memory access: offset and size.
#[derive(Copy, Clone)]
pub struct AllocRange {
    pub start: Size,
    pub size: Size,
}

impl fmt::Debug for AllocRange {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "[{:#x}..{:#x}]", self.start.bytes(), self.end().bytes())
    }
}

/// Free-starting constructor for less syntactic overhead.
#[inline(always)]
pub fn alloc_range(start: Size, size: Size) -> AllocRange {
    AllocRange { start, size }
}

impl From<Range<Size>> for AllocRange {
    #[inline]
    fn from(r: Range<Size>) -> Self {
        alloc_range(r.start, r.end - r.start) // `Size` subtraction (overflow-checked)
    }
}

impl From<Range<usize>> for AllocRange {
    #[inline]
    fn from(r: Range<usize>) -> Self {
        AllocRange::from(Size::from_bytes(r.start)..Size::from_bytes(r.end))
    }
}

impl AllocRange {
    #[inline(always)]
    pub fn end(self) -> Size {
        self.start + self.size // This does overflow checking.
    }

    /// Returns the `subrange` within this range; panics if it is not a subrange.
    #[inline]
    pub fn subrange(self, subrange: AllocRange) -> AllocRange {
        let sub_start = self.start + subrange.start;
        let range = alloc_range(sub_start, subrange.size);
        assert!(range.end() <= self.end(), "access outside the bounds for given AllocRange");
        range
    }
}

// The constructors are all without extra; the extra gets added by a machine hook later.
impl<Prov: Provenance, Bytes: AllocBytes> Allocation<Prov, (), Bytes> {
    /// Creates an allocation from an existing `Bytes` value - this is needed for miri FFI support
    pub fn from_raw_bytes(bytes: Bytes, align: Align, mutability: Mutability) -> Self {
        let size = Size::from_bytes(bytes.len());
        Self {
            bytes,
            provenance: ProvenanceMap::new(),
            init_mask: InitMask::new(size, true),
            align,
            mutability,
            extra: (),
        }
    }

    /// Creates an allocation initialized by the given bytes
    pub fn from_bytes<'a>(
        slice: impl Into<Cow<'a, [u8]>>,
        align: Align,
        mutability: Mutability,
    ) -> Self {
        let bytes = Bytes::from_bytes(slice, align);
        let size = Size::from_bytes(bytes.len());
        Self {
            bytes,
            provenance: ProvenanceMap::new(),
            init_mask: InitMask::new(size, true),
            align,
            mutability,
            extra: (),
        }
    }

    pub fn from_bytes_byte_aligned_immutable<'a>(slice: impl Into<Cow<'a, [u8]>>) -> Self {
        Allocation::from_bytes(slice, Align::ONE, Mutability::Not)
    }

    fn uninit_inner<R>(size: Size, align: Align, fail: impl FnOnce() -> R) -> Result<Self, R> {
        // We raise an error if we cannot create the allocation on the host.
        // This results in an error that can happen non-deterministically, since the memory
        // available to the compiler can change between runs. Normally queries are always
        // deterministic. However, we can be non-deterministic here because all uses of const
        // evaluation (including ConstProp!) will make compilation fail (via hard error
        // or ICE) upon encountering a `MemoryExhausted` error.
        let bytes = Bytes::zeroed(size, align).ok_or_else(fail)?;

        Ok(Allocation {
            bytes,
            provenance: ProvenanceMap::new(),
            init_mask: InitMask::new(size, false),
            align,
            mutability: Mutability::Mut,
            extra: (),
        })
    }

    /// Try to create an Allocation of `size` bytes, failing if there is not enough memory
    /// available to the compiler to do so.
    pub fn try_uninit<'tcx>(size: Size, align: Align) -> InterpResult<'tcx, Self> {
        Self::uninit_inner(size, align, || {
            ty::tls::with(|tcx| tcx.dcx().delayed_bug("exhausted memory during interpretation"));
            InterpError::ResourceExhaustion(ResourceExhaustionInfo::MemoryExhausted).into()
        })
    }

    /// Try to create an Allocation of `size` bytes, panics if there is not enough memory
    /// available to the compiler to do so.
    ///
    /// Example use case: To obtain an Allocation filled with specific data,
    /// first call this function and then call write_scalar to fill in the right data.
    pub fn uninit(size: Size, align: Align) -> Self {
        match Self::uninit_inner(size, align, || {
            panic!("Allocation::uninit called with panic_on_fail had allocation failure");
        }) {
            Ok(x) => x,
            Err(x) => x,
        }
    }
}

impl<Bytes: AllocBytes> Allocation<CtfeProvenance, (), Bytes> {
    /// Adjust allocation from the ones in `tcx` to a custom Machine instance
    /// with a different `Provenance` and `Extra` type.
    pub fn adjust_from_tcx<Prov: Provenance, Extra, Err>(
        self,
        cx: &impl HasDataLayout,
        extra: Extra,
        mut adjust_ptr: impl FnMut(Pointer<CtfeProvenance>) -> Result<Pointer<Prov>, Err>,
    ) -> Result<Allocation<Prov, Extra, Bytes>, Err> {
        let mut bytes = self.bytes;
        // Adjust provenance of pointers stored in this allocation.
        let mut new_provenance = Vec::with_capacity(self.provenance.ptrs().len());
        let ptr_size = cx.data_layout().pointer_size.bytes_usize();
        let endian = cx.data_layout().endian;
        for &(offset, alloc_id) in self.provenance.ptrs().iter() {
            let idx = offset.bytes_usize();
            let ptr_bytes = &mut bytes[idx..idx + ptr_size];
            let bits = read_target_uint(endian, ptr_bytes).unwrap();
            let (ptr_prov, ptr_offset) =
                adjust_ptr(Pointer::new(alloc_id, Size::from_bytes(bits)))?.into_parts();
            write_target_uint(endian, ptr_bytes, ptr_offset.bytes().into()).unwrap();
            new_provenance.push((offset, ptr_prov));
        }
        // Create allocation.
        Ok(Allocation {
            bytes,
            provenance: ProvenanceMap::from_presorted_ptrs(new_provenance),
            init_mask: self.init_mask,
            align: self.align,
            mutability: self.mutability,
            extra,
        })
    }
}

/// Raw accessors. Provide access to otherwise private bytes.
impl<Prov: Provenance, Extra, Bytes: AllocBytes> Allocation<Prov, Extra, Bytes> {
    pub fn len(&self) -> usize {
        self.bytes.len()
    }

    pub fn size(&self) -> Size {
        Size::from_bytes(self.len())
    }

    /// Looks at a slice which may contain uninitialized bytes or provenance. This differs
    /// from `get_bytes_with_uninit_and_ptr` in that it does no provenance checks (even on the
    /// edges) at all.
    /// This must not be used for reads affecting the interpreter execution.
    pub fn inspect_with_uninit_and_ptr_outside_interpreter(&self, range: Range<usize>) -> &[u8] {
        &self.bytes[range]
    }

    /// Returns the mask indicating which bytes are initialized.
    pub fn init_mask(&self) -> &InitMask {
        &self.init_mask
    }

    /// Returns the provenance map.
    pub fn provenance(&self) -> &ProvenanceMap<Prov> {
        &self.provenance
    }
}

/// Byte accessors.
impl<Prov: Provenance, Extra, Bytes: AllocBytes> Allocation<Prov, Extra, Bytes> {
    /// This is the entirely abstraction-violating way to just grab the raw bytes without
    /// caring about provenance or initialization.
    ///
    /// This function also guarantees that the resulting pointer will remain stable
    /// even when new allocations are pushed to the `HashMap`. `mem_copy_repeatedly` relies
    /// on that.
    #[inline]
    pub fn get_bytes_unchecked(&self, range: AllocRange) -> &[u8] {
        &self.bytes[range.start.bytes_usize()..range.end().bytes_usize()]
    }

    /// Checks that these bytes are initialized, and then strip provenance (if possible) and return
    /// them.
    ///
    /// It is the caller's responsibility to check bounds and alignment beforehand.
    /// Most likely, you want to use the `PlaceTy` and `OperandTy`-based methods
    /// on `InterpCx` instead.
    #[inline]
    pub fn get_bytes_strip_provenance(
        &self,
        cx: &impl HasDataLayout,
        range: AllocRange,
    ) -> AllocResult<&[u8]> {
        self.init_mask.is_range_initialized(range).map_err(|uninit_range| {
            AllocError::InvalidUninitBytes(Some(BadBytesAccess {
                access: range,
                bad: uninit_range,
            }))
        })?;
        if !Prov::OFFSET_IS_ADDR {
            if !self.provenance.range_empty(range, cx) {
                // Find the provenance.
                let (offset, _prov) = self
                    .provenance
                    .range_get_ptrs(range, cx)
                    .first()
                    .copied()
                    .expect("there must be provenance somewhere here");
                let start = offset.max(range.start); // the pointer might begin before `range`!
                let end = (offset + cx.pointer_size()).min(range.end()); // the pointer might end after `range`!
                return Err(AllocError::ReadPointerAsInt(Some(BadBytesAccess {
                    access: range,
                    bad: AllocRange::from(start..end),
                })));
            }
        }
        Ok(self.get_bytes_unchecked(range))
    }

    /// This is the entirely abstraction-violating way to just get mutable access to the raw bytes.
    /// Just calling this already marks everything as defined and removes provenance, so be sure to
    /// actually overwrite all the data there!
    ///
    /// It is the caller's responsibility to check bounds and alignment beforehand.
    /// Most likely, you want to use the `PlaceTy` and `OperandTy`-based methods
    /// on `InterpCx` instead.
    pub fn get_bytes_unchecked_for_overwrite(
        &mut self,
        cx: &impl HasDataLayout,
        range: AllocRange,
    ) -> AllocResult<&mut [u8]> {
        self.mark_init(range, true);
        self.provenance.clear(range, cx)?;

        Ok(&mut self.bytes[range.start.bytes_usize()..range.end().bytes_usize()])
    }

    /// A raw pointer variant of `get_bytes_unchecked_for_overwrite` that avoids invalidating existing immutable aliases
    /// into this memory.
    pub fn get_bytes_unchecked_for_overwrite_ptr(
        &mut self,
        cx: &impl HasDataLayout,
        range: AllocRange,
    ) -> AllocResult<*mut [u8]> {
        self.mark_init(range, true);
        self.provenance.clear(range, cx)?;

        assert!(range.end().bytes_usize() <= self.bytes.len()); // need to do our own bounds-check
        // Cruciall, we go via `AllocBytes::as_mut_ptr`, not `AllocBytes::deref_mut`.
        let begin_ptr = self.bytes.as_mut_ptr().wrapping_add(range.start.bytes_usize());
        let len = range.end().bytes_usize() - range.start.bytes_usize();
        Ok(ptr::slice_from_raw_parts_mut(begin_ptr, len))
    }

    /// This gives direct mutable access to the entire buffer, just exposing their internal state
    /// without reseting anything. Directly exposes `AllocBytes::as_mut_ptr`. Only works if
    /// `OFFSET_IS_ADDR` is true.
    pub fn get_bytes_unchecked_raw_mut(&mut self) -> *mut u8 {
        assert!(Prov::OFFSET_IS_ADDR);
        self.bytes.as_mut_ptr()
    }
}

/// Reading and writing.
impl<Prov: Provenance, Extra, Bytes: AllocBytes> Allocation<Prov, Extra, Bytes> {
    /// Sets the init bit for the given range.
    fn mark_init(&mut self, range: AllocRange, is_init: bool) {
        if range.size.bytes() == 0 {
            return;
        }
        assert!(self.mutability == Mutability::Mut);
        self.init_mask.set_range(range, is_init);
    }

    /// Reads a *non-ZST* scalar.
    ///
    /// If `read_provenance` is `true`, this will also read provenance; otherwise (if the machine
    /// supports that) provenance is entirely ignored.
    ///
    /// ZSTs can't be read because in order to obtain a `Pointer`, we need to check
    /// for ZSTness anyway due to integer pointers being valid for ZSTs.
    ///
    /// It is the caller's responsibility to check bounds and alignment beforehand.
    /// Most likely, you want to call `InterpCx::read_scalar` instead of this method.
    pub fn read_scalar(
        &self,
        cx: &impl HasDataLayout,
        range: AllocRange,
        read_provenance: bool,
    ) -> AllocResult<Scalar<Prov>> {
        // First and foremost, if anything is uninit, bail.
        if self.init_mask.is_range_initialized(range).is_err() {
            return Err(AllocError::InvalidUninitBytes(None));
        }

        // Get the integer part of the result. We HAVE TO check provenance before returning this!
        let bytes = self.get_bytes_unchecked(range);
        let bits = read_target_uint(cx.data_layout().endian, bytes).unwrap();

        if read_provenance {
            assert_eq!(range.size, cx.data_layout().pointer_size);

            // When reading data with provenance, the easy case is finding provenance exactly where we
            // are reading, then we can put data and provenance back together and return that.
            if let Some(prov) = self.provenance.get_ptr(range.start) {
                // Now we can return the bits, with their appropriate provenance.
                let ptr = Pointer::new(prov, Size::from_bytes(bits));
                return Ok(Scalar::from_pointer(ptr, cx));
            }

            // If we can work on pointers byte-wise, join the byte-wise provenances.
            if Prov::OFFSET_IS_ADDR {
                let mut prov = self.provenance.get(range.start, cx);
                for offset in Size::from_bytes(1)..range.size {
                    let this_prov = self.provenance.get(range.start + offset, cx);
                    prov = Prov::join(prov, this_prov);
                }
                // Now use this provenance.
                let ptr = Pointer::new(prov, Size::from_bytes(bits));
                return Ok(Scalar::from_maybe_pointer(ptr, cx));
            } else {
                // Without OFFSET_IS_ADDR, the only remaining case we can handle is total absence of
                // provenance.
                if self.provenance.range_empty(range, cx) {
                    return Ok(Scalar::from_uint(bits, range.size));
                }
                // Else we have mixed provenance, that doesn't work.
                return Err(AllocError::ReadPartialPointer(range.start));
            }
        } else {
            // We are *not* reading a pointer.
            // If we can just ignore provenance or there is none, that's easy.
            if Prov::OFFSET_IS_ADDR || self.provenance.range_empty(range, cx) {
                // We just strip provenance.
                return Ok(Scalar::from_uint(bits, range.size));
            }
            // There is some provenance and we don't have OFFSET_IS_ADDR. This doesn't work.
            return Err(AllocError::ReadPointerAsInt(None));
        }
    }

    /// Writes a *non-ZST* scalar.
    ///
    /// ZSTs can't be read because in order to obtain a `Pointer`, we need to check
    /// for ZSTness anyway due to integer pointers being valid for ZSTs.
    ///
    /// It is the caller's responsibility to check bounds and alignment beforehand.
    /// Most likely, you want to call `InterpCx::write_scalar` instead of this method.
    pub fn write_scalar(
        &mut self,
        cx: &impl HasDataLayout,
        range: AllocRange,
        val: Scalar<Prov>,
    ) -> AllocResult {
        assert!(self.mutability == Mutability::Mut);

        // `to_bits_or_ptr_internal` is the right method because we just want to store this data
        // as-is into memory. This also double-checks that `val.size()` matches `range.size`.
        let (bytes, provenance) = match val.to_bits_or_ptr_internal(range.size)? {
            Right(ptr) => {
                let (provenance, offset) = ptr.into_parts();
                (u128::from(offset.bytes()), Some(provenance))
            }
            Left(data) => (data, None),
        };

        let endian = cx.data_layout().endian;
        // Yes we do overwrite all the bytes in `dst`.
        let dst = self.get_bytes_unchecked_for_overwrite(cx, range)?;
        write_target_uint(endian, dst, bytes).unwrap();

        // See if we have to also store some provenance.
        if let Some(provenance) = provenance {
            assert_eq!(range.size, cx.data_layout().pointer_size);
            self.provenance.insert_ptr(range.start, provenance, cx);
        }

        Ok(())
    }

    /// Write "uninit" to the given memory range.
    pub fn write_uninit(&mut self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult {
        self.mark_init(range, false);
        self.provenance.clear(range, cx)?;
        return Ok(());
    }

    /// Applies a previously prepared provenance copy.
    /// The affected range, as defined in the parameters to `provenance().prepare_copy` is expected
    /// to be clear of provenance.
    ///
    /// This is dangerous to use as it can violate internal `Allocation` invariants!
    /// It only exists to support an efficient implementation of `mem_copy_repeatedly`.
    pub fn provenance_apply_copy(&mut self, copy: ProvenanceCopy<Prov>) {
        self.provenance.apply_copy(copy)
    }

    /// Applies a previously prepared copy of the init mask.
    ///
    /// This is dangerous to use as it can violate internal `Allocation` invariants!
    /// It only exists to support an efficient implementation of `mem_copy_repeatedly`.
    pub fn init_mask_apply_copy(&mut self, copy: InitCopy, range: AllocRange, repeat: u64) {
        self.init_mask.apply_copy(copy, range, repeat)
    }
}