1 //! The virtual memory representation of the MIR interpreter.
4 use std::convert::TryFrom;
6 use std::ops::{Deref, DerefMut, Range};
9 use rustc_ast::Mutability;
10 use rustc_data_structures::sorted_map::SortedMap;
11 use rustc_target::abi::{Align, HasDataLayout, Size};
14 read_target_uint, write_target_uint, AllocId, InterpError, InterpResult, Pointer,
15 ResourceExhaustionInfo, Scalar, ScalarMaybeUninit, UndefinedBehaviorInfo, UninitBytesAccess,
19 /// This type represents an Allocation in the Miri/CTFE core engine.
21 /// Its public API is rather low-level, working directly with allocation offsets and a custom error
22 /// type to account for the lack of an AllocId on this level. The Miri/CTFE core engine `memory`
23 /// module provides higher-level access.
24 #[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash, TyEncodable, TyDecodable)]
26 pub struct Allocation<Tag = (), Extra = ()> {
27 /// The actual bytes of the allocation.
28 /// Note that the bytes of a pointer represent the offset of the pointer.
30 /// Maps from byte addresses to extra data for each pointer.
31 /// Only the first byte of a pointer is inserted into the map; i.e.,
32 /// every entry in this map applies to `pointer_size` consecutive bytes starting
33 /// at the given offset.
34 relocations: Relocations<Tag>,
35 /// Denotes which part of this allocation is initialized.
37 /// The alignment of the allocation to detect unaligned reads.
38 /// (`Align` guarantees that this is a power of two.)
40 /// `true` if the allocation is mutable.
41 /// Also used by codegen to determine if a static should be put into mutable memory,
42 /// which happens for `static mut` and `static` with interior mutability.
43 pub mutability: Mutability,
44 /// Extra state for the machine.
48 /// We have our own error type that does not know about the `AllocId`; that information
49 /// is added when converting to `InterpError`.
52 /// Encountered a pointer where we needed raw bytes.
54 /// Using uninitialized data where it is not allowed.
55 InvalidUninitBytes(Option<UninitBytesAccess>),
57 pub type AllocResult<T = ()> = Result<T, AllocError>;
60 pub fn to_interp_error<'tcx>(self, alloc_id: AllocId) -> InterpError<'tcx> {
62 AllocError::ReadPointerAsBytes => {
63 InterpError::Unsupported(UnsupportedOpInfo::ReadPointerAsBytes)
65 AllocError::InvalidUninitBytes(info) => InterpError::UndefinedBehavior(
66 UndefinedBehaviorInfo::InvalidUninitBytes(info.map(|b| (alloc_id, b))),
72 /// The information that makes up a memory access: offset and size.
73 #[derive(Copy, Clone, Debug)]
74 pub struct AllocRange {
79 /// Free-starting constructor for less syntactic overhead.
81 pub fn alloc_range(start: Size, size: Size) -> AllocRange {
82 AllocRange { start, size }
87 pub fn end(self) -> Size {
88 self.start + self.size // This does overflow checking.
91 /// Returns the `subrange` within this range; panics if it is not a subrange.
93 pub fn subrange(self, subrange: AllocRange) -> AllocRange {
94 let sub_start = self.start + subrange.start;
95 let range = alloc_range(sub_start, subrange.size);
96 assert!(range.end() <= self.end(), "access outside the bounds for given AllocRange");
101 // The constructors are all without extra; the extra gets added by a machine hook later.
102 impl<Tag> Allocation<Tag> {
103 /// Creates an allocation initialized by the given bytes
104 pub fn from_bytes<'a>(
105 slice: impl Into<Cow<'a, [u8]>>,
107 mutability: Mutability,
109 let bytes = slice.into().into_owned();
110 let size = Size::from_bytes(bytes.len());
113 relocations: Relocations::new(),
114 init_mask: InitMask::new(size, true),
121 pub fn from_bytes_byte_aligned_immutable<'a>(slice: impl Into<Cow<'a, [u8]>>) -> Self {
122 Allocation::from_bytes(slice, Align::ONE, Mutability::Not)
125 /// Try to create an Allocation of `size` bytes, failing if there is not enough memory
126 /// available to the compiler to do so.
127 pub fn uninit(size: Size, align: Align) -> InterpResult<'static, Self> {
128 let mut bytes = Vec::new();
129 bytes.try_reserve(size.bytes_usize()).map_err(|_| {
130 InterpError::ResourceExhaustion(ResourceExhaustionInfo::MemoryExhausted)
132 bytes.resize(size.bytes_usize(), 0);
136 relocations: Relocations::new(),
137 init_mask: InitMask::new(size, false),
139 mutability: Mutability::Mut,
145 impl Allocation<()> {
146 /// Add Tag and Extra fields
147 pub fn with_tags_and_extra<T, E>(
149 mut tagger: impl FnMut(AllocId) -> T,
151 ) -> Allocation<T, E> {
154 relocations: Relocations::from_presorted(
157 // The allocations in the relocations (pointers stored *inside* this allocation)
158 // all get the base pointer tag.
159 .map(|&(offset, ((), alloc))| {
160 let tag = tagger(alloc);
161 (offset, (tag, alloc))
165 init_mask: self.init_mask,
167 mutability: self.mutability,
173 /// Raw accessors. Provide access to otherwise private bytes.
174 impl<Tag, Extra> Allocation<Tag, Extra> {
175 pub fn len(&self) -> usize {
179 pub fn size(&self) -> Size {
180 Size::from_bytes(self.len())
183 /// Looks at a slice which may describe uninitialized bytes or describe a relocation. This differs
184 /// from `get_bytes_with_uninit_and_ptr` in that it does no relocation checks (even on the
186 /// This must not be used for reads affecting the interpreter execution.
187 pub fn inspect_with_uninit_and_ptr_outside_interpreter(&self, range: Range<usize>) -> &[u8] {
191 /// Returns the mask indicating which bytes are initialized.
192 pub fn init_mask(&self) -> &InitMask {
196 /// Returns the relocation list.
197 pub fn relocations(&self) -> &Relocations<Tag> {
203 impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
204 /// The last argument controls whether we error out when there are uninitialized
205 /// or pointer bytes. You should never call this, call `get_bytes` or
206 /// `get_bytes_with_uninit_and_ptr` instead,
208 /// This function also guarantees that the resulting pointer will remain stable
209 /// even when new allocations are pushed to the `HashMap`. `copy_repeatedly` relies
212 /// It is the caller's responsibility to check bounds and alignment beforehand.
213 fn get_bytes_internal(
215 cx: &impl HasDataLayout,
217 check_init_and_ptr: bool,
218 ) -> AllocResult<&[u8]> {
219 if check_init_and_ptr {
220 self.check_init(range)?;
221 self.check_relocations(cx, range)?;
223 // We still don't want relocations on the *edges*.
224 self.check_relocation_edges(cx, range)?;
227 Ok(&self.bytes[range.start.bytes_usize()..range.end().bytes_usize()])
230 /// Checks that these bytes are initialized and not pointer bytes, and then return them
233 /// It is the caller's responsibility to check bounds and alignment beforehand.
234 /// Most likely, you want to use the `PlaceTy` and `OperandTy`-based methods
235 /// on `InterpCx` instead.
237 pub fn get_bytes(&self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult<&[u8]> {
238 self.get_bytes_internal(cx, range, true)
241 /// It is the caller's responsibility to handle uninitialized and pointer bytes.
242 /// However, this still checks that there are no relocations on the *edges*.
244 /// It is the caller's responsibility to check bounds and alignment beforehand.
246 pub fn get_bytes_with_uninit_and_ptr(
248 cx: &impl HasDataLayout,
250 ) -> AllocResult<&[u8]> {
251 self.get_bytes_internal(cx, range, false)
254 /// Just calling this already marks everything as defined and removes relocations,
255 /// so be sure to actually put data there!
257 /// It is the caller's responsibility to check bounds and alignment beforehand.
258 /// Most likely, you want to use the `PlaceTy` and `OperandTy`-based methods
259 /// on `InterpCx` instead.
260 pub fn get_bytes_mut(&mut self, cx: &impl HasDataLayout, range: AllocRange) -> &mut [u8] {
261 self.mark_init(range, true);
262 self.clear_relocations(cx, range);
264 &mut self.bytes[range.start.bytes_usize()..range.end().bytes_usize()]
267 /// A raw pointer variant of `get_bytes_mut` that avoids invalidating existing aliases into this memory.
268 pub fn get_bytes_mut_ptr(&mut self, cx: &impl HasDataLayout, range: AllocRange) -> *mut [u8] {
269 self.mark_init(range, true);
270 self.clear_relocations(cx, range);
272 assert!(range.end().bytes_usize() <= self.bytes.len()); // need to do our own bounds-check
273 let begin_ptr = self.bytes.as_mut_ptr().wrapping_add(range.start.bytes_usize());
274 let len = range.end().bytes_usize() - range.start.bytes_usize();
275 ptr::slice_from_raw_parts_mut(begin_ptr, len)
279 /// Reading and writing.
280 impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
281 /// Validates that `ptr.offset` and `ptr.offset + size` do not point to the middle of a
282 /// relocation. If `allow_uninit_and_ptr` is `false`, also enforces that the memory in the
283 /// given range contains neither relocations nor uninitialized bytes.
286 cx: &impl HasDataLayout,
288 allow_uninit_and_ptr: bool,
290 // Check bounds and relocations on the edges.
291 self.get_bytes_with_uninit_and_ptr(cx, range)?;
292 // Check uninit and ptr.
293 if !allow_uninit_and_ptr {
294 self.check_init(range)?;
295 self.check_relocations(cx, range)?;
300 /// Reads a *non-ZST* scalar.
302 /// ZSTs can't be read because in order to obtain a `Pointer`, we need to check
303 /// for ZSTness anyway due to integer pointers being valid for ZSTs.
305 /// It is the caller's responsibility to check bounds and alignment beforehand.
306 /// Most likely, you want to call `InterpCx::read_scalar` instead of this method.
309 cx: &impl HasDataLayout,
311 ) -> AllocResult<ScalarMaybeUninit<Tag>> {
312 // `get_bytes_unchecked` tests relocation edges.
313 let bytes = self.get_bytes_with_uninit_and_ptr(cx, range)?;
314 // Uninit check happens *after* we established that the alignment is correct.
315 // We must not return `Ok()` for unaligned pointers!
316 if self.is_init(range).is_err() {
317 // This inflates uninitialized bytes to the entire scalar, even if only a few
318 // bytes are uninitialized.
319 return Ok(ScalarMaybeUninit::Uninit);
321 // Now we do the actual reading.
322 let bits = read_target_uint(cx.data_layout().endian, bytes).unwrap();
323 // See if we got a pointer.
324 if range.size != cx.data_layout().pointer_size {
326 // *Now*, we better make sure that the inside is free of relocations too.
327 self.check_relocations(cx, range)?;
330 if let Some(&(tag, alloc_id)) = self.relocations.get(&range.start) {
331 let ptr = Pointer::new_with_tag(alloc_id, Size::from_bytes(bits), tag);
332 return Ok(ScalarMaybeUninit::Scalar(ptr.into()));
335 // We don't. Just return the bits.
336 Ok(ScalarMaybeUninit::Scalar(Scalar::from_uint(bits, range.size)))
339 /// Writes a *non-ZST* scalar.
341 /// ZSTs can't be read because in order to obtain a `Pointer`, we need to check
342 /// for ZSTness anyway due to integer pointers being valid for ZSTs.
344 /// It is the caller's responsibility to check bounds and alignment beforehand.
345 /// Most likely, you want to call `InterpCx::write_scalar` instead of this method.
348 cx: &impl HasDataLayout,
350 val: ScalarMaybeUninit<Tag>,
352 let val = match val {
353 ScalarMaybeUninit::Scalar(scalar) => scalar,
354 ScalarMaybeUninit::Uninit => {
355 self.mark_init(range, false);
360 let bytes = match val.to_bits_or_ptr(range.size, cx) {
361 Err(val) => u128::from(val.offset.bytes()),
365 let endian = cx.data_layout().endian;
366 let dst = self.get_bytes_mut(cx, range);
367 write_target_uint(endian, dst, bytes).unwrap();
369 // See if we have to also write a relocation.
370 if let Scalar::Ptr(val) = val {
371 self.relocations.insert(range.start, (val.tag, val.alloc_id));
379 impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
380 /// Returns all relocations overlapping with the given pointer-offset pair.
381 pub fn get_relocations(
383 cx: &impl HasDataLayout,
385 ) -> &[(Size, (Tag, AllocId))] {
386 // We have to go back `pointer_size - 1` bytes, as that one would still overlap with
387 // the beginning of this range.
388 let start = range.start.bytes().saturating_sub(cx.data_layout().pointer_size.bytes() - 1);
389 self.relocations.range(Size::from_bytes(start)..range.end())
392 /// Checks that there are no relocations overlapping with the given range.
394 fn check_relocations(&self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult {
395 if self.get_relocations(cx, range).is_empty() {
398 Err(AllocError::ReadPointerAsBytes)
402 /// Removes all relocations inside the given range.
403 /// If there are relocations overlapping with the edges, they
404 /// are removed as well *and* the bytes they cover are marked as
405 /// uninitialized. This is a somewhat odd "spooky action at a distance",
406 /// but it allows strictly more code to run than if we would just error
407 /// immediately in that case.
408 fn clear_relocations(&mut self, cx: &impl HasDataLayout, range: AllocRange) {
409 // Find the start and end of the given range and its outermost relocations.
410 let (first, last) = {
411 // Find all relocations overlapping the given range.
412 let relocations = self.get_relocations(cx, range);
413 if relocations.is_empty() {
418 relocations.first().unwrap().0,
419 relocations.last().unwrap().0 + cx.data_layout().pointer_size,
422 let start = range.start;
423 let end = range.end();
425 // Mark parts of the outermost relocations as uninitialized if they partially fall outside the
428 self.init_mask.set_range(first, start, false);
431 self.init_mask.set_range(end, last, false);
434 // Forget all the relocations.
435 self.relocations.remove_range(first..last);
438 /// Errors if there are relocations overlapping with the edges of the
439 /// given memory range.
441 fn check_relocation_edges(&self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult {
442 self.check_relocations(cx, alloc_range(range.start, Size::ZERO))?;
443 self.check_relocations(cx, alloc_range(range.end(), Size::ZERO))?;
448 /// Uninitialized bytes.
449 impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
450 /// Checks whether the given range is entirely initialized.
452 /// Returns `Ok(())` if it's initialized. Otherwise returns the range of byte
453 /// indexes of the first contiguous uninitialized access.
454 fn is_init(&self, range: AllocRange) -> Result<(), Range<Size>> {
455 self.init_mask.is_range_initialized(range.start, range.end()) // `Size` addition
458 /// Checks that a range of bytes is initialized. If not, returns the `InvalidUninitBytes`
459 /// error which will report the first range of bytes which is uninitialized.
460 fn check_init(&self, range: AllocRange) -> AllocResult {
461 self.is_init(range).or_else(|idx_range| {
462 Err(AllocError::InvalidUninitBytes(Some(UninitBytesAccess {
463 access_offset: range.start,
464 access_size: range.size,
465 uninit_offset: idx_range.start,
466 uninit_size: idx_range.end - idx_range.start, // `Size` subtraction
471 pub fn mark_init(&mut self, range: AllocRange, is_init: bool) {
472 if range.size.bytes() == 0 {
475 self.init_mask.set_range(range.start, range.end(), is_init);
479 /// Run-length encoding of the uninit mask.
480 /// Used to copy parts of a mask multiple times to another allocation.
481 pub struct InitMaskCompressed {
482 /// Whether the first range is initialized.
484 /// The lengths of ranges that are run-length encoded.
485 /// The initialization state of the ranges alternate starting with `initial`.
486 ranges: smallvec::SmallVec<[u64; 1]>,
489 impl InitMaskCompressed {
490 pub fn no_bytes_init(&self) -> bool {
491 // The `ranges` are run-length encoded and of alternating initialization state.
492 // So if `ranges.len() > 1` then the second block is an initialized range.
493 !self.initial && self.ranges.len() == 1
497 /// Transferring the initialization mask to other allocations.
498 impl<Tag, Extra> Allocation<Tag, Extra> {
499 /// Creates a run-length encoding of the initialization mask.
500 pub fn compress_uninit_range(&self, src: Pointer<Tag>, size: Size) -> InitMaskCompressed {
501 // Since we are copying `size` bytes from `src` to `dest + i * size` (`for i in 0..repeat`),
502 // a naive initialization mask copying algorithm would repeatedly have to read the initialization mask from
503 // the source and write it to the destination. Even if we optimized the memory accesses,
504 // we'd be doing all of this `repeat` times.
505 // Therefore we precompute a compressed version of the initialization mask of the source value and
506 // then write it back `repeat` times without computing any more information from the source.
508 // A precomputed cache for ranges of initialized / uninitialized bits
509 // 0000010010001110 will become
510 // `[5, 1, 2, 1, 3, 3, 1]`,
511 // where each element toggles the state.
513 let mut ranges = smallvec::SmallVec::<[u64; 1]>::new();
514 let initial = self.init_mask.get(src.offset);
516 let mut cur = initial;
518 for i in 1..size.bytes() {
519 // FIXME: optimize to bitshift the current uninitialized block's bits and read the top bit.
520 if self.init_mask.get(src.offset + Size::from_bytes(i)) == cur {
523 ranges.push(cur_len);
529 ranges.push(cur_len);
531 InitMaskCompressed { ranges, initial }
534 /// Applies multiple instances of the run-length encoding to the initialization mask.
535 pub fn mark_compressed_init_range(
537 defined: &InitMaskCompressed,
542 // An optimization where we can just overwrite an entire range of initialization
543 // bits if they are going to be uniformly `1` or `0`.
544 if defined.ranges.len() <= 1 {
545 self.init_mask.set_range_inbounds(
547 dest.offset + size * repeat, // `Size` operations
553 for mut j in 0..repeat {
555 j += dest.offset.bytes();
556 let mut cur = defined.initial;
557 for range in &defined.ranges {
560 self.init_mask.set_range_inbounds(
561 Size::from_bytes(old_j),
572 #[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, TyEncodable, TyDecodable)]
573 pub struct Relocations<Tag = (), Id = AllocId>(SortedMap<Size, (Tag, Id)>);
575 impl<Tag, Id> Relocations<Tag, Id> {
576 pub fn new() -> Self {
577 Relocations(SortedMap::new())
580 // The caller must guarantee that the given relocations are already sorted
581 // by address and contain no duplicates.
582 pub fn from_presorted(r: Vec<(Size, (Tag, Id))>) -> Self {
583 Relocations(SortedMap::from_presorted_elements(r))
587 impl<Tag> Deref for Relocations<Tag> {
588 type Target = SortedMap<Size, (Tag, AllocId)>;
590 fn deref(&self) -> &Self::Target {
595 impl<Tag> DerefMut for Relocations<Tag> {
596 fn deref_mut(&mut self) -> &mut Self::Target {
601 /// A partial, owned list of relocations to transfer into another allocation.
602 pub struct AllocationRelocations<Tag> {
603 relative_relocations: Vec<(Size, (Tag, AllocId))>,
606 impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
607 pub fn prepare_relocation_copy(
609 cx: &impl HasDataLayout,
613 ) -> AllocationRelocations<Tag> {
614 let relocations = self.get_relocations(cx, src);
615 if relocations.is_empty() {
616 return AllocationRelocations { relative_relocations: Vec::new() };
620 let mut new_relocations = Vec::with_capacity(relocations.len() * (count as usize));
623 new_relocations.extend(relocations.iter().map(|&(offset, reloc)| {
624 // compute offset for current repetition
625 let dest_offset = dest + size * i; // `Size` operations
627 // shift offsets from source allocation to destination allocation
628 (offset + dest_offset) - src.start, // `Size` operations
634 AllocationRelocations { relative_relocations: new_relocations }
637 /// Applies a relocation copy.
638 /// The affected range, as defined in the parameters to `prepare_relocation_copy` is expected
639 /// to be clear of relocations.
640 pub fn mark_relocation_range(&mut self, relocations: AllocationRelocations<Tag>) {
641 self.relocations.insert_presorted(relocations.relative_relocations);
645 ////////////////////////////////////////////////////////////////////////////////
646 // Uninitialized byte tracking
647 ////////////////////////////////////////////////////////////////////////////////
651 /// A bitmask where each bit refers to the byte with the same index. If the bit is `true`, the byte
652 /// is initialized. If it is `false` the byte is uninitialized.
653 #[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord, Hash, TyEncodable, TyDecodable)]
654 #[derive(HashStable)]
655 pub struct InitMask {
661 pub const BLOCK_SIZE: u64 = 64;
663 pub fn new(size: Size, state: bool) -> Self {
664 let mut m = InitMask { blocks: vec![], len: Size::ZERO };
669 /// Checks whether the range `start..end` (end-exclusive) is entirely initialized.
671 /// Returns `Ok(())` if it's initialized. Otherwise returns a range of byte
672 /// indexes for the first contiguous span of the uninitialized access.
674 pub fn is_range_initialized(&self, start: Size, end: Size) -> Result<(), Range<Size>> {
676 return Err(self.len..end);
679 // FIXME(oli-obk): optimize this for allocations larger than a block.
680 let idx = (start.bytes()..end.bytes()).map(Size::from_bytes).find(|&i| !self.get(i));
684 let uninit_end = (idx.bytes()..end.bytes())
685 .map(Size::from_bytes)
686 .find(|&i| self.get(i))
694 pub fn set_range(&mut self, start: Size, end: Size, new_state: bool) {
697 self.grow(end - len, new_state);
699 self.set_range_inbounds(start, end, new_state);
702 pub fn set_range_inbounds(&mut self, start: Size, end: Size, new_state: bool) {
703 let (blocka, bita) = bit_index(start);
704 let (blockb, bitb) = bit_index(end);
705 if blocka == blockb {
706 // First set all bits except the first `bita`,
707 // then unset the last `64 - bitb` bits.
708 let range = if bitb == 0 {
711 (u64::MAX << bita) & (u64::MAX >> (64 - bitb))
714 self.blocks[blocka] |= range;
716 self.blocks[blocka] &= !range;
720 // across block boundaries
722 // Set `bita..64` to `1`.
723 self.blocks[blocka] |= u64::MAX << bita;
724 // Set `0..bitb` to `1`.
726 self.blocks[blockb] |= u64::MAX >> (64 - bitb);
728 // Fill in all the other blocks (much faster than one bit at a time).
729 for block in (blocka + 1)..blockb {
730 self.blocks[block] = u64::MAX;
733 // Set `bita..64` to `0`.
734 self.blocks[blocka] &= !(u64::MAX << bita);
735 // Set `0..bitb` to `0`.
737 self.blocks[blockb] &= !(u64::MAX >> (64 - bitb));
739 // Fill in all the other blocks (much faster than one bit at a time).
740 for block in (blocka + 1)..blockb {
741 self.blocks[block] = 0;
747 pub fn get(&self, i: Size) -> bool {
748 let (block, bit) = bit_index(i);
749 (self.blocks[block] & (1 << bit)) != 0
753 pub fn set(&mut self, i: Size, new_state: bool) {
754 let (block, bit) = bit_index(i);
755 self.set_bit(block, bit, new_state);
759 fn set_bit(&mut self, block: usize, bit: usize, new_state: bool) {
761 self.blocks[block] |= 1 << bit;
763 self.blocks[block] &= !(1 << bit);
767 pub fn grow(&mut self, amount: Size, new_state: bool) {
768 if amount.bytes() == 0 {
771 let unused_trailing_bits =
772 u64::try_from(self.blocks.len()).unwrap() * Self::BLOCK_SIZE - self.len.bytes();
773 if amount.bytes() > unused_trailing_bits {
774 let additional_blocks = amount.bytes() / Self::BLOCK_SIZE + 1;
776 // FIXME(oli-obk): optimize this by repeating `new_state as Block`.
777 iter::repeat(0).take(usize::try_from(additional_blocks).unwrap()),
780 let start = self.len;
782 self.set_range_inbounds(start, start + amount, new_state); // `Size` operation
787 fn bit_index(bits: Size) -> (usize, usize) {
788 let bits = bits.bytes();
789 let a = bits / InitMask::BLOCK_SIZE;
790 let b = bits % InitMask::BLOCK_SIZE;
791 (usize::try_from(a).unwrap(), usize::try_from(b).unwrap())