1 #[cfg(feature = "stack-cache")]
4 use rustc_data_structures::fx::FxHashSet;
6 use crate::borrow_tracker::{
7 stacked_borrows::{Item, Permission},
10 use crate::ProvenanceExtra;
12 /// Exactly what cache size we should use is a difficult tradeoff. There will always be some
13 /// workload which has a `BorTag` working set which exceeds the size of the cache, and ends up
14 /// falling back to linear searches of the borrow stack very often.
15 /// The cost of making this value too large is that the loop in `Stack::insert` which ensures the
16 /// entries in the cache stay correct after an insert becomes expensive.
17 #[cfg(feature = "stack-cache")]
18 const CACHE_LEN: usize = 32;
20 /// Extra per-location state.
21 #[derive(Clone, Debug)]
23 /// Used *mostly* as a stack; never empty.
25 /// * Above a `SharedReadOnly` there can only be more `SharedReadOnly`.
26 /// * Except for `Untagged`, no tag occurs in the stack more than once.
28 /// If this is `Some(id)`, then the actual current stack is unknown. This can happen when
29 /// wildcard pointers are used to access this location. What we do know is that `borrows` are at
30 /// the top of the stack, and below it are arbitrarily many items whose `tag` is strictly less
32 /// When the bottom is unknown, `borrows` always has a `SharedReadOnly` or `Unique` at the bottom;
33 /// we never have the unknown-to-known boundary in an SRW group.
34 unknown_bottom: Option<BorTag>,
36 /// A small LRU cache of searches of the borrow stack.
37 #[cfg(feature = "stack-cache")]
39 /// On a read, we need to disable all `Unique` above the granting item. We can avoid most of
40 /// this scan by keeping track of the region of the borrow stack that may contain `Unique`s.
41 #[cfg(feature = "stack-cache")]
42 unique_range: Range<usize>,
46 pub fn retain(&mut self, tags: &FxHashSet<BorTag>) {
47 let mut first_removed = None;
49 // We never consider removing the bottom-most tag. For stacks without an unknown
50 // bottom this preserves the base tag.
51 // Note that the algorithm below is based on considering the tag at read_idx - 1,
52 // so precisely considering the tag at index 0 for removal when we have an unknown
53 // bottom would complicate the implementation. The simplification of not considering
54 // it does not have a significant impact on the degree to which the GC mititages
57 let mut write_idx = read_idx;
58 while read_idx < self.borrows.len() {
59 let left = self.borrows[read_idx - 1];
60 let this = self.borrows[read_idx];
61 let should_keep = match this.perm() {
62 // SharedReadWrite is the simplest case, if it's unreachable we can just remove it.
63 Permission::SharedReadWrite => tags.contains(&this.tag()),
64 // Only retain a Disabled tag if it is terminating a SharedReadWrite block.
65 Permission::Disabled => left.perm() == Permission::SharedReadWrite,
66 // Unique and SharedReadOnly can terminate a SharedReadWrite block, so only remove
67 // them if they are both unreachable and not directly after a SharedReadWrite.
68 Permission::Unique | Permission::SharedReadOnly =>
69 left.perm() == Permission::SharedReadWrite || tags.contains(&this.tag()),
73 if read_idx != write_idx {
74 self.borrows[write_idx] = self.borrows[read_idx];
77 } else if first_removed.is_none() {
78 first_removed = Some(read_idx);
83 self.borrows.truncate(write_idx);
85 #[cfg(not(feature = "stack-cache"))]
86 drop(first_removed); // This is only needed for the stack-cache
88 #[cfg(feature = "stack-cache")]
89 if let Some(first_removed) = first_removed {
90 // Either end of unique_range may have shifted, all we really know is that we can't
91 // have introduced a new Unique.
92 if !self.unique_range.is_empty() {
93 self.unique_range = 0..self.len();
96 // Replace any Items which have been collected with the base item, a known-good value.
97 for i in 0..CACHE_LEN {
98 if self.cache.idx[i] >= first_removed {
99 self.cache.items[i] = self.borrows[0];
100 self.cache.idx[i] = 0;
107 /// A very small cache of searches of a borrow stack, mapping `Item`s to their position in said stack.
109 /// It may seem like maintaining this cache is a waste for small stacks, but
110 /// (a) iterating over small fixed-size arrays is super fast, and (b) empirically this helps *a lot*,
111 /// probably because runtime is dominated by large stacks.
112 #[cfg(feature = "stack-cache")]
113 #[derive(Clone, Debug)]
115 items: [Item; CACHE_LEN], // Hot in find_granting
116 idx: [usize; CACHE_LEN], // Hot in grant
119 #[cfg(feature = "stack-cache")]
121 /// When a tag is used, we call this function to add or refresh it in the cache.
123 /// We use the position in the cache to represent how recently a tag was used; the first position
124 /// is the most recently used tag. So an add shifts every element towards the end, and inserts
125 /// the new element at the start. We lose the last element.
126 /// This strategy is effective at keeping the most-accessed items in the cache, but it costs a
127 /// linear shift across the entire cache when we add a new tag.
128 fn add(&mut self, idx: usize, item: Item) {
129 self.items.copy_within(0..CACHE_LEN - 1, 1);
130 self.items[0] = item;
131 self.idx.copy_within(0..CACHE_LEN - 1, 1);
136 impl PartialEq for Stack {
137 fn eq(&self, other: &Self) -> bool {
138 // All the semantics of Stack are in self.borrows, everything else is caching
139 self.borrows == other.borrows
146 /// Panics if any of the caching mechanisms have broken,
147 /// - The StackCache indices don't refer to the parallel items,
148 /// - There are no Unique items outside of first_unique..last_unique
149 #[cfg(all(feature = "stack-cache", debug_assertions))]
150 fn verify_cache_consistency(&self) {
151 // Only a full cache needs to be valid. Also see the comments in find_granting_cache
152 // and set_unknown_bottom.
153 if self.borrows.len() >= CACHE_LEN {
154 for (tag, stack_idx) in self.cache.items.iter().zip(self.cache.idx.iter()) {
155 assert_eq!(self.borrows[*stack_idx], *tag);
159 // Check that all Unique items fall within unique_range.
160 for (idx, item) in self.borrows.iter().enumerate() {
161 if item.perm() == Permission::Unique {
163 self.unique_range.contains(&idx),
171 // Check that the unique_range is a valid index into the borrow stack.
172 // This asserts that the unique_range's start <= end.
173 let _uniques = &self.borrows[self.unique_range.clone()];
175 // We cannot assert that the unique range is precise.
176 // Both ends may shift around when `Stack::retain` is called. Additionally,
177 // when we pop items within the unique range, setting the end of the range precisely
178 // requires doing a linear search of the borrow stack, which is exactly the kind of
179 // operation that all this caching exists to avoid.
182 /// Find the item granting the given kind of access to the given tag, and return where
183 /// it is on the stack. For wildcard tags, the given index is approximate, but if *no*
184 /// index is given it means the match was *not* in the known part of the stack.
185 /// `Ok(None)` indicates it matched the "unknown" part of the stack.
186 /// `Err` indicates it was not found.
187 pub(super) fn find_granting(
190 tag: ProvenanceExtra,
191 exposed_tags: &FxHashSet<BorTag>,
192 ) -> Result<Option<usize>, ()> {
193 #[cfg(all(feature = "stack-cache", debug_assertions))]
194 self.verify_cache_consistency();
196 let ProvenanceExtra::Concrete(tag) = tag else {
197 // Handle the wildcard case.
198 // Go search the stack for an exposed tag.
202 .enumerate() // we also need to know *where* in the stack
203 .rev() // search top-to-bottom
204 .find_map(|(idx, item)| {
205 // If the item fits and *might* be this wildcard, use it.
206 if item.perm().grants(access) && exposed_tags.contains(&item.tag()) {
213 return Ok(Some(idx));
215 // If we couldn't find it in the stack, check the unknown bottom.
216 return if self.unknown_bottom.is_some() { Ok(None) } else { Err(()) };
219 if let Some(idx) = self.find_granting_tagged(access, tag) {
220 return Ok(Some(idx));
223 // Couldn't find it in the stack; but if there is an unknown bottom it might be there.
224 let found = self.unknown_bottom.is_some_and(|unknown_limit| {
225 tag < unknown_limit // unknown_limit is an upper bound for what can be in the unknown bottom.
227 if found { Ok(None) } else { Err(()) }
230 fn find_granting_tagged(&mut self, access: AccessKind, tag: BorTag) -> Option<usize> {
231 #[cfg(feature = "stack-cache")]
232 if let Some(idx) = self.find_granting_cache(access, tag) {
236 // If we didn't find the tag in the cache, fall back to a linear search of the
237 // whole stack, and add the tag to the cache.
238 for (stack_idx, item) in self.borrows.iter().enumerate().rev() {
239 if tag == item.tag() && item.perm().grants(access) {
240 #[cfg(feature = "stack-cache")]
241 self.cache.add(stack_idx, *item);
242 return Some(stack_idx);
248 #[cfg(feature = "stack-cache")]
249 fn find_granting_cache(&mut self, access: AccessKind, tag: BorTag) -> Option<usize> {
250 // This looks like a common-sense optimization; we're going to do a linear search of the
251 // cache or the borrow stack to scan the shorter of the two. This optimization is miniscule
252 // and this check actually ensures we do not access an invalid cache.
253 // When a stack is created and when items are removed from the top of the borrow stack, we
254 // need some valid value to populate the cache. In both cases, we try to use the bottom
255 // item. But when the stack is cleared in `set_unknown_bottom` there is nothing we could
256 // place in the cache that is correct. But due to the way we populate the cache in
257 // `StackCache::add`, we know that when the borrow stack has grown larger than the cache,
258 // every slot in the cache is valid.
259 if self.borrows.len() <= CACHE_LEN {
262 // Search the cache for the tag we're looking up
263 let cache_idx = self.cache.items.iter().position(|t| t.tag() == tag)?;
264 let stack_idx = self.cache.idx[cache_idx];
265 // If we found the tag, look up its position in the stack to see if it grants
266 // the required permission
267 if self.cache.items[cache_idx].perm().grants(access) {
268 // If it does, and it's not already in the most-recently-used position, re-insert it at
269 // the most-recently-used position. This technically reduces the efficiency of the
270 // cache by duplicating elements, but current benchmarks do not seem to benefit from
271 // avoiding this duplication.
272 // But if the tag is in position 1, avoiding the duplicating add is trivial.
273 // If it does, and it's not already in the most-recently-used position, move it there.
274 // Except if the tag is in position 1, this is equivalent to just a swap, so do that.
276 self.cache.items.swap(0, 1);
277 self.cache.idx.swap(0, 1);
278 } else if cache_idx > 1 {
279 self.cache.add(stack_idx, self.cache.items[cache_idx]);
283 // Tag is in the cache, but it doesn't grant the required permission
288 pub fn insert(&mut self, new_idx: usize, new: Item) {
289 self.borrows.insert(new_idx, new);
291 #[cfg(feature = "stack-cache")]
292 self.insert_cache(new_idx, new);
295 #[cfg(feature = "stack-cache")]
296 fn insert_cache(&mut self, new_idx: usize, new: Item) {
297 // Adjust the possibly-unique range if an insert occurs before or within it
298 if self.unique_range.start >= new_idx {
299 self.unique_range.start += 1;
301 if self.unique_range.end >= new_idx {
302 self.unique_range.end += 1;
304 if new.perm() == Permission::Unique {
305 // If this is the only Unique, set the range to contain just the new item.
306 if self.unique_range.is_empty() {
307 self.unique_range = new_idx..new_idx + 1;
309 // We already have other Unique items, expand the range to include the new item
310 self.unique_range.start = self.unique_range.start.min(new_idx);
311 self.unique_range.end = self.unique_range.end.max(new_idx + 1);
315 // The above insert changes the meaning of every index in the cache >= new_idx, so now
316 // we need to find every one of those indexes and increment it.
317 // But if the insert is at the end (equivalent to a push), we can skip this step because
318 // it didn't change the position of any other items.
319 if new_idx != self.borrows.len() - 1 {
320 for idx in &mut self.cache.idx {
327 // This primes the cache for the next access, which is almost always the just-added tag.
328 self.cache.add(new_idx, new);
330 #[cfg(debug_assertions)]
331 self.verify_cache_consistency();
334 /// Construct a new `Stack` using the passed `Item` as the base tag.
335 pub fn new(item: Item) -> Self {
338 unknown_bottom: None,
339 #[cfg(feature = "stack-cache")]
340 cache: StackCache { idx: [0; CACHE_LEN], items: [item; CACHE_LEN] },
341 #[cfg(feature = "stack-cache")]
342 unique_range: if item.perm() == Permission::Unique { 0..1 } else { 0..0 },
346 pub fn get(&self, idx: usize) -> Option<Item> {
347 self.borrows.get(idx).cloned()
350 #[allow(clippy::len_without_is_empty)] // Stacks are never empty
351 pub fn len(&self) -> usize {
355 pub fn unknown_bottom(&self) -> Option<BorTag> {
359 pub fn set_unknown_bottom(&mut self, tag: BorTag) {
360 // We clear the borrow stack but the lookup cache doesn't support clearing per se. Instead,
361 // there is a check explained in `find_granting_cache` which protects against accessing the
362 // cache when it has been cleared and not yet refilled.
363 self.borrows.clear();
364 self.unknown_bottom = Some(tag);
365 #[cfg(feature = "stack-cache")]
367 self.unique_range = 0..0;
371 /// Find all `Unique` elements in this borrow stack above `granting_idx`, pass a copy of them
372 /// to the `visitor`, then set their `Permission` to `Disabled`.
373 pub fn disable_uniques_starting_at(
375 disable_start: usize,
376 mut visitor: impl FnMut(Item) -> crate::InterpResult<'tcx>,
377 ) -> crate::InterpResult<'tcx> {
378 #[cfg(feature = "stack-cache")]
379 let unique_range = self.unique_range.clone();
380 #[cfg(not(feature = "stack-cache"))]
381 let unique_range = 0..self.len();
383 if disable_start <= unique_range.end {
384 let lower = unique_range.start.max(disable_start);
385 let upper = unique_range.end;
386 for item in &mut self.borrows[lower..upper] {
387 if item.perm() == Permission::Unique {
388 log::trace!("access: disabling item {:?}", item);
390 item.set_permission(Permission::Disabled);
391 // Also update all copies of this item in the cache.
392 #[cfg(feature = "stack-cache")]
393 for it in &mut self.cache.items {
394 if it.tag() == item.tag() {
395 it.set_permission(Permission::Disabled);
402 #[cfg(feature = "stack-cache")]
403 if disable_start <= self.unique_range.start {
404 // We disabled all Unique items
405 self.unique_range.start = 0;
406 self.unique_range.end = 0;
408 // Truncate the range to only include items up to the index that we started disabling
410 self.unique_range.end = self.unique_range.end.min(disable_start);
413 #[cfg(all(feature = "stack-cache", debug_assertions))]
414 self.verify_cache_consistency();
419 /// Produces an iterator which iterates over `range` in reverse, and when dropped removes that
420 /// range of `Item`s from this `Stack`.
421 pub fn pop_items_after<V: FnMut(Item) -> crate::InterpResult<'tcx>>(
425 ) -> crate::InterpResult<'tcx> {
426 while self.borrows.len() > start {
427 let item = self.borrows.pop().unwrap();
431 #[cfg(feature = "stack-cache")]
432 if !self.borrows.is_empty() {
433 // After we remove from the borrow stack, every aspect of our caching may be invalid, but it is
434 // also possible that the whole cache is still valid. So we call this method to repair what
435 // aspects of the cache are now invalid, instead of resetting the whole thing to a trivially
436 // valid default state.
437 let base_tag = self.borrows[0];
440 // Remove invalid entries from the cache by rotating them to the end of the cache, then
441 // keep track of how many invalid elements there are and overwrite them with the base tag.
442 // The base tag here serves as a harmless default value.
443 for _ in 0..CACHE_LEN - 1 {
444 if self.cache.idx[cursor] >= start {
445 self.cache.idx[cursor..CACHE_LEN - removed].rotate_left(1);
446 self.cache.items[cursor..CACHE_LEN - removed].rotate_left(1);
452 for i in CACHE_LEN - removed - 1..CACHE_LEN {
453 self.cache.idx[i] = 0;
454 self.cache.items[i] = base_tag;
457 if start <= self.unique_range.start {
458 // We removed all the Unique items
459 self.unique_range = 0..0;
461 // Ensure the range doesn't extend past the new top of the stack
462 self.unique_range.end = self.unique_range.end.min(start);
465 self.unique_range = 0..0;
468 #[cfg(all(feature = "stack-cache", debug_assertions))]
469 self.verify_cache_consistency();