1 use crate::stacked_borrows::{AccessKind, Item, Permission, SbTag, SbTagExtra};
2 use rustc_data_structures::fx::FxHashSet;
3 #[cfg(feature = "stack-cache")]
6 /// Exactly what cache size we should use is a difficult tradeoff. There will always be some
7 /// workload which has a `SbTag` working set which exceeds the size of the cache, and ends up
8 /// falling back to linear searches of the borrow stack very often.
9 /// The cost of making this value too large is that the loop in `Stack::insert` which ensures the
10 /// entries in the cache stay correct after an insert becomes expensive.
11 #[cfg(feature = "stack-cache")]
12 const CACHE_LEN: usize = 32;
14 /// Extra per-location state.
15 #[derive(Clone, Debug)]
17 /// Used *mostly* as a stack; never empty.
19 /// * Above a `SharedReadOnly` there can only be more `SharedReadOnly`.
20 /// * Except for `Untagged`, no tag occurs in the stack more than once.
22 /// If this is `Some(id)`, then the actual current stack is unknown. This can happen when
23 /// wildcard pointers are used to access this location. What we do know is that `borrows` are at
24 /// the top of the stack, and below it are arbitrarily many items whose `tag` is strictly less
26 /// When the bottom is unknown, `borrows` always has a `SharedReadOnly` or `Unique` at the bottom;
27 /// we never have the unknown-to-known boundary in an SRW group.
28 unknown_bottom: Option<SbTag>,
30 /// A small LRU cache of searches of the borrow stack.
31 #[cfg(feature = "stack-cache")]
33 /// On a read, we need to disable all `Unique` above the granting item. We can avoid most of
34 /// this scan by keeping track of the region of the borrow stack that may contain `Unique`s.
35 #[cfg(feature = "stack-cache")]
36 unique_range: Range<usize>,
39 /// A very small cache of searches of the borrow stack
40 /// This maps items to locations in the borrow stack. Any use of this still needs to do a
41 /// probably-cold random access into the borrow stack to figure out what `Permission` an
42 /// `SbTag` grants. We could avoid this by also storing the `Permission` in the cache, but
43 /// most lookups into the cache are immediately followed by access of the full borrow stack anyway.
45 /// It may seem like maintaining this cache is a waste for small stacks, but
46 /// (a) iterating over small fixed-size arrays is super fast, and (b) empirically this helps *a lot*,
47 /// probably because runtime is dominated by large stacks.
48 #[cfg(feature = "stack-cache")]
49 #[derive(Clone, Debug)]
51 items: [Item; CACHE_LEN], // Hot in find_granting
52 idx: [usize; CACHE_LEN], // Hot in grant
55 #[cfg(feature = "stack-cache")]
57 /// When a tag is used, we call this function to add or refresh it in the cache.
59 /// We use the position in the cache to represent how recently a tag was used; the first position
60 /// is the most recently used tag. So an add shifts every element towards the end, and inserts
61 /// the new element at the start. We lose the last element.
62 /// This strategy is effective at keeping the most-accessed items in the cache, but it costs a
63 /// linear shift across the entire cache when we add a new tag.
64 fn add(&mut self, idx: usize, item: Item) {
65 self.items.copy_within(0..CACHE_LEN - 1, 1);
67 self.idx.copy_within(0..CACHE_LEN - 1, 1);
72 impl PartialEq for Stack {
73 fn eq(&self, other: &Self) -> bool {
74 // All the semantics of Stack are in self.borrows, everything else is caching
75 self.borrows == other.borrows
82 /// Panics if any of the caching mechanisms have broken,
83 /// - The StackCache indices don't refer to the parallel items,
84 /// - There are no Unique items outside of first_unique..last_unique
85 #[cfg(feature = "expensive-debug-assertions")]
86 fn verify_cache_consistency(&self) {
87 // Only a full cache needs to be valid. Also see the comments in find_granting_cache
88 // and set_unknown_bottom.
89 if self.borrows.len() >= CACHE_LEN {
90 for (tag, stack_idx) in self.cache.items.iter().zip(self.cache.idx.iter()) {
91 assert_eq!(self.borrows[*stack_idx], *tag);
95 for (idx, item) in self.borrows.iter().enumerate() {
96 if item.perm() == Permission::Unique {
98 self.unique_range.contains(&idx),
107 /// Find the item granting the given kind of access to the given tag, and return where
108 /// it is on the stack. For wildcard tags, the given index is approximate, but if *no*
109 /// index is given it means the match was *not* in the known part of the stack.
110 /// `Ok(None)` indicates it matched the "unknown" part of the stack.
111 /// `Err` indicates it was not found.
112 pub(super) fn find_granting(
116 exposed_tags: &FxHashSet<SbTag>,
117 ) -> Result<Option<usize>, ()> {
118 #[cfg(feature = "expensive-debug-assertions")]
119 self.verify_cache_consistency();
121 let SbTagExtra::Concrete(tag) = tag else {
122 // Handle the wildcard case.
123 // Go search the stack for an exposed tag.
127 .enumerate() // we also need to know *where* in the stack
128 .rev() // search top-to-bottom
129 .find_map(|(idx, item)| {
130 // If the item fits and *might* be this wildcard, use it.
131 if item.perm().grants(access) && exposed_tags.contains(&item.tag()) {
138 return Ok(Some(idx));
140 // If we couldn't find it in the stack, check the unknown bottom.
141 return if self.unknown_bottom.is_some() { Ok(None) } else { Err(()) };
144 if let Some(idx) = self.find_granting_tagged(access, tag) {
145 return Ok(Some(idx));
148 // Couldn't find it in the stack; but if there is an unknown bottom it might be there.
149 let found = self.unknown_bottom.is_some_and(|&unknown_limit| {
150 tag.0 < unknown_limit.0 // unknown_limit is an upper bound for what can be in the unknown bottom.
152 if found { Ok(None) } else { Err(()) }
155 fn find_granting_tagged(&mut self, access: AccessKind, tag: SbTag) -> Option<usize> {
156 #[cfg(feature = "stack-cache")]
157 if let Some(idx) = self.find_granting_cache(access, tag) {
161 // If we didn't find the tag in the cache, fall back to a linear search of the
162 // whole stack, and add the tag to the cache.
163 for (stack_idx, item) in self.borrows.iter().enumerate().rev() {
164 if tag == item.tag() && item.perm().grants(access) {
165 #[cfg(feature = "stack-cache")]
166 self.cache.add(stack_idx, *item);
167 return Some(stack_idx);
173 #[cfg(feature = "stack-cache")]
174 fn find_granting_cache(&mut self, access: AccessKind, tag: SbTag) -> Option<usize> {
175 // This looks like a common-sense optimization; we're going to do a linear search of the
176 // cache or the borrow stack to scan the shorter of the two. This optimization is miniscule
177 // and this check actually ensures we do not access an invalid cache.
178 // When a stack is created and when items are removed from the top of the borrow stack, we
179 // need some valid value to populate the cache. In both cases, we try to use the bottom
180 // item. But when the stack is cleared in `set_unknown_bottom` there is nothing we could
181 // place in the cache that is correct. But due to the way we populate the cache in
182 // `StackCache::add`, we know that when the borrow stack has grown larger than the cache,
183 // every slot in the cache is valid.
184 if self.borrows.len() <= CACHE_LEN {
187 // Search the cache for the tag we're looking up
188 let cache_idx = self.cache.items.iter().position(|t| t.tag() == tag)?;
189 let stack_idx = self.cache.idx[cache_idx];
190 // If we found the tag, look up its position in the stack to see if it grants
191 // the required permission
192 if self.cache.items[cache_idx].perm().grants(access) {
193 // If it does, and it's not already in the most-recently-used position, re-insert it at
194 // the most-recently-used position. This technically reduces the efficiency of the
195 // cache by duplicating elements, but current benchmarks do not seem to benefit from
196 // avoiding this duplication.
197 // But if the tag is in position 1, avoiding the duplicating add is trivial.
198 // If it does, and it's not already in the most-recently-used position, move it there.
199 // Except if the tag is in position 1, this is equivalent to just a swap, so do that.
201 self.cache.items.swap(0, 1);
202 self.cache.idx.swap(0, 1);
203 } else if cache_idx > 1 {
204 self.cache.add(stack_idx, self.cache.items[cache_idx]);
208 // Tag is in the cache, but it doesn't grant the required permission
213 pub fn insert(&mut self, new_idx: usize, new: Item) {
214 self.borrows.insert(new_idx, new);
216 #[cfg(feature = "stack-cache")]
217 self.insert_cache(new_idx, new);
220 #[cfg(feature = "stack-cache")]
221 fn insert_cache(&mut self, new_idx: usize, new: Item) {
222 // Adjust the possibly-unique range if an insert occurs before or within it
223 if self.unique_range.start >= new_idx {
224 self.unique_range.start += 1;
226 if self.unique_range.end >= new_idx {
227 self.unique_range.end += 1;
229 if new.perm() == Permission::Unique {
230 // Make sure the possibly-unique range contains the new borrow
231 self.unique_range.start = self.unique_range.start.min(new_idx);
232 self.unique_range.end = self.unique_range.end.max(new_idx + 1);
235 // The above insert changes the meaning of every index in the cache >= new_idx, so now
236 // we need to find every one of those indexes and increment it.
237 // But if the insert is at the end (equivalent to a push), we can skip this step because
238 // it didn't change the position of any other items.
239 if new_idx != self.borrows.len() - 1 {
240 for idx in &mut self.cache.idx {
247 // This primes the cache for the next access, which is almost always the just-added tag.
248 self.cache.add(new_idx, new);
250 #[cfg(feature = "expensive-debug-assertions")]
251 self.verify_cache_consistency();
254 /// Construct a new `Stack` using the passed `Item` as the base tag.
255 pub fn new(item: Item) -> Self {
258 unknown_bottom: None,
259 #[cfg(feature = "stack-cache")]
260 cache: StackCache { idx: [0; CACHE_LEN], items: [item; CACHE_LEN] },
261 #[cfg(feature = "stack-cache")]
262 unique_range: if item.perm() == Permission::Unique { 0..1 } else { 0..0 },
266 pub fn get(&self, idx: usize) -> Option<Item> {
267 self.borrows.get(idx).cloned()
270 #[allow(clippy::len_without_is_empty)] // Stacks are never empty
271 pub fn len(&self) -> usize {
275 pub fn unknown_bottom(&self) -> Option<SbTag> {
279 pub fn set_unknown_bottom(&mut self, tag: SbTag) {
280 // We clear the borrow stack but the lookup cache doesn't support clearing per se. Instead,
281 // there is a check explained in `find_granting_cache` which protects against accessing the
282 // cache when it has been cleared and not yet refilled.
283 self.borrows.clear();
284 self.unknown_bottom = Some(tag);
287 /// Find all `Unique` elements in this borrow stack above `granting_idx`, pass a copy of them
288 /// to the `visitor`, then set their `Permission` to `Disabled`.
289 pub fn disable_uniques_starting_at<V: FnMut(Item) -> crate::InterpResult<'tcx>>(
291 disable_start: usize,
293 ) -> crate::InterpResult<'tcx> {
294 #[cfg(feature = "stack-cache")]
295 let unique_range = self.unique_range.clone();
296 #[cfg(not(feature = "stack-cache"))]
297 let unique_range = 0..self.len();
299 if disable_start <= unique_range.end {
300 let lower = unique_range.start.max(disable_start);
301 let upper = (unique_range.end + 1).min(self.borrows.len());
302 for item in &mut self.borrows[lower..upper] {
303 if item.perm() == Permission::Unique {
304 log::trace!("access: disabling item {:?}", item);
306 item.set_permission(Permission::Disabled);
307 // Also update all copies of this item in the cache.
308 for it in &mut self.cache.items {
309 if it.tag() == item.tag() {
310 it.set_permission(Permission::Disabled);
317 #[cfg(feature = "stack-cache")]
318 if disable_start < self.unique_range.start {
319 // We disabled all Unique items
320 self.unique_range.start = 0;
321 self.unique_range.end = 0;
323 // Truncate the range to disable_start. This is + 2 because we are only removing
324 // elements after disable_start, and this range does not include the end.
325 self.unique_range.end = self.unique_range.end.min(disable_start + 1);
328 #[cfg(feature = "expensive-debug-assertions")]
329 self.verify_cache_consistency();
334 /// Produces an iterator which iterates over `range` in reverse, and when dropped removes that
335 /// range of `Item`s from this `Stack`.
336 pub fn pop_items_after<V: FnMut(Item) -> crate::InterpResult<'tcx>>(
340 ) -> crate::InterpResult<'tcx> {
341 while self.borrows.len() > start {
342 let item = self.borrows.pop().unwrap();
346 #[cfg(feature = "stack-cache")]
347 if !self.borrows.is_empty() {
348 // After we remove from the borrow stack, every aspect of our caching may be invalid, but it is
349 // also possible that the whole cache is still valid. So we call this method to repair what
350 // aspects of the cache are now invalid, instead of resetting the whole thing to a trivially
351 // valid default state.
352 let base_tag = self.borrows[0];
355 // Remove invalid entries from the cache by rotating them to the end of the cache, then
356 // keep track of how many invalid elements there are and overwrite them with the base tag.
357 // The base tag here serves as a harmless default value.
358 for _ in 0..CACHE_LEN - 1 {
359 if self.cache.idx[cursor] >= start {
360 self.cache.idx[cursor..CACHE_LEN - removed].rotate_left(1);
361 self.cache.items[cursor..CACHE_LEN - removed].rotate_left(1);
367 for i in CACHE_LEN - removed - 1..CACHE_LEN {
368 self.cache.idx[i] = 0;
369 self.cache.items[i] = base_tag;
372 if start < self.unique_range.start.saturating_sub(1) {
373 // We removed all the Unique items
374 self.unique_range = 0..0;
376 // Ensure the range doesn't extend past the new top of the stack
377 self.unique_range.end = self.unique_range.end.min(start + 1);
380 self.unique_range = 0..0;
383 #[cfg(feature = "expensive-debug-assertions")]
384 self.verify_cache_consistency();