miri/borrow_tracker/stacked_borrows/stack.rs
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#[cfg(feature = "stack-cache")]
use std::ops::Range;
use rustc_data_structures::fx::FxHashSet;
use tracing::trace;
use crate::borrow_tracker::stacked_borrows::{Item, Permission};
use crate::borrow_tracker::{AccessKind, BorTag};
use crate::{InterpResult, ProvenanceExtra, interp_ok};
/// Exactly what cache size we should use is a difficult trade-off. There will always be some
/// workload which has a `BorTag` working set which exceeds the size of the cache, and ends up
/// falling back to linear searches of the borrow stack very often.
/// The cost of making this value too large is that the loop in `Stack::insert` which ensures the
/// entries in the cache stay correct after an insert becomes expensive.
#[cfg(feature = "stack-cache")]
const CACHE_LEN: usize = 32;
/// Extra per-location state.
#[derive(Clone, Debug)]
pub struct Stack {
/// Used *mostly* as a stack; never empty.
/// Invariants:
/// * Above a `SharedReadOnly` there can only be more `SharedReadOnly`.
/// * Except for `Untagged`, no tag occurs in the stack more than once.
borrows: Vec<Item>,
/// If this is `Some(id)`, then the actual current stack is unknown. This can happen when
/// wildcard pointers are used to access this location. What we do know is that `borrows` are at
/// the top of the stack, and below it are arbitrarily many items whose `tag` is strictly less
/// than `id`.
/// When the bottom is unknown, `borrows` always has a `SharedReadOnly` or `Unique` at the bottom;
/// we never have the unknown-to-known boundary in an SRW group.
unknown_bottom: Option<BorTag>,
/// A small LRU cache of searches of the borrow stack.
#[cfg(feature = "stack-cache")]
cache: StackCache,
/// On a read, we need to disable all `Unique` above the granting item. We can avoid most of
/// this scan by keeping track of the region of the borrow stack that may contain `Unique`s.
#[cfg(feature = "stack-cache")]
unique_range: Range<usize>,
}
impl Stack {
pub fn retain(&mut self, tags: &FxHashSet<BorTag>) {
let mut first_removed = None;
// We never consider removing the bottom-most tag. For stacks without an unknown
// bottom this preserves the root tag.
// Note that the algorithm below is based on considering the tag at read_idx - 1,
// so precisely considering the tag at index 0 for removal when we have an unknown
// bottom would complicate the implementation. The simplification of not considering
// it does not have a significant impact on the degree to which the GC mitigates
// memory growth.
let mut read_idx = 1;
let mut write_idx = read_idx;
while read_idx < self.borrows.len() {
let left = self.borrows[read_idx - 1];
let this = self.borrows[read_idx];
let should_keep = match this.perm() {
// SharedReadWrite is the simplest case, if it's unreachable we can just remove it.
Permission::SharedReadWrite => tags.contains(&this.tag()),
// Only retain a Disabled tag if it is terminating a SharedReadWrite block.
Permission::Disabled => left.perm() == Permission::SharedReadWrite,
// Unique and SharedReadOnly can terminate a SharedReadWrite block, so only remove
// them if they are both unreachable and not directly after a SharedReadWrite.
Permission::Unique | Permission::SharedReadOnly =>
left.perm() == Permission::SharedReadWrite || tags.contains(&this.tag()),
};
if should_keep {
if read_idx != write_idx {
self.borrows[write_idx] = self.borrows[read_idx];
}
write_idx += 1;
} else if first_removed.is_none() {
first_removed = Some(read_idx);
}
read_idx += 1;
}
self.borrows.truncate(write_idx);
#[cfg(not(feature = "stack-cache"))]
let _unused = first_removed; // This is only needed for the stack-cache
#[cfg(feature = "stack-cache")]
if let Some(first_removed) = first_removed {
// Either end of unique_range may have shifted, all we really know is that we can't
// have introduced a new Unique.
if !self.unique_range.is_empty() {
self.unique_range = 0..self.len();
}
// Replace any Items which have been collected with the root item, a known-good value.
for i in 0..CACHE_LEN {
if self.cache.idx[i] >= first_removed {
self.cache.items[i] = self.borrows[0];
self.cache.idx[i] = 0;
}
}
}
}
}
/// A very small cache of searches of a borrow stack, mapping `Item`s to their position in said stack.
///
/// It may seem like maintaining this cache is a waste for small stacks, but
/// (a) iterating over small fixed-size arrays is super fast, and (b) empirically this helps *a lot*,
/// probably because runtime is dominated by large stacks.
#[cfg(feature = "stack-cache")]
#[derive(Clone, Debug)]
struct StackCache {
items: [Item; CACHE_LEN], // Hot in find_granting
idx: [usize; CACHE_LEN], // Hot in grant
}
#[cfg(feature = "stack-cache")]
impl StackCache {
/// When a tag is used, we call this function to add or refresh it in the cache.
///
/// We use the position in the cache to represent how recently a tag was used; the first position
/// is the most recently used tag. So an add shifts every element towards the end, and inserts
/// the new element at the start. We lose the last element.
/// This strategy is effective at keeping the most-accessed items in the cache, but it costs a
/// linear shift across the entire cache when we add a new tag.
fn add(&mut self, idx: usize, item: Item) {
self.items.copy_within(0..CACHE_LEN - 1, 1);
self.items[0] = item;
self.idx.copy_within(0..CACHE_LEN - 1, 1);
self.idx[0] = idx;
}
}
impl PartialEq for Stack {
fn eq(&self, other: &Self) -> bool {
let Stack {
borrows,
unknown_bottom,
// The cache is ignored for comparison.
#[cfg(feature = "stack-cache")]
cache: _,
#[cfg(feature = "stack-cache")]
unique_range: _,
} = self;
*borrows == other.borrows && *unknown_bottom == other.unknown_bottom
}
}
impl Eq for Stack {}
impl<'tcx> Stack {
/// Panics if any of the caching mechanisms have broken,
/// - The StackCache indices don't refer to the parallel items,
/// - There are no Unique items outside of first_unique..last_unique
#[cfg(feature = "stack-cache-consistency-check")]
fn verify_cache_consistency(&self) {
// Only a full cache needs to be valid. Also see the comments in find_granting_cache
// and set_unknown_bottom.
if self.borrows.len() >= CACHE_LEN {
for (tag, stack_idx) in self.cache.items.iter().zip(self.cache.idx.iter()) {
assert_eq!(self.borrows[*stack_idx], *tag);
}
}
// Check that all Unique items fall within unique_range.
for (idx, item) in self.borrows.iter().enumerate() {
if item.perm() == Permission::Unique {
assert!(
self.unique_range.contains(&idx),
"{:?} {:?}",
self.unique_range,
self.borrows
);
}
}
// Check that the unique_range is a valid index into the borrow stack.
// This asserts that the unique_range's start <= end.
let _uniques = &self.borrows[self.unique_range.clone()];
// We cannot assert that the unique range is precise.
// Both ends may shift around when `Stack::retain` is called. Additionally,
// when we pop items within the unique range, setting the end of the range precisely
// requires doing a linear search of the borrow stack, which is exactly the kind of
// operation that all this caching exists to avoid.
}
/// Find the item granting the given kind of access to the given tag, and return where
/// it is on the stack. For wildcard tags, the given index is approximate, but if *no*
/// index is given it means the match was *not* in the known part of the stack.
/// `Ok(None)` indicates it matched the "unknown" part of the stack.
/// `Err` indicates it was not found.
pub(super) fn find_granting(
&mut self,
access: AccessKind,
tag: ProvenanceExtra,
exposed_tags: &FxHashSet<BorTag>,
) -> Result<Option<usize>, ()> {
#[cfg(feature = "stack-cache-consistency-check")]
self.verify_cache_consistency();
let ProvenanceExtra::Concrete(tag) = tag else {
// Handle the wildcard case.
// Go search the stack for an exposed tag.
if let Some(idx) = self
.borrows
.iter()
.enumerate() // we also need to know *where* in the stack
.rev() // search top-to-bottom
.find_map(|(idx, item)| {
// If the item fits and *might* be this wildcard, use it.
if item.perm().grants(access) && exposed_tags.contains(&item.tag()) {
Some(idx)
} else {
None
}
})
{
return Ok(Some(idx));
}
// If we couldn't find it in the stack, check the unknown bottom.
return if self.unknown_bottom.is_some() { Ok(None) } else { Err(()) };
};
if let Some(idx) = self.find_granting_tagged(access, tag) {
return Ok(Some(idx));
}
// Couldn't find it in the stack; but if there is an unknown bottom it might be there.
let found = self.unknown_bottom.is_some_and(|unknown_limit| {
tag < unknown_limit // unknown_limit is an upper bound for what can be in the unknown bottom.
});
if found { Ok(None) } else { Err(()) }
}
fn find_granting_tagged(&mut self, access: AccessKind, tag: BorTag) -> Option<usize> {
#[cfg(feature = "stack-cache")]
if let Some(idx) = self.find_granting_cache(access, tag) {
return Some(idx);
}
// If we didn't find the tag in the cache, fall back to a linear search of the
// whole stack, and add the tag to the cache.
for (stack_idx, item) in self.borrows.iter().enumerate().rev() {
if tag == item.tag() && item.perm().grants(access) {
#[cfg(feature = "stack-cache")]
self.cache.add(stack_idx, *item);
return Some(stack_idx);
}
}
None
}
#[cfg(feature = "stack-cache")]
fn find_granting_cache(&mut self, access: AccessKind, tag: BorTag) -> Option<usize> {
// This looks like a common-sense optimization; we're going to do a linear search of the
// cache or the borrow stack to scan the shorter of the two. This optimization is minuscule
// and this check actually ensures we do not access an invalid cache.
// When a stack is created and when items are removed from the top of the borrow stack, we
// need some valid value to populate the cache. In both cases, we try to use the bottom
// item. But when the stack is cleared in `set_unknown_bottom` there is nothing we could
// place in the cache that is correct. But due to the way we populate the cache in
// `StackCache::add`, we know that when the borrow stack has grown larger than the cache,
// every slot in the cache is valid.
if self.borrows.len() <= CACHE_LEN {
return None;
}
// Search the cache for the tag we're looking up
let cache_idx = self.cache.items.iter().position(|t| t.tag() == tag)?;
let stack_idx = self.cache.idx[cache_idx];
// If we found the tag, look up its position in the stack to see if it grants
// the required permission
if self.cache.items[cache_idx].perm().grants(access) {
// If it does, and it's not already in the most-recently-used position, re-insert it at
// the most-recently-used position. This technically reduces the efficiency of the
// cache by duplicating elements, but current benchmarks do not seem to benefit from
// avoiding this duplication.
// But if the tag is in position 1, avoiding the duplicating add is trivial.
// If it does, and it's not already in the most-recently-used position, move it there.
// Except if the tag is in position 1, this is equivalent to just a swap, so do that.
if cache_idx == 1 {
self.cache.items.swap(0, 1);
self.cache.idx.swap(0, 1);
} else if cache_idx > 1 {
self.cache.add(stack_idx, self.cache.items[cache_idx]);
}
Some(stack_idx)
} else {
// Tag is in the cache, but it doesn't grant the required permission
None
}
}
pub fn insert(&mut self, new_idx: usize, new: Item) {
self.borrows.insert(new_idx, new);
#[cfg(feature = "stack-cache")]
self.insert_cache(new_idx, new);
}
#[cfg(feature = "stack-cache")]
fn insert_cache(&mut self, new_idx: usize, new: Item) {
// Adjust the possibly-unique range if an insert occurs before or within it
if self.unique_range.start >= new_idx {
self.unique_range.start += 1;
}
if self.unique_range.end >= new_idx {
self.unique_range.end += 1;
}
if new.perm() == Permission::Unique {
// If this is the only Unique, set the range to contain just the new item.
if self.unique_range.is_empty() {
self.unique_range = new_idx..new_idx + 1;
} else {
// We already have other Unique items, expand the range to include the new item
self.unique_range.start = self.unique_range.start.min(new_idx);
self.unique_range.end = self.unique_range.end.max(new_idx + 1);
}
}
// The above insert changes the meaning of every index in the cache >= new_idx, so now
// we need to find every one of those indexes and increment it.
// But if the insert is at the end (equivalent to a push), we can skip this step because
// it didn't change the position of any other items.
if new_idx != self.borrows.len() - 1 {
for idx in &mut self.cache.idx {
if *idx >= new_idx {
*idx += 1;
}
}
}
// This primes the cache for the next access, which is almost always the just-added tag.
self.cache.add(new_idx, new);
#[cfg(feature = "stack-cache-consistency-check")]
self.verify_cache_consistency();
}
/// Construct a new `Stack` using the passed `Item` as the root tag.
pub fn new(item: Item) -> Self {
Stack {
borrows: vec![item],
unknown_bottom: None,
#[cfg(feature = "stack-cache")]
cache: StackCache { idx: [0; CACHE_LEN], items: [item; CACHE_LEN] },
#[cfg(feature = "stack-cache")]
unique_range: if item.perm() == Permission::Unique { 0..1 } else { 0..0 },
}
}
pub fn get(&self, idx: usize) -> Option<Item> {
self.borrows.get(idx).cloned()
}
#[allow(clippy::len_without_is_empty)] // Stacks are never empty
pub fn len(&self) -> usize {
self.borrows.len()
}
pub fn unknown_bottom(&self) -> Option<BorTag> {
self.unknown_bottom
}
pub fn set_unknown_bottom(&mut self, tag: BorTag) {
// We clear the borrow stack but the lookup cache doesn't support clearing per se. Instead,
// there is a check explained in `find_granting_cache` which protects against accessing the
// cache when it has been cleared and not yet refilled.
self.borrows.clear();
self.unknown_bottom = Some(tag);
#[cfg(feature = "stack-cache")]
{
self.unique_range = 0..0;
}
}
/// Find all `Unique` elements in this borrow stack above `granting_idx`, pass a copy of them
/// to the `visitor`, then set their `Permission` to `Disabled`.
pub fn disable_uniques_starting_at(
&mut self,
disable_start: usize,
mut visitor: impl FnMut(Item) -> InterpResult<'tcx>,
) -> InterpResult<'tcx> {
#[cfg(feature = "stack-cache")]
let unique_range = self.unique_range.clone();
#[cfg(not(feature = "stack-cache"))]
let unique_range = 0..self.len();
if disable_start <= unique_range.end {
let lower = unique_range.start.max(disable_start);
let upper = unique_range.end;
for item in &mut self.borrows[lower..upper] {
if item.perm() == Permission::Unique {
trace!("access: disabling item {:?}", item);
visitor(*item)?;
item.set_permission(Permission::Disabled);
// Also update all copies of this item in the cache.
#[cfg(feature = "stack-cache")]
for it in &mut self.cache.items {
if it.tag() == item.tag() {
it.set_permission(Permission::Disabled);
}
}
}
}
}
#[cfg(feature = "stack-cache")]
if disable_start <= self.unique_range.start {
// We disabled all Unique items
self.unique_range.start = 0;
self.unique_range.end = 0;
} else {
// Truncate the range to only include items up to the index that we started disabling
// at.
self.unique_range.end = self.unique_range.end.min(disable_start);
}
#[cfg(feature = "stack-cache-consistency-check")]
self.verify_cache_consistency();
interp_ok(())
}
/// Produces an iterator which iterates over `range` in reverse, and when dropped removes that
/// range of `Item`s from this `Stack`.
pub fn pop_items_after<V: FnMut(Item) -> InterpResult<'tcx>>(
&mut self,
start: usize,
mut visitor: V,
) -> InterpResult<'tcx> {
while self.borrows.len() > start {
let item = self.borrows.pop().unwrap();
visitor(item)?;
}
#[cfg(feature = "stack-cache")]
if !self.borrows.is_empty() {
// After we remove from the borrow stack, every aspect of our caching may be invalid, but it is
// also possible that the whole cache is still valid. So we call this method to repair what
// aspects of the cache are now invalid, instead of resetting the whole thing to a trivially
// valid default state.
let base_tag = self.borrows[0];
let mut removed = 0;
let mut cursor = 0;
// Remove invalid entries from the cache by rotating them to the end of the cache, then
// keep track of how many invalid elements there are and overwrite them with the root tag.
// The root tag here serves as a harmless default value.
for _ in 0..CACHE_LEN - 1 {
if self.cache.idx[cursor] >= start {
self.cache.idx[cursor..CACHE_LEN - removed].rotate_left(1);
self.cache.items[cursor..CACHE_LEN - removed].rotate_left(1);
removed += 1;
} else {
cursor += 1;
}
}
for i in CACHE_LEN - removed - 1..CACHE_LEN {
self.cache.idx[i] = 0;
self.cache.items[i] = base_tag;
}
if start <= self.unique_range.start {
// We removed all the Unique items
self.unique_range = 0..0;
} else {
// Ensure the range doesn't extend past the new top of the stack
self.unique_range.end = self.unique_range.end.min(start);
}
} else {
self.unique_range = 0..0;
}
#[cfg(feature = "stack-cache-consistency-check")]
self.verify_cache_consistency();
interp_ok(())
}
}