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Explain cache behavior a bit better, clean up diff

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This commit is contained in:
Ben Kimock 2022-07-02 19:44:55 -04:00
parent 7cdbf98f57
commit f3b479d556
4 changed files with 21 additions and 16 deletions
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1
Cargo.toml
View File

@ -53,5 +53,6 @@ harness = false
[features]
default = ["stack-cache"]
# Will be enabled on CI via `--all-features`.
expensive-debug-assertions = []
stack-cache = []

4
src/lib.rs
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@ -90,8 +90,8 @@ pub use crate::mono_hash_map::MonoHashMap;
pub use crate::operator::EvalContextExt as OperatorEvalContextExt;
pub use crate::range_map::RangeMap;
pub use crate::stacked_borrows::{
stack::Stack, CallId, EvalContextExt as StackedBorEvalContextExt, Item, Permission, PtrId,
SbTag, SbTagExtra, Stacks,
stack::Stack, CallId, EvalContextExt as StackedBorEvalContextExt, Item, Permission, SbTag,
SbTagExtra, Stacks,
};
pub use crate::sync::{CondvarId, EvalContextExt as SyncEvalContextExt, MutexId, RwLockId};
pub use crate::thread::{

10
src/stacked_borrows.rs
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@ -26,7 +26,6 @@ use diagnostics::{AllocHistory, TagHistory};
pub mod stack;
use stack::Stack;
pub type PtrId = NonZeroU64;
pub type CallId = NonZeroU64;
// Even reading memory can have effects on the stack, so we need a `RefCell` here.
@ -479,7 +478,7 @@ impl<'tcx> Stack {
)
})?;
// Step 2: Remove all items. Also checks for protectors.
// Step 2: Consider all items removed. This checks for protectors.
for idx in (0..self.len()).rev() {
let item = self.get(idx).unwrap();
Stack::item_popped(&item, None, global, alloc_history)?;
@ -579,8 +578,8 @@ impl<'tcx> Stacks {
/// Creates new stack with initial tag.
fn new(size: Size, perm: Permission, tag: SbTag) -> Self {
let item = Item { perm, tag, protector: None };
let stack = Stack::new(item);
Stacks {
stacks: RangeMap::new(size, stack),
history: AllocHistory::new(),
@ -826,14 +825,11 @@ trait EvalContextPrivExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
// We have to use shared references to alloc/memory_extra here since
// `visit_freeze_sensitive` needs to access the global state.
let extra = this.get_alloc_extra(alloc_id)?;
let mut stacked_borrows = extra
.stacked_borrows
.as_ref()
.expect("we should have Stacked Borrows data")
.borrow_mut();
let mut current_span = this.machine.current_span();
this.visit_freeze_sensitive(place, size, |mut range, frozen| {
// Adjust range.
range.start += base_offset;
@ -858,7 +854,7 @@ trait EvalContextPrivExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
item,
(alloc_id, range, offset),
&mut global,
&mut current_span,
current_span,
history,
exposed_tags,
)

22
src/stacked_borrows/stack.rs
View File

@ -27,8 +27,7 @@ pub struct Stack {
/// we never have the unknown-to-known boundary in an SRW group.
unknown_bottom: Option<SbTag>,
/// A small LRU cache of searches of the borrow stack. This only caches accesses of `Tagged`,
/// because those are unique in the stack.
/// 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
@ -42,6 +41,11 @@ pub struct Stack {
/// probably-cold random access into the borrow stack to figure out what `Permission` an
/// `SbTag` grants. We could avoid this by also storing the `Permission` in the cache, but
/// most lookups into the cache are immediately followed by access of the full borrow stack anyway.
///
/// 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.
/// arrays is super fast
#[cfg(feature = "stack-cache")]
#[derive(Clone, Debug)]
struct StackCache {
@ -81,7 +85,9 @@ impl<'tcx> Stack {
/// - There are no Unique tags outside of first_unique..last_unique
#[cfg(feature = "expensive-debug-assertions")]
fn verify_cache_consistency(&self) {
if self.borrows.len() > CACHE_LEN {
// 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.tags.iter().zip(self.cache.idx.iter()) {
assert_eq!(self.borrows[*stack_idx].tag, *tag);
}
@ -154,7 +160,7 @@ impl<'tcx> Stack {
}
// 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 stack.
// 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")]
@ -185,8 +191,11 @@ impl<'tcx> Stack {
// If we found the tag, look up its position in the stack to see if it grants
// the required permission
if self.borrows[stack_idx].perm.grants(access) {
// 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 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 cache_idx == 1 {
self.cache.tags.swap(0, 1);
self.cache.idx.swap(0, 1);
@ -287,7 +296,6 @@ impl<'tcx> Stack {
let unique_range = 0..self.len();
if disable_start <= unique_range.end {
// add 1 so we don't disable the granting item
let lower = unique_range.start.max(disable_start);
let upper = (unique_range.end + 1).min(self.borrows.len());
for item in &mut self.borrows[lower..upper] {