Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

Optimizing Stacked Borrows (part 1?): Cache locations of Tags in a Borrow Stack #1935

Merged
merged 5 commits into from
Jul 3, 2022
Merged

Optimizing Stacked Borrows (part 1?): Cache locations of Tags in a Borrow Stack #1935

Show file tree
Hide file tree
Changes from 2 commits
Commits
Show all changes
5 commits
Select commit Hold shift + click to select a range
3bbcafe
Cache lookups into the borrow stack
saethlin Dec 8, 2021
17acc3f
Document implementation a bit, add some fast paths
saethlin Jul 2, 2022
7cdbf98
Explain the behavior of the cache upon clear
saethlin Jul 2, 2022
f3b479d
Explain cache behavior a bit better, clean up diff
saethlin Jul 2, 2022
b004a03
Typo
saethlin Jul 3, 2022
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
5 changes: 5 additions & 0 deletions Cargo.toml
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -50,3 +50,8 @@ rustc_private = true
[[test]]
name = "compiletest"
harness = false

[features]
default = ["stack-cache"]
expensive-debug-assertions = []
saethlin marked this conversation as resolved.
Show resolved Hide resolved
stack-cache = []
4 changes: 2 additions & 2 deletions src/lib.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -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::{
CallId, EvalContextExt as StackedBorEvalContextExt, Item, Permission, SbTag, SbTagExtra, Stack,
Stacks,
stack::Stack, CallId, EvalContextExt as StackedBorEvalContextExt, Item, Permission, PtrId,
SbTag, SbTagExtra, Stacks,
};
pub use crate::sync::{CondvarId, EvalContextExt as SyncEvalContextExt, MutexId, RwLockId};
pub use crate::thread::{
Expand Down
139 changes: 34 additions & 105 deletions src/stacked_borrows.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -23,6 +23,10 @@ use crate::*;
pub mod diagnostics;
use diagnostics::{AllocHistory, TagHistory};

pub mod stack;
use stack::Stack;

pub type PtrId = NonZeroU64;
saethlin marked this conversation as resolved.
Show resolved Hide resolved
pub type CallId = NonZeroU64;

// Even reading memory can have effects on the stack, so we need a `RefCell` here.
Expand Down Expand Up @@ -111,23 +115,6 @@ impl fmt::Debug for Item {
}
}

/// Extra per-location state.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct Stack {
/// Used *mostly* as a stack; never empty.
/// Invariants:
/// * Above a `SharedReadOnly` there can only be more `SharedReadOnly`.
/// * 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<SbTag>,
}

/// Extra per-allocation state.
#[derive(Clone, Debug)]
pub struct Stacks {
Expand Down Expand Up @@ -297,65 +284,10 @@ impl Permission {

/// Core per-location operations: access, dealloc, reborrow.
impl<'tcx> Stack {
/// 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.
fn find_granting(
&self,
access: AccessKind,
tag: SbTagExtra,
exposed_tags: &FxHashSet<SbTag>,
) -> Result<Option<usize>, ()> {
let SbTagExtra::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.borrows
.iter()
.enumerate() // we also need to know *where* in the stack
.rev() // search top-to-bottom
// Return permission of first item that grants access.
// We require a permission with the right tag, ensuring U3 and F3.
.find_map(|(idx, item)| {
if tag == item.tag && item.perm.grants(access) { Some(idx) } else { None }
})
{
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.0 < unknown_limit.0 // unknown_limit is an upper bound for what can be in the unknown bottom.
});
if found { Ok(None) } else { Err(()) }
}

/// Find the first write-incompatible item above the given one --
/// i.e, find the height to which the stack will be truncated when writing to `granting`.
fn find_first_write_incompatible(&self, granting: usize) -> usize {
let perm = self.borrows[granting].perm;
let perm = self.get(granting).unwrap().perm;
match perm {
Permission::SharedReadOnly => bug!("Cannot use SharedReadOnly for writing"),
Permission::Disabled => bug!("Cannot use Disabled for anything"),
Expand All @@ -366,7 +298,7 @@ impl<'tcx> Stack {
Permission::SharedReadWrite => {
// The SharedReadWrite *just* above us are compatible, to skip those.
let mut idx = granting + 1;
while let Some(item) = self.borrows.get(idx) {
while let Some(item) = self.get(idx) {
if item.perm == Permission::SharedReadWrite {
// Go on.
idx += 1;
Expand Down Expand Up @@ -461,16 +393,16 @@ impl<'tcx> Stack {
// There is a SRW group boundary between the unknown and the known, so everything is incompatible.
0
};
for item in self.borrows.drain(first_incompatible_idx..).rev() {
trace!("access: popping item {:?}", item);
self.pop_items_after(first_incompatible_idx, |item| {
Stack::item_popped(
&item,
Some((tag, alloc_range, offset, access)),
global,
alloc_history,
)?;
alloc_history.log_invalidation(item.tag, alloc_range, current_span);
}
Ok(())
})?;
} else {
// On a read, *disable* all `Unique` above the granting item. This ensures U2 for read accesses.
// The reason this is not following the stack discipline (by removing the first Unique and
Expand All @@ -487,44 +419,39 @@ impl<'tcx> Stack {
// We are reading from something in the unknown part. That means *all* `Unique` we know about are dead now.
0
};
for idx in (first_incompatible_idx..self.borrows.len()).rev() {
let item = &mut self.borrows[idx];

if item.perm == Permission::Unique {
trace!("access: disabling item {:?}", item);
Stack::item_popped(
item,
Some((tag, alloc_range, offset, access)),
global,
alloc_history,
)?;
item.perm = Permission::Disabled;
alloc_history.log_invalidation(item.tag, alloc_range, current_span);
}
}
self.disable_uniques_starting_at(first_incompatible_idx, |item| {
Stack::item_popped(
&item,
Some((tag, alloc_range, offset, access)),
global,
alloc_history,
)?;
alloc_history.log_invalidation(item.tag, alloc_range, current_span);
Ok(())
})?;
}

// If this was an approximate action, we now collapse everything into an unknown.
if granting_idx.is_none() || matches!(tag, SbTagExtra::Wildcard) {
// Compute the upper bound of the items that remain.
// (This is why we did all the work above: to reduce the items we have to consider here.)
let mut max = NonZeroU64::new(1).unwrap();
for item in &self.borrows {
for i in 0..self.len() {
let item = self.get(i).unwrap();
// Skip disabled items, they cannot be matched anyway.
if !matches!(item.perm, Permission::Disabled) {
// We are looking for a strict upper bound, so add 1 to this tag.
max = cmp::max(item.tag.0.checked_add(1).unwrap(), max);
}
}
if let Some(unk) = self.unknown_bottom {
if let Some(unk) = self.unknown_bottom() {
max = cmp::max(unk.0, max);
}
// Use `max` as new strict upper bound for everything.
trace!(
"access: forgetting stack to upper bound {max} due to wildcard or unknown access"
);
self.borrows.clear();
self.unknown_bottom = Some(SbTag(max));
self.set_unknown_bottom(SbTag(max));
}

// Done.
Expand Down Expand Up @@ -553,7 +480,8 @@ impl<'tcx> Stack {
})?;

// Step 2: Remove all items. Also checks for protectors.
saethlin marked this conversation as resolved.
Show resolved Hide resolved
saethlin marked this conversation as resolved.
Show resolved Hide resolved
for item in self.borrows.drain(..).rev() {
for idx in (0..self.len()).rev() {
let item = self.get(idx).unwrap();
Stack::item_popped(&item, None, global, alloc_history)?;
}
Ok(())
Expand Down Expand Up @@ -601,8 +529,7 @@ impl<'tcx> Stack {
// The new thing is SRW anyway, so we cannot push it "on top of the unkown part"
// (for all we know, it might join an SRW group inside the unknown).
trace!("reborrow: forgetting stack entirely due to SharedReadWrite reborrow from wildcard or unknown");
self.borrows.clear();
self.unknown_bottom = Some(global.next_ptr_tag);
self.set_unknown_bottom(global.next_ptr_tag);
return Ok(());
};

Expand All @@ -629,19 +556,18 @@ impl<'tcx> Stack {
// on top of `derived_from`, and we want the new item at the top so that we
// get the strongest possible guarantees.
// This ensures U1 and F1.
self.borrows.len()
self.len()
};

// Put the new item there. As an optimization, deduplicate if it is equal to one of its new neighbors.
// `new_idx` might be 0 if we just cleared the entire stack.
if self.borrows.get(new_idx) == Some(&new)
|| (new_idx > 0 && self.borrows[new_idx - 1] == new)
if self.get(new_idx) == Some(new) || (new_idx > 0 && self.get(new_idx - 1).unwrap() == new)
{
// Optimization applies, done.
trace!("reborrow: avoiding adding redundant item {:?}", new);
} else {
trace!("reborrow: adding item {:?}", new);
self.borrows.insert(new_idx, new);
self.insert(new_idx, new);
}
Ok(())
}
Expand All @@ -653,8 +579,8 @@ 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 { borrows: vec![item], unknown_bottom: None };

let stack = Stack::new(item);
Stacks {
stacks: RangeMap::new(size, stack),
history: AllocHistory::new(),
Expand Down Expand Up @@ -900,11 +826,14 @@ 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();
saethlin marked this conversation as resolved.
Show resolved Hide resolved

this.visit_freeze_sensitive(place, size, |mut range, frozen| {
// Adjust range.
range.start += base_offset;
Expand All @@ -929,7 +858,7 @@ trait EvalContextPrivExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
item,
(alloc_id, range, offset),
&mut global,
current_span,
&mut current_span,
history,
exposed_tags,
)
Expand Down
5 changes: 4 additions & 1 deletion src/stacked_borrows/diagnostics.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -185,7 +185,10 @@ fn operation_summary(

fn error_cause(stack: &Stack, tag: SbTagExtra) -> &'static str {
if let SbTagExtra::Concrete(tag) = tag {
if stack.borrows.iter().any(|item| item.tag == tag && item.perm != Permission::Disabled) {
if (0..stack.len())
.map(|i| stack.get(i).unwrap())
.any(|item| item.tag == tag && item.perm != Permission::Disabled)
{
", but that tag only grants SharedReadOnly permission for this location"
} else {
", but that tag does not exist in the borrow stack for this location"
Expand Down
Loading