bjoernager/rust
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Use a ref-counted pointer for ownership of the predecessor cache

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...instead of a `LockGuard` which means the lock is held for longer than
necessary.
This commit is contained in:
Dylan MacKenzie 2020-04-21 10:15:07 -07:00
parent 740f228b5b
commit 59c746030a
1 changed files with 23 additions and 14 deletions
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37
src/librustc_middle/mir/predecessors.rs
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@ -1,5 +1,5 @@
use rustc_data_structures::stable_hasher::{HashStable, StableHasher}; use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
use rustc_data_structures::sync::{Lock, LockGuard, MappedLockGuard}; use rustc_data_structures::sync::{Lock, Lrc};
use rustc_index::vec::IndexVec; use rustc_index::vec::IndexVec;
use rustc_serialize as serialize; use rustc_serialize as serialize;
use smallvec::SmallVec; use smallvec::SmallVec;
@ -11,7 +11,7 @@ pub type Predecessors = IndexVec<BasicBlock, SmallVec<[BasicBlock; 4]>>;
#[derive(Clone, Debug)] #[derive(Clone, Debug)]
pub struct PredecessorCache { pub struct PredecessorCache {
cache: Lock<Option<Predecessors>>, cache: Lock<Option<Lrc<Predecessors>>>,
} }
impl PredecessorCache { impl PredecessorCache {
@ -20,30 +20,39 @@ impl PredecessorCache {
PredecessorCache { cache: Lock::new(None) } PredecessorCache { cache: Lock::new(None) }
} }
/// Invalidates the predecessor cache.
///
/// Invalidating the predecessor cache requires mutating the MIR, which in turn requires a
/// unique reference (`&mut`) to the `mir::Body`. Because of this, we can assume that all
/// callers of `invalidate` have a unique reference to the MIR and thus to the predecessor
/// cache. This means we don't actually need to take a lock when `invalidate` is called.
#[inline] #[inline]
pub fn invalidate(&mut self) { pub fn invalidate(&mut self) {
*self.cache.get_mut() = None; *self.cache.get_mut() = None;
} }
/// Returns a ref-counted smart pointer containing the predecessor graph for this MIR.
///
/// We use ref-counting instead of a mapped `LockGuard` here to ensure that the lock for
/// `cache` is only held inside this function. As long as no other locks are taken while
/// computing the predecessor graph, deadlock is impossible.
#[inline] #[inline]
pub fn compute( pub fn compute(
&self, &self,
basic_blocks: &IndexVec<BasicBlock, BasicBlockData<'_>>, basic_blocks: &IndexVec<BasicBlock, BasicBlockData<'_>>,
) -> MappedLockGuard<'_, Predecessors> { ) -> Lrc<Predecessors> {
LockGuard::map(self.cache.lock(), |cache| { Lrc::clone(self.cache.lock().get_or_insert_with(|| {
cache.get_or_insert_with(|| { let mut preds = IndexVec::from_elem(SmallVec::new(), basic_blocks);
let mut preds = IndexVec::from_elem(SmallVec::new(), basic_blocks); for (bb, data) in basic_blocks.iter_enumerated() {
for (bb, data) in basic_blocks.iter_enumerated() { if let Some(term) = &data.terminator {
if let Some(term) = &data.terminator { for &succ in term.successors() {
for &succ in term.successors() { preds[succ].push(bb);
preds[succ].push(bb);
}
} }
} }
}
preds Lrc::new(preds)
}) }))
})
} }
} }