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perf: walk liveness backwards in Conflicts::build

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This commit is contained in:
Jonas Schievink 2020-09-12 15:10:38 +02:00
parent 7dbc7f76e1
commit 402f863d8a
1 changed files with 40 additions and 25 deletions
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65
compiler/rustc_mir/src/transform/dest_prop.rs
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@ -100,10 +100,7 @@ use crate::{
transform::{MirPass, MirSource},
util::{dump_mir, PassWhere},
};
use dataflow::{
impls::{MaybeInitializedLocals, MaybeLiveLocals},
ResultsCursor,
};
use dataflow::impls::{MaybeInitializedLocals, MaybeLiveLocals};
use rustc_data_structures::unify::{InPlaceUnificationTable, UnifyKey};
use rustc_index::{
bit_set::{BitMatrix, BitSet},
@ -382,16 +379,11 @@ impl Conflicts {
body.local_decls.len(),
);
let mut record_conflicts =
|init: &ResultsCursor<'_, '_, MaybeInitializedLocals>,
live: &ResultsCursor<'_, '_, MaybeLiveLocals>| {
let mut requires_storage = init.get().clone();
requires_storage.intersect(live.get());
for local in requires_storage.iter() {
conflicts.union_row_with(&requires_storage, local);
}
};
let mut record_conflicts = |new_conflicts: &BitSet<_>| {
for local in new_conflicts.iter() {
conflicts.union_row_with(&new_conflicts, local);
}
};
let def_id = source.def_id();
let mut init = MaybeInitializedLocals
@ -457,27 +449,50 @@ impl Conflicts {
},
);
let mut relevant_locals = Vec::new();
// Visit only reachable basic blocks. The exact order is not important.
for (block, data) in traversal::preorder(body) {
// Observe the dataflow state *before* all possible locations (statement or terminator) in
// each basic block...
// We need to observe the dataflow state *before* all possible locations (statement or
// terminator) in each basic block, and then observe the state *after* the terminator
// effect is applied. As long as neither `init` nor `borrowed` has a "before" effect,
// we will observe all possible dataflow states.
// Since liveness is a backwards analysis, we need to walk the results backwards. To do
// that, we first collect in the `MaybeInitializedLocals` results in a forwards
// traversal.
relevant_locals.resize_with(data.statements.len() + 1, || {
BitSet::new_empty(body.local_decls.len())
});
// First, go forwards for `MaybeInitializedLocals`.
for statement_index in 0..=data.statements.len() {
let loc = Location { block, statement_index };
trace!("record conflicts at {:?}", loc);
init.seek_before_primary_effect(loc);
live.seek_after_primary_effect(loc);
// FIXME: liveness is backwards, so this is slow
record_conflicts(&init, &live);
relevant_locals[statement_index].clone_from(init.get());
}
// Now, go backwards and union with the liveness results.
for statement_index in (0..=data.statements.len()).rev() {
let loc = Location { block, statement_index };
live.seek_after_primary_effect(loc);
relevant_locals[statement_index].intersect(live.get());
trace!("record conflicts at {:?}", loc);
record_conflicts(&relevant_locals[statement_index]);
}
// ...and then observe the state *after* the terminator effect is applied. As long as
// neither `init` nor `borrowed` has a "before" effect, we will observe all possible
// dataflow states here or in the loop above.
trace!("record conflicts at end of {:?}", block);
init.seek_to_block_end(block);
live.seek_to_block_end(block);
record_conflicts(&init, &live);
let mut conflicts = init.get().clone();
conflicts.intersect(live.get());
trace!("record conflicts at end of {:?}", block);
record_conflicts(&conflicts);
}
Self { matrix: conflicts, unify_cache: BitSet::new_empty(body.local_decls.len()) }