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Limit number of tracked places, and some other perf improvements

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This commit is contained in:
Jannis Christopher Köhl 2022-10-25 21:54:39 +02:00
parent da4a40f816
commit 630e17d3e4
2 changed files with 66 additions and 14 deletions
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23
compiler/rustc_mir_transform/src/dataflow_const_prop.rs
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@ -15,6 +15,8 @@ use rustc_span::DUMMY_SP;
use crate::MirPass;
const TRACKING_LIMIT: usize = 1000;
pub struct DataflowConstProp;
impl<'tcx> MirPass<'tcx> for DataflowConstProp {
@ -22,18 +24,33 @@ impl<'tcx> MirPass<'tcx> for DataflowConstProp {
sess.mir_opt_level() >= 1
}
#[instrument(skip_all level = "debug")]
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
// Decide which places to track during the analysis.
let map = Map::from_filter(tcx, body, Ty::is_scalar);
// We want to have a somewhat linear runtime w.r.t. the number of statements/terminators.
// Let's call this number `n`. Dataflow analysis has `O(h*n)` transfer function
// applications, where `h` is the height of the lattice. Because the height of our lattice
// is linear w.r.t. the number of tracked places, this is `O(tracked_places * n)`. However,
// because every transfer function application could traverse the whole map, this becomes
// `O(num_nodes * tracked_places * n)` in terms of time complexity. Since the number of
// map nodes is strongly correlated to the number of tracked places, this becomes more or
// less `O(n)` if we place a constant limit on the number of tracked places.
if map.tracked_places() > TRACKING_LIMIT {
debug!("aborted dataflow const prop due to too many tracked places");
return;
}
// Perform the actual dataflow analysis.
let analysis = ConstAnalysis::new(tcx, body, map);
let results = analysis.wrap().into_engine(tcx, body).iterate_to_fixpoint();
let results = debug_span!("analyze")
.in_scope(|| analysis.wrap().into_engine(tcx, body).iterate_to_fixpoint());
// Collect results and patch the body afterwards.
let mut visitor = CollectAndPatch::new(tcx, &results.analysis.0.map);
results.visit_reachable_with(body, &mut visitor);
visitor.visit_body(body);
debug_span!("collect").in_scope(|| results.visit_reachable_with(body, &mut visitor));
debug_span!("patch").in_scope(|| visitor.visit_body(body));
}
}