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Auto merge of #127036 - cjgillot:sparse-state, r=oli-obk

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Make jump threading state sparse

Continuation of https://github.com/rust-lang/rust/pull/127024

Both dataflow const-prop and jump threading involve cloning the state vector a lot. This PR replaces the data structure by a sparse vector, considering:
- that jump threading state is typically very sparse (at most 1 or 2 set entries);
- that dataflow const-prop is disabled by default;
- that place/value map is very eager, and prone to creating an overly large state.

The first commit is shared with the previous PR to avoid needless conflicts.

r? `@oli-obk`
This commit is contained in:
bors 2024-07-03 18:52:04 +00:00
commit 2b90614e94
3 changed files with 141 additions and 80 deletions
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19
compiler/rustc_mir_transform/src/jump_threading.rs
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@ -47,6 +47,7 @@ use rustc_middle::mir::visit::Visitor;
use rustc_middle::mir::*;
use rustc_middle::ty::layout::LayoutOf;
use rustc_middle::ty::{self, ScalarInt, TyCtxt};
use rustc_mir_dataflow::lattice::HasBottom;
use rustc_mir_dataflow::value_analysis::{Map, PlaceIndex, State, TrackElem};
use rustc_span::DUMMY_SP;
use rustc_target::abi::{TagEncoding, Variants};
@ -158,9 +159,17 @@ impl Condition {
}
}
#[derive(Copy, Clone, Debug, Default)]
#[derive(Copy, Clone, Debug)]
struct ConditionSet<'a>(&'a [Condition]);
impl HasBottom for ConditionSet<'_> {
const BOTTOM: Self = ConditionSet(&[]);
fn is_bottom(&self) -> bool {
self.0.is_empty()
}
}
impl<'a> ConditionSet<'a> {
fn iter(self) -> impl Iterator<Item = Condition> + 'a {
self.0.iter().copied()
@ -177,7 +186,7 @@ impl<'a> ConditionSet<'a> {
impl<'tcx, 'a> TOFinder<'tcx, 'a> {
fn is_empty(&self, state: &State<ConditionSet<'a>>) -> bool {
state.all(|cs| cs.0.is_empty())
state.all_bottom()
}
/// Recursion entry point to find threading opportunities.
@ -198,7 +207,7 @@ impl<'tcx, 'a> TOFinder<'tcx, 'a> {
debug!(?discr);
let cost = CostChecker::new(self.tcx, self.param_env, None, self.body);
let mut state = State::new(ConditionSet::default(), self.map);
let mut state = State::new_reachable();
let conds = if let Some((value, then, else_)) = targets.as_static_if() {
let value = ScalarInt::try_from_uint(value, discr_layout.size)?;
@ -255,7 +264,7 @@ impl<'tcx, 'a> TOFinder<'tcx, 'a> {
// _1 = 5 // Whatever happens here, it won't change the result of a `SwitchInt`.
// _1 = 6
if let Some((lhs, tail)) = self.mutated_statement(stmt) {
state.flood_with_tail_elem(lhs.as_ref(), tail, self.map, ConditionSet::default());
state.flood_with_tail_elem(lhs.as_ref(), tail, self.map, ConditionSet::BOTTOM);
}
}
@ -609,7 +618,7 @@ impl<'tcx, 'a> TOFinder<'tcx, 'a> {
// We can recurse through this terminator.
let mut state = state();
if let Some(place_to_flood) = place_to_flood {
state.flood_with(place_to_flood.as_ref(), self.map, ConditionSet::default());
state.flood_with(place_to_flood.as_ref(), self.map, ConditionSet::BOTTOM);
}
self.find_opportunity(bb, state, cost.clone(), depth + 1);
}