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Remove the use of the union-find structure during elaboration #7922

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Merged
elliottt merged 2 commits into from
Feb 13, 2024
Merged

Remove the use of the union-find structure during elaboration #7922

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4fc3903
Remove the use of the union-find structure during elaboration
elliottt Feb 12, 2024
6e876b3
Document the eclass union subtree invariant
elliottt Feb 13, 2024
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1 change: 0 additions & 1 deletion cranelift/codegen/src/egraph.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -619,7 +619,6 @@ impl<'a> EgraphPass<'a> {
&self.domtree_children,
self.loop_analysis,
&mut self.remat_values,
&mut self.eclasses,
&mut self.stats,
);
elaborator.elaborate();
Expand Down
37 changes: 14 additions & 23 deletions cranelift/codegen/src/egraph/elaborate.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,6 @@ use crate::ir::{Block, Function, Inst, Value, ValueDef};
use crate::loop_analysis::{Loop, LoopAnalysis};
use crate::scoped_hash_map::ScopedHashMap;
use crate::trace;
use crate::unionfind::UnionFind;
use alloc::vec::Vec;
use cranelift_entity::{packed_option::ReservedValue, SecondaryMap};
use smallvec::{smallvec, SmallVec};
Expand All @@ -22,7 +21,6 @@ pub(crate) struct Elaborator<'a> {
domtree: &'a DominatorTree,
domtree_children: &'a DomTreeWithChildren,
loop_analysis: &'a LoopAnalysis,
eclasses: &'a mut UnionFind<Value>,
/// Map from Value that is produced by a pure Inst (and was thus
/// not in the side-effecting skeleton) to the value produced by
/// an elaborated inst (placed in the layout) to whose results we
Expand All @@ -40,6 +38,11 @@ pub(crate) struct Elaborator<'a> {
/// is already placed in the Layout. If so, we duplicate, and
/// insert non-identity mappings from the original inst's results
/// to the cloned inst's results.
///
/// Note that as values may refer to unions that represent a subset
/// of a larger eclass, it's not valid to walk towards the root of a
/// union tree: doing so would potentially equate values that fall
/// on different branches of the dominator tree.
value_to_elaborated_value: ScopedHashMap<Value, ElaboratedValue>,
/// Map from Value to the best (lowest-cost) Value in its eclass
/// (tree of union value-nodes).
Expand Down Expand Up @@ -138,7 +141,6 @@ impl<'a> Elaborator<'a> {
domtree_children: &'a DomTreeWithChildren,
loop_analysis: &'a LoopAnalysis,
remat_values: &'a FxHashSet<Value>,
eclasses: &'a mut UnionFind<Value>,
stats: &'a mut Stats,
) -> Self {
let num_values = func.dfg.num_values();
Expand All @@ -150,7 +152,6 @@ impl<'a> Elaborator<'a> {
domtree,
domtree_children,
loop_analysis,
eclasses,
value_to_elaborated_value: ScopedHashMap::with_capacity(num_values),
value_to_best_value,
loop_stack: smallvec![],
Expand Down Expand Up @@ -392,14 +393,6 @@ impl<'a> Elaborator<'a> {
let value = self.func.dfg.resolve_aliases(value);

self.stats.elaborate_visit_node += 1;
let canonical_value = self.eclasses.find_and_update(value);
debug_assert_ne!(canonical_value, Value::reserved_value());
trace!(
"elaborate: value {} canonical {} before {}",
value,
canonical_value,
before
);

// Get the best option; we use `value` (latest
// value) here so we have a full view of the
Expand All @@ -409,7 +402,7 @@ impl<'a> Elaborator<'a> {
trace!("elaborate: value {} -> best {}", value, best_value);
debug_assert_ne!(best_value, Value::reserved_value());

if let Some(elab_val) = self.value_to_elaborated_value.get(&canonical_value) {
if let Some(elab_val) = self.value_to_elaborated_value.get(&best_value) {
// Value is available; use it.
trace!("elaborate: value {} -> {:?}", value, elab_val);
self.stats.elaborate_memoize_hit += 1;
Expand Down Expand Up @@ -635,16 +628,14 @@ impl<'a> Elaborator<'a> {
value: new_result,
in_block: insert_block,
};
let canonical_result = self.eclasses.find_and_update(result);
let best_result = self.value_to_best_value[result];
self.value_to_elaborated_value.insert_if_absent_with_depth(
canonical_result,
best_result.1,
elab_value,
scope_depth,
);

self.eclasses.add(new_result);
self.eclasses.union(result, new_result);
self.value_to_best_value[new_result] = self.value_to_best_value[result];
self.value_to_best_value[new_result] = best_result;

trace!(
" -> cloned inst has new result {} for orig {}",
Expand All @@ -663,9 +654,9 @@ impl<'a> Elaborator<'a> {
value: result,
in_block: insert_block,
};
let canonical_result = self.eclasses.find_and_update(result);
let best_result = self.value_to_best_value[result];
self.value_to_elaborated_value.insert_if_absent_with_depth(
canonical_result,
best_result.1,
elab_value,
scope_depth,
);
Expand All @@ -675,7 +666,7 @@ impl<'a> Elaborator<'a> {
};

// Place the inst just before `before`.
debug_assert!(
assert!(
is_pure_for_egraph(self.func, inst),
"something has gone very wrong if we are elaborating effectful \
instructions, they should have remained in the skeleton"
Expand Down Expand Up @@ -757,9 +748,9 @@ impl<'a> Elaborator<'a> {
// map now.
for &result in self.func.dfg.inst_results(inst) {
trace!(" -> result {}", result);
let canonical_result = self.eclasses.find_and_update(result);
let best_result = self.value_to_best_value[result];
self.value_to_elaborated_value.insert_if_absent(
canonical_result,
best_result.1,
ElaboratedValue {
in_block: block,
value: result,
Expand Down