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React Compiler: Store aliasing values in an inline set (#37366)

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97.6% of the value sets tracked per identifier in mutation / aliasing
inference hold exactly one element, but each was a `FxHashSet`, meaning
each was a heap allocation.
Because the inference code retains a full state in each basic block,
these single-element hashsets were a major contributor to peak memory
allocation.

This replaces them with a small inline set inspired by smolvec /
tinyvec. Five values are stored inline, and any more spill to the heap.
This was only needed in **0.02%** of sets in my data corpus.

This also makes iteration order match the TS implementation. TS uses
`Set` and iterates in insertion order; the Fx set iterated in hash
order. This brings the two behaviors in line.

| Benchmark | Peak allocation | Allocation count | Wall time |

|------------------|-----------------------------|------------------|-----------|
| legacy/image.tsx | 33.40 -> 28.07 MiB (-16.0%) | -66.7% | -28.8% |
| next-client | 33.40 -> 28.07 (-16.0%) | -42.0% | -15.7% |
| devtools | 16.29 -> 14.25 (-12.5%) | -19.9% | -7.3% |
| fixtures | 9.41 -> 7.90 (-16.0%) | -7.3% | -3.3% |
| next-examples | 4.85 -> 4.85 ( 0.0%) | -4.8% | -1.6% |
This commit is contained in:
Andrew Imm
2026-09-01 22:10:29 -07:00
committed by GitHub
parent 0209ef8a77
commit 469c4ec4d3
1 changed files with 116 additions and 25 deletions
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compiler/crates/react_compiler_inference/src/infer_mutation_aliasing_effects.rs
+116 -25
View File
@@ -346,6 +346,104 @@ impl ValueReasonSet {
// InferenceState
// =============================================================================
/// Number of `ValueId`s a [`ValueIdSet`] holds before spilling to the heap.
///
/// Measured against a corpus of real projects containing over 10,000 files,
/// 97.6% of these sets hold a single value, 99.7% hold at most two, and 99.98%
/// at most five. Five entries is coincidentally "free" since it fits in the
/// overhead of the `Vec` we're replacing, so it's a natural cut-off.
const VALUE_ID_INLINE_CAPACITY: usize = 5;
/// An insertion-ordered set of `ValueId`s.
///
/// This was previously an `FxHashSet`, which allocated for every entry of
/// [`InferenceState::variables`] — and since the inference fixpoint retains a
/// state per block, those overwhelmingly single-element sets were the largest
/// remaining source of peak heap in the pass. Almost all of them now live
/// inline, similar to a `tinyvec` / `smolvec`.
///
/// It maintains insertion order, bringing it closer to the original TS
/// implementation, which uses a `Set` and iterates it in insertion order.
#[derive(Debug, Clone)]
enum ValueIdSet {
Inline {
items: [ValueId; VALUE_ID_INLINE_CAPACITY],
len: u8,
},
Spilled(Box<[ValueId]>),
}
impl Default for ValueIdSet {
fn default() -> Self {
ValueIdSet::Inline {
items: [ValueId(0); VALUE_ID_INLINE_CAPACITY],
len: 0,
}
}
}
impl ValueIdSet {
fn single(value: ValueId) -> Self {
let mut set = Self::default();
set.insert(value);
set
}
fn as_slice(&self) -> &[ValueId] {
match self {
ValueIdSet::Inline { items, len } => &items[..*len as usize],
ValueIdSet::Spilled(values) => values,
}
}
fn iter(&self) -> impl Iterator<Item = ValueId> + '_ {
self.as_slice().iter().copied()
}
fn contains(&self, value: ValueId) -> bool {
self.as_slice().contains(&value)
}
fn is_empty(&self) -> bool {
self.as_slice().is_empty()
}
/// Appends `value` if not already present, spilling to the heap once the
/// inline capacity is exhausted.
fn insert(&mut self, value: ValueId) {
if self.contains(value) {
return;
}
match self {
ValueIdSet::Inline { items, len } if (*len as usize) < VALUE_ID_INLINE_CAPACITY => {
items[*len as usize] = value;
*len += 1;
}
ValueIdSet::Inline { items, len } => {
let mut values = items[..*len as usize].to_vec();
values.push(value);
*self = ValueIdSet::Spilled(values.into_boxed_slice());
}
ValueIdSet::Spilled(values) => {
// A boxed slice has no spare capacity, so growing reallocates.
// Only 0.02% of sets ever spill at all, which is what makes the
// trade worthwhile — see the note on `Spilled` above.
let mut grown = std::mem::take(values).into_vec();
grown.push(value);
*values = grown.into_boxed_slice();
}
}
}
/// Adds every member of `other`, keeping `self`'s order and appending
/// newcomers in `other`'s order.
fn union_with(&mut self, other: &ValueIdSet) {
for value in other.iter() {
self.insert(value);
}
}
}
/// The abstract state tracked during inference.
/// Uses interior mutability via a struct with direct fields (no Rc needed since
/// we always have exclusive access in the pass).
@@ -355,7 +453,7 @@ struct InferenceState {
/// The kind of each value, based on its allocation site
values: FxHashMap<ValueId, AbstractValue>,
/// The set of values pointed to by each identifier
variables: FxHashMap<IdentifierId, FxHashSet<ValueId>>,
variables: FxHashMap<IdentifierId, ValueIdSet>,
/// Tracks uninitialized identifier access errors (matches TS invariant).
/// Uses Cell so it can be set from `&self` methods like `kind()`.
/// Stores (IdentifierId, usage_loc) where usage_loc is the source location
@@ -392,8 +490,8 @@ impl InferenceState {
}
};
let mut merged_kind: Option<AbstractValue> = None;
for value_id in values {
let kind = match self.values.get(value_id) {
for value_id in values.iter() {
let kind = match self.values.get(&value_id) {
Some(k) => k,
None => continue,
};
@@ -413,9 +511,8 @@ impl InferenceState {
}
fn define(&mut self, place_id: IdentifierId, value_id: ValueId) {
let mut set = FxHashSet::default();
set.insert(value_id);
self.variables.insert(place_id, set);
self.variables
.insert(place_id, ValueIdSet::single(value_id));
}
fn assign(&mut self, into: IdentifierId, from: IdentifierId) {
@@ -425,8 +522,7 @@ impl InferenceState {
// Create a stable value for uninitialized identifiers
// Use a deterministic ID based on the from identifier
let vid = ValueId(from.0 | 0x80000000);
let mut set = FxHashSet::default();
set.insert(vid);
let set = ValueIdSet::single(vid);
if !self.values.contains_key(&vid) {
self.values.insert(
vid,
@@ -451,7 +547,8 @@ impl InferenceState {
Some(v) => v.clone(),
None => return,
};
let merged: FxHashSet<ValueId> = prev_values.union(&new_values).copied().collect();
let mut merged = prev_values;
merged.union_with(&new_values);
self.variables.insert(place, merged);
}
@@ -461,7 +558,7 @@ impl InferenceState {
fn values_for(&self, place_id: IdentifierId) -> Vec<ValueId> {
match self.variables.get(&place_id) {
Some(values) => values.iter().copied().collect(),
Some(values) => values.iter().collect(),
None => Vec::new(),
}
}
@@ -470,8 +567,8 @@ impl InferenceState {
fn kind_opt(&self, place_id: IdentifierId) -> Option<AbstractValue> {
let values = self.variables.get(&place_id)?;
let mut merged_kind: Option<AbstractValue> = None;
for value_id in values {
let kind = self.values.get(value_id)?;
for value_id in values.iter() {
let kind = self.values.get(&value_id)?;
merged_kind = Some(match merged_kind {
Some(prev) => merge_abstract_values(&prev, kind),
None => kind.clone(),
@@ -558,7 +655,7 @@ impl InferenceState {
fn merge(&self, other: &InferenceState) -> Option<InferenceState> {
let mut next_values: Option<FxHashMap<ValueId, AbstractValue>> = None;
let mut next_variables: Option<FxHashMap<IdentifierId, FxHashSet<ValueId>>> = None;
let mut next_variables: Option<FxHashMap<IdentifierId, ValueIdSet>> = None;
// Merge values present in both
for (id, this_value) in &self.values {
@@ -583,17 +680,11 @@ impl InferenceState {
// Merge variables present in both
for (id, this_values) in &self.variables {
if let Some(other_values) = other.variables.get(id) {
let mut has_new = false;
for ov in other_values {
if !this_values.contains(ov) {
has_new = true;
break;
}
}
let has_new = other_values.iter().any(|ov| !this_values.contains(ov));
if has_new {
let nvars = next_variables.get_or_insert_with(|| self.variables.clone());
let merged: FxHashSet<ValueId> =
this_values.union(other_values).copied().collect();
let mut merged = this_values.clone();
merged.union_with(other_values);
nvars.insert(*id, merged);
}
}
@@ -623,11 +714,11 @@ impl InferenceState {
phi_place_id: IdentifierId,
phi_operands: &IndexMap<BlockId, Place, FxBuildHasher>,
) {
let mut values: FxHashSet<ValueId> = FxHashSet::default();
let mut values = ValueIdSet::default();
for (_, operand) in phi_operands {
if let Some(operand_values) = self.variables.get(&operand.identifier) {
for v in operand_values {
values.insert(*v);
for v in operand_values.iter() {
values.insert(v);
}
}
// If not found, it's a backedge that will be handled later by merge