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LibJS: Avoid IteratorRecord GC-allocation in GetIterator instruction

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With this change, `GetIterator` no longer GC-allocates an
`IteratorRecord`. Instead, it stores the iterator record fields in
bytecode registers. This avoids per-iteration allocations in patterns
like: `for (let [x] of array) {}`.

`IteratorRecord` now inherits from `IteratorRecordImpl`, which holds the
iteration state. This allows the existing iteration helpers
(`iterator_next()`, `iterator_step()`, etc.) operate on both the
GC-allocated and the register-backed forms.

Microbenchmarks:
1.1x array-destructuring-assignment-rest.js
1.226x array-destructuring-assignment.js
This commit is contained in:
Aliaksandr Kalenik 2025-10-27 19:46:54 +01:00 • committed by Alexander Kalenik
parent a4e3890c05
commit 646457099c
Notes: github-actions[bot] 2025-11-02 19:06:58 +00:00
18 changed files with 252 additions and 235 deletions
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49
Libraries/LibJS/Bytecode/ASTCodegen.cpp
View file

@ -1434,8 +1434,10 @@ static Bytecode::CodeGenerationErrorOr<void> generate_array_binding_pattern_byte
auto is_iterator_exhausted = generator.allocate_register();
generator.emit_mov(is_iterator_exhausted, generator.add_constant(Value(false)));
auto iterator = generator.allocate_register();
generator.emit<Bytecode::Op::GetIterator>(iterator, input_array);
auto iterator_object = generator.allocate_register();
auto iterator_next_method = generator.allocate_register();
auto iterator_done_property = generator.allocate_register();
generator.emit<Bytecode::Op::GetIterator>(iterator_object, iterator_next_method, iterator_done_property, input_array);
bool first = true;
auto assign_value_to_alias = [&](auto& alias, ScopedOperand value) {
@ -1469,7 +1471,7 @@ static Bytecode::CodeGenerationErrorOr<void> generate_array_binding_pattern_byte
if (first) {
// The iterator has not been called, and is thus known to be not exhausted
generator.emit<Bytecode::Op::IteratorToArray>(value, iterator);
generator.emit<Bytecode::Op::IteratorToArray>(value, iterator_object, iterator_next_method, iterator_done_property);
} else {
auto& if_exhausted_block = generator.make_block();
auto& if_not_exhausted_block = generator.make_block();
@ -1487,7 +1489,7 @@ static Bytecode::CodeGenerationErrorOr<void> generate_array_binding_pattern_byte
generator.emit<Bytecode::Op::Jump>(Bytecode::Label { continuation_block });
generator.switch_to_basic_block(if_not_exhausted_block);
generator.emit<Bytecode::Op::IteratorToArray>(value, iterator);
generator.emit<Bytecode::Op::IteratorToArray>(value, iterator_object, iterator_next_method, iterator_done_property);
generator.emit<Bytecode::Op::Jump>(Bytecode::Label { continuation_block });
generator.switch_to_basic_block(continuation_block);
@ -1510,7 +1512,7 @@ static Bytecode::CodeGenerationErrorOr<void> generate_array_binding_pattern_byte
}
auto value = generator.allocate_register();
generator.emit<Bytecode::Op::IteratorNextUnpack>(value, is_iterator_exhausted, iterator);
generator.emit<Bytecode::Op::IteratorNextUnpack>(value, is_iterator_exhausted, iterator_object, iterator_next_method, iterator_done_property);
// We still have to check for exhaustion here. If the iterator is exhausted,
// we need to bail before trying to get the value
@ -1571,7 +1573,7 @@ static Bytecode::CodeGenerationErrorOr<void> generate_array_binding_pattern_byte
Bytecode::Label { not_done_block });
generator.switch_to_basic_block(not_done_block);
generator.emit<Bytecode::Op::IteratorClose>(iterator, Completion::Type::Normal, Optional<Value> {});
generator.emit<Bytecode::Op::IteratorClose>(iterator_object, iterator_next_method, iterator_done_property, Completion::Type::Normal, Optional<Value> {});
generator.emit<Bytecode::Op::Jump>(Bytecode::Label { done_block });
generator.switch_to_basic_block(done_block);
@ -1980,17 +1982,12 @@ Bytecode::CodeGenerationErrorOr<Optional<ScopedOperand>> YieldExpression::genera
auto value = TRY(m_argument->generate_bytecode(generator)).value();
// 4. Let iteratorRecord be ? GetIterator(value, generatorKind).
auto iterator_record = generator.allocate_register();
auto iterator_hint = generator.is_in_async_generator_function() ? IteratorHint::Async : IteratorHint::Sync;
generator.emit<Bytecode::Op::GetIterator>(iterator_record, value, iterator_hint);
// 5. Let iterator be iteratorRecord.[[Iterator]].
auto iterator = generator.allocate_register();
generator.emit<Bytecode::Op::GetObjectFromIteratorRecord>(iterator, iterator_record);
// Cache iteratorRecord.[[NextMethod]] for use in step 7.a.i.
auto next_method = generator.allocate_register();
generator.emit<Bytecode::Op::GetNextMethodFromIteratorRecord>(next_method, iterator_record);
auto iterator_done_property = generator.allocate_register();
auto iterator_hint = generator.is_in_async_generator_function() ? IteratorHint::Async : IteratorHint::Sync;
generator.emit<Bytecode::Op::GetIterator>(iterator, next_method, iterator_done_property, value, iterator_hint);
// 6. Let received be NormalCompletion(undefined).
// See get_received_completion_type_and_value above.
@ -2165,11 +2162,11 @@ Bytecode::CodeGenerationErrorOr<Optional<ScopedOperand>> YieldExpression::genera
// 3. If generatorKind is async, perform ? AsyncIteratorClose(iteratorRecord, closeCompletion).
if (generator.is_in_async_generator_function()) {
// FIXME: This performs `await` outside of the generator!
generator.emit<Bytecode::Op::AsyncIteratorClose>(iterator_record, Completion::Type::Normal, Optional<Value> {});
generator.emit<Bytecode::Op::AsyncIteratorClose>(iterator, next_method, done, Completion::Type::Normal, Optional<Value> {});
}
// 4. Else, perform ? IteratorClose(iteratorRecord, closeCompletion).
else {
generator.emit<Bytecode::Op::IteratorClose>(iterator_record, Completion::Type::Normal, Optional<Value> {});
generator.emit<Bytecode::Op::IteratorClose>(iterator, next_method, done, Completion::Type::Normal, Optional<Value> {});
}
// 5. NOTE: The next step throws a TypeError to indicate that there was a yield* protocol violation: iterator does not have a throw method.
@ -3049,7 +3046,9 @@ enum class IterationKind {
struct ForInOfHeadEvaluationResult {
bool is_destructuring { false };
LHSKind lhs_kind { LHSKind::Assignment };
Optional<ScopedOperand> iterator;
Optional<ScopedOperand> iterator_object;
Optional<ScopedOperand> iterator_next_method;
Optional<ScopedOperand> iterator_done_property;
};
static Bytecode::CodeGenerationErrorOr<ForInOfHeadEvaluationResult> for_in_of_head_evaluation(Bytecode::Generator& generator, IterationKind iteration_kind, Variant<NonnullRefPtr<ASTNode const>, NonnullRefPtr<BindingPattern const>> const& lhs, NonnullRefPtr<ASTNode const> const& rhs)
{
@ -3122,7 +3121,9 @@ static Bytecode::CodeGenerationErrorOr<ForInOfHeadEvaluationResult> for_in_of_he
// 5. Let exprValue be ? GetValue(exprRef).
// NOTE: No need to store this anywhere.
auto iterator = generator.allocate_register();
auto iterator_object = generator.allocate_register();
auto iterator_next_method = generator.allocate_register();
auto iterator_done_property = generator.allocate_register();
// 6. If iterationKind is enumerate, then
if (iteration_kind == IterationKind::Enumerate) {
@ -3144,7 +3145,7 @@ static Bytecode::CodeGenerationErrorOr<ForInOfHeadEvaluationResult> for_in_of_he
// c. Let iterator be EnumerateObjectProperties(obj).
// d. Let nextMethod be ! GetV(iterator, "next").
// e. Return the Iterator Record { [[Iterator]]: iterator, [[NextMethod]]: nextMethod, [[Done]]: false }.
generator.emit<Bytecode::Op::GetObjectPropertyIterator>(iterator, object);
generator.emit<Bytecode::Op::GetObjectPropertyIterator>(iterator_object, iterator_next_method, iterator_done_property, object);
}
// 7. Else,
else {
@ -3154,10 +3155,12 @@ static Bytecode::CodeGenerationErrorOr<ForInOfHeadEvaluationResult> for_in_of_he
auto iterator_kind = iteration_kind == IterationKind::AsyncIterate ? IteratorHint::Async : IteratorHint::Sync;
// d. Return ? GetIterator(exprValue, iteratorKind).
generator.emit<Bytecode::Op::GetIterator>(iterator, object, iterator_kind);
generator.emit<Bytecode::Op::GetIterator>(iterator_object, iterator_next_method, iterator_done_property, object, iterator_kind);
}
result.iterator = iterator;
result.iterator_object = iterator_object;
result.iterator_next_method = iterator_next_method;
result.iterator_done_property = iterator_done_property;
return result;
}
@ -3199,7 +3202,7 @@ static Bytecode::CodeGenerationErrorOr<Optional<ScopedOperand>> for_in_of_body_e
auto done = generator.allocate_register();
if (iterator_kind == IteratorHint::Sync) {
generator.emit<Bytecode::Op::IteratorNextUnpack>(next_value, done, *head_result.iterator);
generator.emit<Bytecode::Op::IteratorNextUnpack>(next_value, done, *head_result.iterator_object, *head_result.iterator_next_method, *head_result.iterator_done_property);
auto& loop_continue = generator.make_block();
generator.emit_jump_if(
@ -3209,7 +3212,7 @@ static Bytecode::CodeGenerationErrorOr<Optional<ScopedOperand>> for_in_of_body_e
generator.switch_to_basic_block(loop_continue);
} else {
auto next_result = generator.allocate_register();
generator.emit<Bytecode::Op::IteratorNext>(next_result, *head_result.iterator);
generator.emit<Bytecode::Op::IteratorNext>(next_result, *head_result.iterator_object, *head_result.iterator_next_method, *head_result.iterator_done_property);
// b. If iteratorKind is async, set nextResult to ? Await(nextResult).
auto received_completion = generator.allocate_register();