Implement a complete Rust reimplementation of the LibJS frontend:
lexer, parser, AST, scope collector, and bytecode code generator.
The Rust pipeline is built via Corrosion (CMake-Cargo bridge) and
linked into LibJS as a static library. It is gated behind a build
flag (ENABLE_RUST, on by default except on Windows) and two runtime
environment variables:
- LIBJS_CPP: Use the C++ pipeline instead of Rust
- LIBJS_COMPARE_PIPELINES=1: Run both pipelines in lockstep,
aborting on any difference in AST or bytecode generated.
The C++ side communicates with Rust through a C FFI layer
(RustIntegration.cpp/h) that passes source text to Rust and receives
a populated Executable back via a BytecodeFactory interface.
Per step 13 of ScriptEvaluation in the ECMA-262 spec, the script body
should only be evaluated if GlobalDeclarationInstantiation returned a
normal completion.
This can't currently be triggered since we always create fresh Script
objects, but if we ever start reusing cached executables across
evaluations, this would prevent a subtle bug where the script body
runs despite GDI failing.
AsyncIteratorClose is now fully inlined as bytecode in ASTCodegen.cpp,
using the Await bytecode op to yield naturally. The C++ implementation
used synchronous await() which spins the event loop, violating
assertions when execution contexts are on the stack.
Change the completion_value field from Optional<Value> to Operand
in both IteratorClose and AsyncIteratorClose bytecode instructions.
This allows passing a dynamic value from a register, which is needed
for iterator close on abrupt completion where the exception value
is not known at codegen time.
Remove CodeGenerationError and make all bytecode generation functions
return their results directly instead of wrapping them in
CodeGenerationErrorOr.
For the few remaining sites where codegen encounters an unimplemented
or unexpected AST node, we now use a new emit_todo() helper that emits
a NewTypeError + Throw sequence at compile time (preserving the runtime
behavior) and then switches to a dead basic block so subsequent codegen
for the same function can continue without issue.
This allows us to remove error handling from all callers of the
bytecode compiler, simplifying the code significantly.
After compiling the bytecode executable on first run, null out the
AST (m_parse_node) and clear AnnexB candidates since they are no
longer needed. This frees the memory held by the entire AST for the
script's lifetime.
The parse_node() accessor now returns a nullable pointer. Callers
(js.cpp for AST dumping, Interpreter for first compilation) access
the AST before it is dropped.
Add Script::global_declaration_instantiation() that performs the GDI
algorithm using pre-computed name lists and shared function data
instead of walking the AST.
Runtime checks (has_lexical_declaration, can_declare_global_function,
etc.) remain since they depend on global environment state. AnnexB
iterates pre-collected candidates and calls
set_should_do_additional_annexB_steps() on stored refs.
The Interpreter::run(Script&) now calls the Script method instead of
the Program method.
Replace the runtime uses of formal_parameters() with pre-computed data:
- m_formal_parameter_count stores the parameter count
- m_parameter_names_for_mapped_arguments stores ordered parameter names
for simple parameter lists (used by create_mapped_arguments_object)
Change create_mapped_arguments_object to take Span<Utf16FlyString>
instead of NonnullRefPtr<FunctionParameters const>.
Remove virtual formal_parameters() from FunctionObject as it is no
longer needed.
Replace the ClassExpression const& reference in the NewClass
instruction with a u32 class_blueprint_index. The interpreter now
reads from the ClassBlueprint stored on the Executable and calls
construct_class() instead of the AST-based create_class_constructor().
Literal field initializers (numbers, booleans, null, strings, negated
numbers) are used directly in construct_class() without creating an
ECMAScriptFunctionObject, avoiding function creation overhead for
common field patterns like `x = 0` or `name = "hello"`.
Set class_field_initializer_name on SharedFunctionInstanceData at
codegen time for statically-known field keys (identifiers, private
identifiers, string literals, and numeric literals). For computed
keys, the name is set at runtime in construct_class().
ClassExpression AST nodes are no longer referenced from bytecode.
Replace the FunctionNode const& stored on the NewFunction bytecode
instruction with an index into a table of pre-created
SharedFunctionInstanceData objects on the Executable.
During bytecode compilation, we now eagerly create
SharedFunctionInstanceData for each function that will be
instantiated by NewFunction, and store it on both the FunctionNode
(for caching) and the Executable (for GC tracing).
At runtime, NewFunction simply looks up the SharedFunctionInstanceData
by index and calls create_from_function_data() directly, bypassing
the AST entirely. This removes one of the main reasons the AST had
to stay alive after compilation.
The instantiate_ordinary_function_expression() helper in
Interpreter.cpp is removed as its non-trivial code path (creating a
scope for named function expressions) was dead code -- it was only
called when !has_name(), so the has_own_name branch never executed.
delete super.x and delete super[expr] always throw a ReferenceError
per spec. Instead of deferring this to runtime via DeleteByIdWithThis
and DeleteByValueWithThis instructions, emit the throw directly during
bytecode generation.
Remove the now-unused DeleteByIdWithThis and DeleteByValueWithThis
instructions, and add a NewReferenceError instruction.
This comment referenced the old runtime unwind context stack behavior
where a flag had to be set to prevent yield from going through a
finally statement. That mechanism was removed and finally is now
handled purely through explicit completion records in bytecode.
After replacing the runtime unwind context stack with explicit
completion records for try/finally dispatch, the distinction between
"handler" (catch) and "finalizer" (finally) in the exception handler
table is no longer meaningful at runtime.
handle_exception() checked handler first, then finalizer, but they
did the exact same thing (set the PC). When both were present, the
finalizer was dead code.
Collapse both fields into a single handler_offset (now non-optional,
since an entry always has a target), remove the finalizer concept
from BasicBlock, UnwindContext, and ExceptionHandlers, and simplify
handle_exception() to a direct assignment.
The runtime unwind context stack was pushed by EnterUnwindContext
and popped by LeaveUnwindContext. With both opcodes removed, it is
no longer read or written by anything.
Remove UnwindInfo, the unwind_contexts vector, its GC visit loop,
its copy in ExecutionContext::copy(), and the VERIFY assertions that
referenced it in handle_exception() and catch_exception().
LeaveUnwindContext popped the runtime unwind context stack. With the
stack being removed, all emission sites become dead code. Remove the
opcode and all its emissions.
EnterUnwindContext pushed an UnwindInfo and jumped to entry_point.
Without the stack push, it's just a Jump. Replace the single emission
site with a Jump and remove the opcode entirely.
Replace the saved_lexical_environments stack in ExecutionContextRareData
with explicit register-based environment tracking. Environments are now
stored in registers and restored via SetLexicalEnvironment, making the
environment flow visible in bytecode.
Key changes:
- Add GetLexicalEnvironment and SetLexicalEnvironment opcodes
- CreateLexicalEnvironment takes explicit parent and dst operands
- EnterObjectEnvironment stores new environment in a dst register
- NewClass takes an explicit class_environment operand
- Remove LeaveLexicalEnvironment opcode (instead: SetLexicalEnvironment)
- Remove saved_lexical_environments from ExecutionContextRareData
- Use a reserved register for the saved lexical environment to avoid
dominance issues with lazily-emitted GetLexicalEnvironment
Each finally scope gets two registers (completion_type and
completion_value) that form an explicit completion record. Every path
into the finally body sets these before jumping, and a dispatch chain
after the finally body routes to the correct continuation.
This replaces the old implicit protocol that relied on the exception
register, a saved_return_value register, and a scheduled_jump field
on ExecutionContext, allowing us to remove:
- 5 opcodes (ContinuePendingUnwind, ScheduleJump, LeaveFinally,
RestoreScheduledJump, PrepareYield)
- 1 reserved register (saved_return_value)
- 2 ExecutionContext fields (scheduled_jump, previously_scheduled_jumps)
The spec for PropertyDefinitionEvaluation requires that when evaluating
a property definition with a computed key (PropertyDefinition :
PropertyName : AssignmentExpression), the PropertyName is fully
evaluated (including ToPropertyKey, which calls ToPrimitive) before the
value's AssignmentExpression is evaluated.
Our bytecode compiler was evaluating the key expression first, then
the value expression, and only performing ToPropertyKey later inside
PutByValue at runtime. This meant user-observable side effects from
ToPrimitive (such as calling Symbol.toPrimitive or toString on the key
object) would fire after the value expression had already been
evaluated.
Fix this by using a new ToPrimitiveWithStringHint instruction that
performs ToPrimitive with string hint(!), and emitting it between the
key and value evaluations in ObjectExpression codegen.
After ToPrimitive, the key is already a primitive, so the subsequent
ToPropertyKey inside PutByValue becomes a no-op from the perspective
of user-observable side
effects.
Also update an existing test that was asserting the old (incorrect)
evaluation order, and add comprehensive new tests for computed property
key evaluation order.
Both SetFunctionName and MakeConstructor are already performed by
ECMAScriptFunctionObject::initialize() when the object is created
via create_from_function_node:
- SetFunctionName: The name is passed to SharedFunctionInstanceData,
and initialize() creates the "name" property from it.
- MakeConstructor: has_constructor() returns true for all normal
non-arrow functions, m_constructor_kind defaults to Base, and
m_may_need_lazy_prototype_instantiation handles the prototype
property creation lazily.
The GetGlobal bytecode optimization bypasses the normal environment
record lookup for global variable access. When a global property is
an accessor (getter), the receiver passed to the getter must be the
global object, not undefined.
The spec's Get(O, P) abstract operation is defined as O.[[Get]](P, O),
meaning the object itself is always the receiver. The global
environment's GetBindingValue delegates to its object record's
GetBindingValue, which calls Get(bindingObject, N), so the receiver
should be the binding object (the global object).
Both the cached path (calling the getter directly from get_direct)
and the non-cached path (calling internal_get) were passing
js_undefined() as the receiver. This caused strict-mode getters on
global properties to receive undefined as their this-value instead
of globalThis.
Notably, the corresponding SetGlobal paths already correctly passed
&binding_object for setter calls.
The i32 multiplication fast path in Mul::execute_impl was producing
+0 instead of -0 when one operand was negative and the other was
zero (e.g. `var a = -1, b = 0; a * b`).
This happened because i32 can't represent -0, so `Value(0)` was
always positive zero. We now fall through to the double path when
the i32 result is zero, which correctly handles the sign.
Also add comprehensive multiplication tests covering negative zero,
basic arithmetic, large integers, type coercion, NaN, and Infinity.
There is no need to concat empty string literals when building template
literals. Now strings will only be concatenated if they need to be.
To handle the edge case where the first segment is not a string
literal, a new `ToString` op code has been added to ensure the value is
a string concatenating more strings.
In addition, basic const folding is now supported for template literal
constants (templates with no interpolated values), which is commonly
used for multi-line string constants.
Previously, when direct eval() was called, we would mark the entire
environment chain as "permanently screwed by eval", disabling variable
access caching all the way up to the global scope.
This was overly conservative. According to the ECMAScript specification,
a sloppy direct eval() can only inject var declarations into its
containing function's variable environment - it cannot inject variables
into parent function scopes.
This patch makes two changes:
1. Stop propagating the "screwed by eval" flag at function boundaries.
When set_permanently_screwed_by_eval() hits a FunctionEnvironment or
GlobalEnvironment, it no longer continues to outer environments.
2. Check each environment during cache lookup traversal. If any
environment in the path is marked as screwed, we bail to the slow
path. This catches the case where we're inside a function with eval
and have a cached coordinate pointing to an outer scope.
The second change is necessary because eval can create local variables
that shadow outer bindings. When looking up a variable from inside a
function that called eval, we can't trust cached coordinates that point
to outer scopes, since eval may have created a closer binding.
This improves performance for code with nested functions where an inner
function uses eval but parent functions perform many variable accesses.
The parent functions can now use cached environment coordinates.
All 29 new tests verify behavior matches V8.
Every function call allocates an ExecutionContext with a trailing array
of Values for registers, locals, constants, and arguments. Previously,
the constructor would initialize all slots to js_special_empty_value(),
but constant slots were then immediately overwritten by the interpreter
copying in values from the Executable before execution began.
To eliminate this redundant initialization, we rearrange the layout from
[registers | constants | locals] to [registers | locals | constants].
This groups registers and locals together at the front, allowing us to
initialize only those slots while leaving constant slots uninitialized
until they're populated with their actual values.
This reduces the per-call initialization cost from O(registers + locals
+ constants) to O(registers + locals).
Also tightens up the types involved (size_t -> u32) and adds VERIFYs to
guard against overflow when computing the combined slot counts, and to
ensure the total fits within the 29-bit operand index field.
When a function creates object literals with simple property names,
we now cache the resulting shape after the first instantiation. On
subsequent calls, we create the object with the cached shape directly
and write property values at their known offsets.
This avoids repeated shape transitions and property offset lookups
for a common JavaScript pattern.
The optimization uses two new bytecode instructions:
- CacheObjectShape: Captures the final shape after object construction
- InitObjectLiteralProperty: Writes properties using cached offsets
Only "simple" object literals are optimized (string literal keys with
simple value expressions). Complex cases like computed properties,
getters/setters, and spread elements use the existing slow path.
3.4x speedup on a microbenchmark that repeatedly instantiates an object
literal with 26 properties. Small progressions on various benchmarks.
We know the length they're gonna end up with up front since we're
instantiating array literals. Pre-sizing them allows us to skip
incremental resizing of the property storage.
This adds visit_edges(Cell::Visitor&) methods to various helper structs
that contain GC pointers, and makes sure they are called from owning
GC-heap-allocated objects as needed.
These were found by our Clang plugin after expanding its capabilities.
The added rules will be enforced by CI going forward.
This resolves a FIXME in its code generation, particularly for:
- Caching the template object
- Setting the correct property attributes
- Freezing the resulting objects
This allows archive.org to load, which uses the Lit library.
The Lit library caches these template objects to determine if a
template has changed, allowing it to determine to do a full template
rerender or only partially update the rendering. Before, we would
always cause a full rerender on update because we didn't return the
same template object.
This caused issues with archive.org's code, I believe particularly with
its router library, where we would constantly detach and reattach nodes
unexpectedly, ending up with the page content not being attached to the
router's custom element.
Instead of storing a list of builtin function objects with the realm,
just move the builtin field from NativeFunction up to FunctionObject.
Now you can ask any FunctionObject for its builtin(), and we no longer
need the get_builtin_value() API.
Fixes 10 test262 tests that were querying the realm builtins at a
bad time.
Regressed in 54b755126c.
Reorder members and use u32 instead of Optional<u32> for things that
didn't actually need the "empty" state other than for assertions.
Reduces memory usage on my x.com home feed by 9.9 MiB.
For StringPrototype functions that defer to RegExpPrototype builtins,
we can skip the generic call stuff (eliding the execution context etc)
and just call the builtin directly.
1.03x speedup on Octane/regexp.js
These were helpful when PropertyKey instantiation happened in the
interpreter, but now that we've moved it to bytecode generation time,
we can use the basic Put*ById* instructions instead.
Instead of creating PropertyKeys on the fly during interpreter
execution, we now store fully-formed ones in the Executable.
This avoids a whole bunch of busywork in property access instructions
and substantially reduces code size bloat.
These instructions are not necessarily rarely used, but they are very
large in terms of code size. By putting them out of line we keep the hot
path of the interpreter smaller and tighter.
