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LibWeb+LibJS: Skip function environment allocation if possible

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If a function has the following properties:
- uses only local variables and registers
- does not use `this`
- does not use `new.target`
- does not use `super`
- does not use direct eval() calls

then it is possible to entirely skip function environment allocation
because it will never be used

This change adds gathering of information whether a function needs to
access `this` from environment and updates `prepare_for_ordinary_call()`
to skip allocation when possible.

For now, this optimisation is too aggressively blocked; e.g. if `this`
is used in a function scope, then all functions in outer scopes have to
allocate an environment. It could be improved in the future, although
this implementation already allows skipping >80% of environment
allocations on Discord, GitHub and Twitter.
This commit is contained in:
Aliaksandr Kalenik 2024-05-03 22:54:12 +02:00 • committed by Andreas Kling
commit 4d5823a5bc
Notes: sideshowbarker 2024-07-17 06:20:50 +09:00
12 changed files with 94 additions and 46 deletions
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2
Userland/Libraries/LibWeb/DOM/EventTarget.cpp
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@ -485,7 +485,7 @@ WebIDL::CallbackType* EventTarget::get_current_value_of_event_handler(FlyString
// 6. Return scope. (NOTE: Not necessary)
auto function = JS::ECMAScriptFunctionObject::create(realm, name.to_deprecated_fly_string(), builder.to_byte_string(), program->body(), program->parameters(), program->function_length(), program->local_variables_names(), scope, nullptr, JS::FunctionKind::Normal, program->is_strict_mode(), program->might_need_arguments_object(), is_arrow_function);
auto function = JS::ECMAScriptFunctionObject::create(realm, name.to_deprecated_fly_string(), builder.to_byte_string(), program->body(), program->parameters(), program->function_length(), program->local_variables_names(), scope, nullptr, JS::FunctionKind::Normal, program->is_strict_mode(), program->uses_this(), program->might_need_arguments_object(), is_arrow_function);
// 10. Remove settings object's realm execution context from the JavaScript execution context stack.
VERIFY(vm.execution_context_stack().last() == &settings_object.realm_execution_context());