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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
parent aede010096
commit 4d5823a5bc
Notes: sideshowbarker 2024-07-17 06:20:50 +09:00 
Author: https://github.com/kalenikaliaksandr
Commit: https://github.com/SerenityOS/serenity/commit/4d5823a5bc
Pull-request: https://github.com/SerenityOS/serenity/pull/24194
12 changed files with 94 additions and 46 deletions
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2
Userland/Libraries/LibWeb/WebDriver/ExecuteScript.cpp
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@@ -266,7 +266,7 @@ static JS::ThrowCompletionOr<JS::Value> execute_a_function_body(Web::Page& page,
// The result of parsing global scope above.
// strict
// The result of parsing strict above.
auto function = JS::ECMAScriptFunctionObject::create(realm, "", move(source_text), function_expression->body(), function_expression->parameters(), function_expression->function_length(), function_expression->local_variables_names(), &global_scope, nullptr, function_expression->kind(), function_expression->is_strict_mode(), function_expression->might_need_arguments_object(), contains_direct_call_to_eval);
auto function = JS::ECMAScriptFunctionObject::create(realm, "", move(source_text), function_expression->body(), function_expression->parameters(), function_expression->function_length(), function_expression->local_variables_names(), &global_scope, nullptr, function_expression->kind(), function_expression->is_strict_mode(), function_expression->uses_this(), function_expression->might_need_arguments_object(), contains_direct_call_to_eval);
// 9. Let completion be Function.[[Call]](window, parameters) with function as the this value.
// NOTE: This is not entirely clear, but I don't think they mean actually passing `function` as