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LibJS: Fast non-local variable access :^)

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This patch introduces the "environment coordinate" concept, which
encodes the distance from a variable access to the binding it ends up
resolving to.

EnvironmentCoordinate has two fields:

    - hops:  The number of hops up the lexical environment chain we have
             to make before getting to the resolved binding.

    - index: The index of the resolved binding within its declarative
             environment record.

Whenever a variable lookup resolves somewhere inside a declarative
environment, we now cache the coordinates and reuse them in subsequent
lookups. This is achieved via a coordinate cache in JS::Identifier.

Note that non-strict direct eval() breaks this optimization and so it
will not be performed if the resolved environment has been permanently
screwed by eval().

This makes variable access *significantly* faster. :^)
This commit is contained in:
Andreas Kling 2021-10-07 01:06:21 +02:00
parent 421845b0cd
commit 41a072bded
8 changed files with 65 additions and 12 deletions
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10
Userland/Libraries/LibJS/Runtime/VM.cpp
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@ -435,7 +435,7 @@ ThrowCompletionOr<void> VM::iterator_binding_initialization(BindingPattern const
}
// 9.1.2.1 GetIdentifierReference ( env, name, strict ), https://tc39.es/ecma262/#sec-getidentifierreference
Reference VM::get_identifier_reference(Environment* environment, FlyString name, bool strict)
Reference VM::get_identifier_reference(Environment* environment, FlyString name, bool strict, size_t hops)
{
// 1. If env is the value null, then
if (!environment) {
@ -448,10 +448,14 @@ Reference VM::get_identifier_reference(Environment* environment, FlyString name,
if (exception())
return {};
Optional<EnvironmentCoordinate> environment_coordinate;
if (index.has_value())
environment_coordinate = EnvironmentCoordinate { .hops = hops, .index = index.value() };
if (exists)
return Reference { *environment, move(name), strict, index };
return Reference { *environment, move(name), strict, environment_coordinate };
else
return get_identifier_reference(environment->outer_environment(), move(name), strict);
return get_identifier_reference(environment->outer_environment(), move(name), strict, hops + 1);
}
// 9.4.2 ResolveBinding ( name [ , env ] ), https://tc39.es/ecma262/#sec-resolvebinding