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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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2
Userland/Libraries/LibJS/Runtime/VM.h
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@ -202,7 +202,7 @@ public:
FlyString unwind_until_label() const { return m_unwind_until_label; }
Reference resolve_binding(FlyString const&, Environment* = nullptr);
Reference get_identifier_reference(Environment*, FlyString, bool strict);
Reference get_identifier_reference(Environment*, FlyString, bool strict, size_t hops = 0);
template<typename T, typename... Args>
void throw_exception(GlobalObject& global_object, Args&&... args)