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LibJS: Fast non-local variable access :^)
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. :^)
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test("basic that non-strict direct eval() prevents non-local access caching", () => {
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function foo(do_eval) {
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var c = 1;
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function bar(do_eval) {
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if (do_eval) eval("var c = 2;");
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return c;
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}
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return bar(do_eval);
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}
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expect(foo(false)).toBe(1);
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expect(foo(true)).toBe(2);
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});
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