What is the meaning of the @overlay macro?

General Usage
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I saw this here: allow `@overlay` for methods with return type declaration by aviatesk · Pull Request #51054 · JuliaLang/julia · GitHub

But I could not find any documentation about it. Does someone know?

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From the source code docs:

This can be used to implement a method override mechanism. Regular compilation will not
consider these methods, and you should customize the compilation flow to look in these
method tables.

Also - it makes a reference to OverlayMethodTable.

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But what would it be good for? Any example?

Or is it just for internal use?

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I do not suggest I studied it or that I have a deep understanding - but from what I’ve searched so far, this seems a good place where you can find some use cases: https://github.com/JuliaLang/julia/blob/master/test/compiler/AbstractInterpreter.jl

Also - keep in mind that the PR referenced by you only extends the macro to work with methods with return types. @overaly has been part of the experimental for a pretty long time.

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It’s generally used for low level replacements of specific functions in the GPU ecosystem, with versions that e.g. don’t throw errors or have some other special GPU implementation.

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6

From userland, you can use the functionality via GitHub - JuliaDebug/CassetteOverlay.jl: An experimental simple method overlay mechanism for Julia.

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