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RFC 0004: HEAP escape analysis #4

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@ysbaddaden ysbaddaden commented Feb 6, 2024

Add an optimization pass to the compiler to detect places where a class instantiation could happen on the stack rather than the GC HEAP.

Preview: https://github.com/crystal-lang/rfcs/blob/rfc-heap-escape-analysis/text/0004-heap-escape-analysis.md

Add an optimization pass to the compiler to detect places where a class instantiation could happen on the stack rather than the GC HEAP.
@ysbaddaden ysbaddaden self-assigned this Feb 6, 2024
@ysbaddaden ysbaddaden marked this pull request as draft February 6, 2024 13:28
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Co-authored-by: Johannes Müller <straightshoota@gmail.com>
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# Drawbacks

Some objects would now be allocated on the stack, which are GC roots that must be scanned conservatively and can't scan precisely. This may have negatively impact GC performance when we implement semi-conservative marking (conservative marking of GC roots _yet_ precise marking of HEAP allocated objects).
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I think it might be possible to reuse a lot of the escape analysis architecture to implement a shadow stack for each fiber which contains only roots. This could be both precise and a far smaller stack than the actual program stack for the GC to scan. Especially given the stack usage will grow when escape analysis is turned on.

I don't know how that tradeoff compares to the LLVM built in stackmap generator, but it's an idea.

@straight-shoota straight-shoota changed the title RFC 0004 : HEAP escape analysis RFC 0004: HEAP escape analysis Feb 8, 2024
Co-authored-by: Linus Sellberg <sellberg@gmail.com>
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4 participants