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Comment by gethly

5 days ago

Most of this should be handled by the compiler already. But it is only 2025, I guess we're just not ready for it.

11 comments

gethly

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jerf  5 days ago

Are you thinking of some sort of annotation the compiler could read and handle?

Because if a compiler starts automatically padding all my structures to put all of the members on their own cache line I'm going to be quite peeved. It would be easy for it to do, yes, but it would be wrong 99%+ of the time.

A far more trenchant complaint is that Go won't automatically sort struct members if necessary to shrink them and you have to apply a linter to get that at linting time if you want it.

danudey  5 days ago

I'm not sure if golang has the same fundamental issues in common use, but in e.g. C you don't want the compiler reordering your structs or adding arbitrary padding because that makes it incompatible with other in-memory representations - e.g. if you're using shared memory with another process that hasn't received the same optimizations, if you're loading raw data into memory/using mmap, etc.

Likewise, one of the examples is moving from an array of structs to a struct of arrays; that's a lot more complex of a code reorganization than you'd want a compiler doing.

It would be good to have a static analyzer that could suggest these changes, but, at least in many cases, you don't want them done automatically.

jandrewrogers  5 days ago

How would that even work? The layout of data structures are constrained by many invariants not visible to the compiler (see also: auto-vectorization). It would be more work and boilerplate to add sufficient annotations to a data structure to enable the compiler to safely modify the layout than just using the layout you want.

  • mappu  4 days ago

    Some languages like Odin, ISPC, and Jai all have annotations that can automatically transform AoS to SoA. A key benefit is you can easily experiment to see if this helps your application, without doing a major refactor.

    In https://github.com/golang/go/issues/64926 it was a bridge-too-far for the Go developers (fair enough) but maybe it could still happen one day.

pphysch  5 days ago

Not really, virtually all these patterns involve tradeoffs that require understanding the data access patterns.

I don't want my compiler adding more padding than bare minimum to every struct. I don't want it transforming an AoS to SoA when I choose AoS to match data access patterns. And so on...

At best Go could add some local directives for compiling these optimizations, but these code changes are really minimal anyways. I would rather see the padding explicitly than some abstract directive.

  • danudey  5 days ago

    I could imagine some kind of compiler declaration in C that would do something like specify break points - sort of like page breaks - for structs, or tell the compiler to automatically pad structs out so that components are on page boundaries, cache line boundaries, etc. Sort of "If we're not properly aligned, add whatever padding you think is best here".

    I guess this is largely provided by std::hardware_destructive_interference_size in C++17, but I'm not sure if there are other language equivalents.

    https://en.cppreference.com/w/cpp/thread/hardware_destructiv...

    • mtklein  5 days ago

      I think this is _Alignas/alignas.

          struct foo {
        _Alignas(64) float x,y;
        _Alignas(64) int     z;
    };
    _Static_assert(sizeof(struct foo) == 192, "");

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