← Back to context

Comment by 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.

3 comments

pphysch

Reply
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, "");

    • danudey  4 days ago

      The example I linked uses alignas, but the key is knowing what value to pass. std::hardware_destructive_interference_size tells you what the current/target hardware's correct align value is, which is the challenge.