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Go Optimization Guide

(goperf.dev)
480 points jedeusus | 1 comments | 31 Mar 25 20:29 UTC | HN request time: 0s | source
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dennis-tra ◴[01 Apr 25 12:15 UTC] No.43545831[source]▶
Can someone explain to me why the compiler can’t do struct-field-alignment? This feels like something that can easily be automated.
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CamouflagedKiwi ◴[01 Apr 25 12:20 UTC] No.43545865[source]▶
Because the order of fields can be significant. It's very relevant for syscalls, and is observable via the reflect package; it'd be strange if the field order was arbitrarily changed (and might change further between releases).

I assume the thinking was that this is pretty easy to optimise if you care, and if it's on by default there'd then have to be some opt-out which there isn't a good mechanism for.

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kbolino ◴[01 Apr 25 12:31 UTC] No.43545977[source]▶
In particular, struct field alignment matches C (even without cgo) and so any change to the default would break a lot of code.
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9rx ◴[01 Apr 25 18:26 UTC] No.43549957[source]▶
> struct field alignment matches C (even without cgo)

The spec defines alignment for numeric types, but that's about it. There is nothing in the spec about struct layout. That is implementation dependent. If you are relying on a particular implementation, you are decidedly in unsafe territory.

> so any change to the default would break a lot of code.

The compiler can disable optimization on cgo calls automatically and most other places where it matters are via the standard library, so it might not be as much as you think. And if you still have a case where it matters, that is what this is for: https://pkg.go.dev/structs#HostLayout

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1. kbolino ◴[01 Apr 25 19:00 UTC] No.43550258{3}[source]▶
That's good to know. I'm not making use of this assumption, but the purego package (came from ebitengine) does. It looks like they're aware of HostLayout [1] but I'm not sure how many other people have gotten the memo (HostLayout didn't exist before Go 1.23).

[1]: https://github.com/ebitengine/purego/issues/259