proposal: cmd/compile: type layout optimization: apply struct field reordering during compile time for efficient memory alignment

[Dentrax]

Context

The goal is to divorce the order of fields in source code from the order of struct fields, then optimize structs by always ordering fields from least to most aligned in memory layout. ¹

Proposal

type Unoptimized struct {
  a int8
  b int16
  c int8
  ...
}

During compile time optimization phase, Go should try to optimize reordering for all structs and their fields.

type LetGoOptimize struct {
  a int8
  b int8
  c int16
  ...
}

• Playground: https://go.dev/play/p/kp3o9k5fKxQ
• Prints: 6, 4, respectively.

golang.org/x/tools already have a linter called fieldalignment that detects structs that would use less memory if their fields were sorted.

Backward Compatibility

I don't have much technical detail about the compiler to cover this section. So waiting your thoughts!

• New go:noreorder compiler directive

The //go:noreorder directive must be followed by a type struct declaration. It specifies that auto reordering should not be applied, overriding the compiler's usual optimization rules.

Similar Works

• Rust - struct field reordering and optimization by @ahicks92

P.S: I couldn't find neither a similar proposal her nor discussions; so feel free to close and drop some references if this is duplicate or have discussed before.

Footnotes

1. https://go101.org/article/memory-layout.html ↩

[seankhliao]

Duplicate of #36606

[AndrewWPhillips]

This is not a duplicate of #36606 as that issue does not propose any reordering of fields. (However, I have seen this issue discussed before and would be surprised if this is not a duplicate of a much earlier issue but I can't seem to find one.)

[dominikh]

It's more likely a duplicate of #10014.