cmd/compile: make compiler reorganize user structs without having user code modify fields so userland readability can come first

[odeke-em]

Coming here from an offline discussion with @aclements, in regards to a new static analysis pass "structslop" that Orijtech, Inc. just released and related issues:

• runtime: sloppy struct field arrangement could be re-arranged for more compact structs and save RAM #42412
• net: sloppy struct field arrangement could be re-arranged for more compact structs #42415
• cmd/compile: rearrange hairyVisitors to reduce struct slop and reclaim 33.33% space wasted #42416

The concern is that by having a static analysis pass that coerces users into statically optimized changes that might have adverse effects, like ruining their readability, ruining co-location and cache line blowing.

A compiler pass as @aclements mentioned would solve these issues, but another benefit would be scalable development whereby every Go developer would get these benefits without having to run a separate pass, it won't interfere with code reviews nor blindly make recommendations that few people can reason about.

/cc @cuonglm @martisch @randall77 @aclements @dr2chase @mknyszek.

[odeke-em]

A somewhat similar proposal that's currently on hold #36606, brought to my attention offline by @rsc.

[rsc]

We can't reorder fields until there is a way to signal "don't reorder fields". That way was going to be and may still be //go:packed (#36606), but that work is put to the side to focus on generics.

Past discussion about reordering struct fields is #10014.

[rsc]

This issue can probably be closed as a duplicate of #36606. I don't think we need to keep both open.

[odeke-em]

SGTM, thank you @rsc for chiming in with the background.