Description
cmd/compile/internal/gc/alg.go func geneq contains this code:
// Find maximal length run of memory-only fields.
size, next := memrun(t, i)
if s := fields[i:next]; len(s) <= 2 {
// Two or fewer fields: use plain field equality.
for _, f := range s {
and(eqfield(np, nq, f.Sym))
}
} else {
// More than two fields: use memequal.
and(eqmem(np, nq, f.Sym, size))
}The idea here is to inline small, simple memory comparisons instead of calling runtime.memequal.
However, using the number of fields isn't a great heuristic. If one of those fields is (say) [64]uint64, inlining it turns into a runtime call anyway. If one of those fields is (say) struct { x, y, z, w uint64 }, we end up inlining four comparisons instead of the intended one. The heuristic fails in the other direction too: If there are eight fields, but each has type byte, we'd be better off doing a single uint64 comparison than calling runtime.memequal.
A better plan would be to calculate how much work we actually have to do and then do it optimally, either with the minimal number of inlined comparisons or with a single runtime call.
Roughly, if we need to compare n bytes, figure out how many comparisons are required to accomplish that, and then use that as a threshold. (We already do something like that in walk.go's walkCompareString when comparing a non-constant string to a constant string.) One complication is alignment. On architectures with alignment requirements, comparing 8 bytes could require (say) a 2 byte comparison + a 4 byte comparison + a 2 byte comparison. The other thing that requires care is pointers: Given struct { f float32; p *int; u uint32 }, we don't want to pull half of p into one register and the other half of p into another register.
Ideally, we would also use this work calculation in our decision about whether to inline equality comparisons in the first place or call a generated routine (walk.go func walkcompare); we currently count fields there, too.
I believe that this would be challenging but possible for someone with relatively little compiler experience, so marking as help wanted (and spelling things out in more detail than usual).