perf: outline logic in Decode to allow for stack allocations

[Jorropo]

I took extra efforts for this to be a backward compatible change, I think DecodedMultihash should return a value struct not a pointer.

I also updated the error type to a value because this allows for 1 instead of 2 allocations when erroring.

name       old time/op    new time/op    delta
Decode-12     102ns ± 3%      18ns ± 3%   -82.47%  (p=0.000 n=9+9)

name       old alloc/op   new alloc/op   delta
Decode-12     64.0B ± 0%      0.0B       -100.00%  (p=0.000 n=10+10)

name       old allocs/op  new allocs/op  delta
Decode-12      1.00 ± 0%      0.00       -100.00%  (p=0.000 n=10+10)

I originally found this problem by benchmarking go-cid:

github.com/ipfs/go-cid.CidFromBytes

/home/hugo/go/pkg/mod/github.com/ipfs/[email protected]/cid.go

  Total:      4.64GB    10.75GB (flat, cum)   100%
    638            .          .           	if len(data) > 2 && data[0] == mh.SHA2_256 && data[1] == 32 {
    639            .          .           		if len(data) < 34 {
    640            .          .           			return 0, Undef, ErrInvalidCid{fmt.Errorf("not enough bytes for cid v0")}
    641            .          .           		}
    642            .          .
    643            .     6.11GB           		h, err := mh.Cast(data[:34])
                                                               _, err := Decode(buf)                                        multihash.go:215

    644            .          .           		if err != nil {
    645            .          .           			return 0, Undef, ErrInvalidCid{err}
    646            .          .           		}

We can see it call mh.Cast and mh.Cast call Decode and instantly drops the DecodedMultihash. The point of this is purely to validate the multihash.

[marten-seemann]

Can we have a test that asserts the 0 allocs are happening?

[github-actions]

Suggested version: v0.2.3

Comparing to: v0.2.2 (diff)

Changes in go.mod file(s):

(empty)

gorelease says:

# summary
Suggested version: v0.2.3

gocompat says:

Your branch is up to date with 'origin/master'.

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