all: avoid copying arrays in loops

[cristaloleg]

Avoid copying arrays of heavy structs (100 bytes and more) in loops. Found by the go-critic linter.
Linter's log:

check-package: $GOPATH/src/github.com/ethereum/go-ethereum/swarm/api/manifest.go:162:2: rangeExprCopy: copy of trie.entries (2056 bytes) can be avoided with &trie.entries
check-package: $GOPATH/src/github.com/ethereum/go-ethereum/swarm/api/manifest.go:311:2: rangeExprCopy: copy of mt.entries (2056 bytes) can be avoided with &mt.entries
check-package: $GOPATH/src/github.com/ethereum/go-ethereum/swarm/api/manifest.go:365:2: rangeExprCopy: copy of mt.entries (2056 bytes) can be avoided with &mt.entries

check-package: $GOPATH/src/github.com/ethereum/go-ethereum/trie/node.go:58:2: rangeExprCopy: copy of n.Children (272 bytes) can be avoided with &n.Children
check-package: $GOPATH/src/github.com/ethereum/go-ethereum/trie/node.go:101:2: rangeExprCopy: copy of n.Children (272 bytes) can be avoided with &n.Children
check-package: $GOPATH/src/github.com/ethereum/go-ethereum/trie/trie.go:359:3: rangeExprCopy: copy of n.Children (272 bytes) can be avoided with &n.Children

check-package: $GOPATH/src/github.com/ethereum/go-ethereum/p2p/discover/table.go:163:2: rangeExprCopy: copy of tab.buckets (136 bytes) can be avoided with &tab.buckets
check-package: $GOPATH/src/github.com/ethereum/go-ethereum/p2p/discover/table.go:511:2: rangeExprCopy: copy of tab.buckets (136 bytes) can be avoided with &tab.buckets
check-package: $GOPATH/src/github.com/ethereum/go-ethereum/p2p/discover/table.go:527:2: rangeExprCopy: copy of tab.buckets (136 bytes) can be avoided with &tab.buckets
check-package: $GOPATH/src/github.com/ethereum/go-ethereum/p2p/discover/table.go:536:2: rangeExprCopy: copy of tab.buckets (136 bytes) can be avoided with &tab.buckets

check-package: $GOPATH/src/github.com/ethereum/go-ethereum/p2p/discover/table_test.go:598:2: rangeExprCopy: copy of tn.dists (6168 bytes) can be avoided with &tn.dists

check-package: $GOPATH/src/github.com/ethereum/go-ethereum/p2p/discv5/table.go:84:2: rangeExprCopy: copy of tab.buckets (2056 bytes) can be avoided with &tab.buckets
check-package: $GOPATH/src/github.com/ethereum/go-ethereum/p2p/discv5/table.go:96:2: rangeExprCopy: copy of tab.buckets (2056 bytes) can be avoided with &tab.buckets
check-package: $GOPATH/src/github.com/ethereum/go-ethereum/p2p/discv5/table.go:124:2: rangeExprCopy: copy of tab.buckets (2056 bytes) can be avoided with &tab.buckets
check-package: $GOPATH/src/github.com/ethereum/go-ethereum/p2p/discv5/table.go:178:2: rangeExprCopy: copy of tab.buckets (2056 bytes) can be avoided with &tab.buckets

check-package: $GOPATH/src/github.com/ethereum/go-ethereum/p2p/discv5/net_test.go:358:2: rangeExprCopy: copy of tn.dists (6168 bytes) can be avoided with &tn.dists

[quasilyte]

It's not slice copy, but rather array copy, if we're talking about rangeExprCopy. 😉

[cristaloleg]

@quasilyte thanks, I've fixed title & description.

[karalabe]

@cristaloleg Are you sure this isn't automatically optimized by the compiler? Would seem really weird if it actually did a full on copy of the arrays.

[quasilyte]

@karalabe I'm sure and tried to fix it in the compiler. Possible to be optimized, but you need to know that loop is never executed from different threads, otherwise you get different results if array is modified. By go specs, it must be a full copy there.

[cristaloleg]

There is an issue for that: golang/go#15812
(Sadly it's postponed from 1.8 till..)

[karalabe]

Fair enough!

[karalabe]

LGTM

[cristaloleg]

Thanks to everyone!