sync: reduce OnceFunc (and variants) heap allocations
The lifetime of the variables are identical; capture
them in a single struct to avoid individual allocations.
The inner closure can also avoid allocation by using the
capture of the outer closure.
Escape analysis for OnceValues:
/go/src/sync/oncefunc.go:74:29: moved to heap: sync.f
/go/src/sync/oncefunc.go:76:3: moved to heap: sync.once
/go/src/sync/oncefunc.go:77:3: moved to heap: sync.valid
/go/src/sync/oncefunc.go:78:3: moved to heap: sync.p
/go/src/sync/oncefunc.go:79:3: moved to heap: sync.r1
/go/src/sync/oncefunc.go:80:3: moved to heap: sync.r2
/go/src/sync/oncefunc.go:82:7: func literal escapes to heap
/go/src/sync/oncefunc.go:83:9: func literal does not escape
/go/src/sync/oncefunc.go:93:9: func literal escapes to heap
After provided changes:
/go/src/sync/oncefunc.go:86:2: moved to heap: sync.d
/go/src/sync/oncefunc.go:96:9: func literal escapes to heap
/go/src/sync/oncefunc.go:99:13: func literal does not escape
/go/src/sync/oncefunc.go:100:10: func literal does not escape
Change-Id: Ib06e650fd427b57e0bdbdf1fe759fe436104ff79
Reviewed-on: https://go-review.googlesource.com/c/go/+/601596
Auto-Submit: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Austin Clements <austin@google.com>