runtime/internal/maps: only conditionally clear groups when sparse
We only want to do the work of clearing slots
if they are full. But we also don't want to do too
much work to figure out whether a slot is full or not,
especially if clearing a slot is cheap.
1) We decide group-by-group instead of slot-by-slot.
If any slot in a group is full, we zero the whole group.
2) If groups are unlikely to be empty, don't bother
testing for it.
3) If groups are 50%/50% likely to be empty, also don't
bother testing, as it confuses the branch predictor. See #75097.
4) But if a group is really large, do the test anyway, as
clearing is expensive.
Fixes #75097
Change-Id: I9191865dd3e0fe887751cffe6082ac27d8d8439c
Reviewed-on: https://go-review.googlesource.com/c/go/+/697876 Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@google.com> Reviewed-by: Michael Pratt <mpratt@google.com> Reviewed-by: Youlin Feng <fengyoulin@live.com>