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runtime: document bmap.tophash

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In particular, it wasn't obvious that some values are special (unless
you also found those special values), so document that it isn't
necessarily a hash value.

Change-Id: Iff292822b44408239e26cd882dc07be6df2c1d38
Reviewed-on: https://go-review.googlesource.com/30143
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
This commit is contained in:
Austin Clements 2016-09-26 13:10:41 -04:00
parent 38f1df66ff
commit bf776a988b
1 changed files with 5 additions and 0 deletions
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5
src/runtime/hashmap.go
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@ -130,6 +130,9 @@ type hmap struct {
// A bucket for a Go map.
type bmap struct {
// tophash generally contains the top byte of the hash value
// for each key in this bucket. If tophash[0] < minTopHash,
// tophash[0] is a bucket evacuation state instead.
tophash [bucketCnt]uint8
// Followed by bucketCnt keys and then bucketCnt values.
// NOTE: packing all the keys together and then all the values together makes the
@ -1079,6 +1082,8 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
// Unlink the overflow buckets & clear key/value to help GC.
if h.flags&oldIterator == 0 {
b = (*bmap)(add(h.oldbuckets, oldbucket*uintptr(t.bucketsize)))
// Preserve b.tophash because the evacuation
// state is maintained there.
memclr(add(unsafe.Pointer(b), dataOffset), uintptr(t.bucketsize)-dataOffset)
}
}