internal/runtime/maps: avoid table lookup on most Iter.Next calls

Speeds up iteration by about 3%.

For #54766.

Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap
Change-Id: I3406376fb8db87306d52e665fcee1f33cf610f24
Reviewed-on: https://go-review.googlesource.com/c/go/+/621115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

diff --git a/src/internal/runtime/maps/table.go b/src/internal/runtime/maps/table.go
index 59d84761c67233690bca336c709957e1146a60f3..bda74ea41bbcae4ec4f257fb9bc33ccfc023b65e 100644 (file)
--- a/src/internal/runtime/maps/table.go
+++ b/src/internal/runtime/maps/table.go
@@ -54,6 +54,9 @@ type table struct {
        // Index of this table in the Map directory. This is the index of the
        // _first_ location in the directory. The table may occur in multiple
        // sequential indicies.
+       //
+       // index is -1 if the table is stale (no longer installed in the
+       // directory).
        index int
 
        // groups is an array of slot groups. Each group holds abi.SwissMapGroupSlots
@@ -710,15 +713,10 @@ func (it *Iter) Next() {
 
        // Continue iteration until we find a full slot.
        for it.dirIdx < it.m.dirLen {
-               // TODO(prattmic): We currently look up the latest table on
-               // every call, even if it.tab is set because the inner loop
-               // checks if it.tab has grown by checking it.tab != newTab.
-               //
-               // We could avoid most of these lookups if we left a flag
-               // behind on the old table to denote that it is stale.
-               dirIdx := int((uint64(it.dirIdx) + it.dirOffset) & uint64(it.m.dirLen-1))
-               newTab := it.m.directoryAt(uintptr(dirIdx))
+               // Find next table.
                if it.tab == nil {
+                       dirIdx := int((uint64(it.dirIdx) + it.dirOffset) & uint64(it.m.dirLen-1))
+                       newTab := it.m.directoryAt(uintptr(dirIdx))
                        if newTab.index != dirIdx {
                                // Normally we skip past all duplicates of the
                                // same entry in the table (see updates to
@@ -781,7 +779,7 @@ func (it *Iter) Next() {
                        // We still use our old table to decide which
                        // keys to lookup in order to avoid returning
                        // the same key twice.
-                       grown := it.tab != newTab
+                       grown := it.tab.index == -1
                        var elem unsafe.Pointer
                        if grown {
                                var ok bool
@@ -956,6 +954,7 @@ func (t *table) split(typ *abi.SwissMapType, m *Map) {
        }
 
        m.installTableSplit(t, left, right)
+       t.index = -1
 }
 
 // grow the capacity of the table by allocating a new table with a bigger array
@@ -999,6 +998,7 @@ func (t *table) grow(typ *abi.SwissMapType, m *Map, newCapacity uint16) {
 
        newTable.checkInvariants(typ, m)
        m.replaceTable(newTable)
+       t.index = -1
 }
 
 // probeSeq maintains the state for a probe sequence that iterates through the