internal/runtime/maps: linear scan of small map

We still use the hash and control word, but loop over all 8 bytes instead of doing the match operation, which ends up being slightly faster when there is only one group. Note that specialized variants added later will avoid hashing at all. For golang#54766. Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest-swissmap Change-Id: I3bb353b023dd6120b6585e87d3efe2f18ac9e1ef Reviewed-on: https://go-review.googlesource.com/c/go/+/611189 Reviewed-by: Keith Randall <[email protected]> Auto-Submit: Michael Pratt <[email protected]> LUCI-TryBot-Result: Go LUCI <[email protected]> Reviewed-by: Keith Randall <[email protected]>
ydnar · Oct 28, 2024 · 808da68 · 808da68
1 parent 77e3d8c
commit 808da68
Showing 1 changed file with 8 additions and 4 deletions.
diff --git a/src/internal/runtime/maps/map.go b/src/internal/runtime/maps/map.go
@@ -381,16 +381,20 @@ func (m *Map) getWithKeySmall(hash uintptr, key unsafe.Pointer) (unsafe.Pointer,
 		data: m.dirPtr,
 	}
 
-	match := g.ctrls().matchH2(h2(hash))
+	h2 := uint8(h2(hash))
+	ctrls := *g.ctrls()
 
-	for match != 0 {
-		i := match.first()
+	for i := uint32(0); i < abi.SwissMapGroupSlots; i++ {
+		c := uint8(ctrls)
+		ctrls >>= 8
+		if c != h2 {
+			continue
+		}
 
 		slotKey := g.key(i)
 		if m.typ.Key.Equal(key, slotKey) {
 			return slotKey, g.elem(i), true
 		}
-		match = match.removeFirst()
 	}
 
 	return nil, nil, false