From: Michael Pratt <mpratt@google.com>
Date: Mon, 19 Aug 2024 21:45:41 +0000 (-0400)
Subject: internal/runtime/maps: linear scan of small map
X-Git-Tag: go1.24rc1~565
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=808da68c1c66e05a04a7d4bc046f27811711d7ff;p=gostls13.git

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 #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 <khr@golang.org>
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
---

diff --git a/src/internal/runtime/maps/map.go b/src/internal/runtime/maps/map.go
index 112fc08e0f..ad6edd65bf 100644
--- 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