sync: prevent (*Map).Range from always escaping

After the change from CL 426074 the Range method on Map always
escape the read variable, leading to an allocation.

Since the compiler doesn't do live-range splitting for local variables we
need to use some hints to only escape in that particular branch.

Fixes #62404

Change-Id: I938a5e593647455fa827e3dd3ed8ea22c7365df1
GitHub-Last-Rev: fcbedb467c
GitHub-Pull-Request: golang/go#62408
Reviewed-on: https://go-review.googlesource.com/c/go/+/524976
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/sync/map.go

@@ -461,7 +461,8 @@ func (m *Map) Range(f func(key, value any) bool) {
 		read = m.loadReadOnly()
 		if read.amended {
 			read = readOnly{m: m.dirty}
-			m.read.Store(&read)
+			copyRead := read
+			m.read.Store(&copyRead)
 			m.dirty = nil
 			m.misses = 0
 		}

src/sync/map_test.go

@@ -5,6 +5,7 @@
 package sync_test
 
 import (
+	"internal/testenv"
 	"math/rand"
 	"reflect"
 	"runtime"
@@ -280,3 +281,16 @@ func TestCompareAndSwap_NonExistingKey(t *testing.T) {
 		t.Fatalf("CompareAndSwap on an non-existing key succeeded")
 	}
 }
+
+func TestMapRangeNoAllocations(t *testing.T) { // Issue 62404
+	testenv.SkipIfOptimizationOff(t)
+	var m sync.Map
+	allocs := testing.AllocsPerRun(10, func() {
+		m.Range(func(key, value any) bool {
+			return true
+		})
+	})
+	if allocs > 0 {
+		t.Errorf("AllocsPerRun of m.Range = %v; want 0", allocs)
+	}
+}