runtime: let Pinner preallocate a reusable ref array

With this change a Pinner preallocates an array of 5 pointers for
references to pinned objects. This reduces allocations when a pinner
is reused with up to 5 pinned objects.

This is a follow-up to CL 367296.

Signed-off-by: Sven Anderson <sven@anderson.de>
Change-Id: Ibea0b9ee4d7e39b0341a1da9d8276a4283e4956d
Reviewed-on: https://go-review.googlesource.com/c/go/+/496275
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>

diff --git a/src/runtime/pinner.go b/src/runtime/pinner.go
index a507a5a3cc767b4c75609b64bb0615da8b7c90fc..94c9e92432b453eb2fab1f7fc3b575776d1378a8 100644 (file)
--- a/src/runtime/pinner.go
+++ b/src/runtime/pinner.go
@@ -28,8 +28,9 @@ type Pinner struct {
 func (p *Pinner) Pin(pointer any) {
        if p.pinner == nil {
                p.pinner = new(pinner)
+               p.refs = p.refStore[:0]
                SetFinalizer(p.pinner, func(i *pinner) {
-                       if i.refs != nil {
+                       if len(i.refs) != 0 {
                                i.unpin() // only required to make the test idempotent
                                pinnerLeakPanic()
                        }
@@ -46,8 +47,14 @@ func (p *Pinner) Unpin() {
        p.pinner.unpin()
 }
 
+const (
+       pinnerSize         = 64
+       pinnerRefStoreSize = (pinnerSize - unsafe.Sizeof([]unsafe.Pointer{})) / unsafe.Sizeof(unsafe.Pointer(nil))
+)
+
 type pinner struct {
-       refs []unsafe.Pointer
+       refs     []unsafe.Pointer
+       refStore [pinnerRefStoreSize]unsafe.Pointer
 }
 
 func (p *pinner) unpin() {
@@ -58,7 +65,8 @@ func (p *pinner) unpin() {
                setPinned(p.refs[i], false)
                p.refs[i] = nil
        }
-       p.refs = nil
+       p.refStore = [pinnerRefStoreSize]unsafe.Pointer{}
+       p.refs = p.refStore[:0]
 }
 
 func pinnerGetPtr(i *any) unsafe.Pointer {