op: don't allocate for each string reference

When storing a string in an interface value that escapes, Go has to heap
allocate space for the string header, as interface values can only store
pointers. In text-heavy applications, this can lead to hundreds of
allocations per frame due to semantic.LabelOp, the primary user of
string-typed references in ops.

Instead of allocating each string header individually, provide a slice
of strings to store string-typed references in, and store pointers into
this slice as the actual references. This only allocates when resizing
the slice's backing array, and averages out to no allocations, as the
backing array gets reused between calls to Ops.Reset.

We introduce two new functions, Write1String and Write2String, which
make use of this new slice for their last argument. We could've
automated this in the existing Write1 and Write2 methods, but that would
require type assertions on each call, and the vast majority of ops do
not make use of strings.

Signed-off-by: Dominik Honnef <dominik@honnef.co>

io/key/key.go

@@ -323,8 +323,7 @@ func (h InputOp) Add(o *op.Ops) {
 	if h.Tag == nil {
 		panic("Tag must be non-nil")
 	}
-	filter := h.Keys
-	data := ops.Write2(&o.Internal, ops.TypeKeyInputLen, h.Tag, &filter)
+	data := ops.Write2String(&o.Internal, ops.TypeKeyInputLen, h.Tag, string(h.Keys))
 	data[0] = byte(ops.TypeKeyInput)
 	data[1] = byte(h.Hint)
 }
@@ -343,7 +342,7 @@ func (h FocusOp) Add(o *op.Ops) {
 }
 
 func (s SnippetOp) Add(o *op.Ops) {
-	data := ops.Write2(&o.Internal, ops.TypeSnippetLen, s.Tag, &s.Text)
+	data := ops.Write2String(&o.Internal, ops.TypeSnippetLen, s.Tag, s.Text)
 	data[0] = byte(ops.TypeSnippet)
 	bo := binary.LittleEndian
 	bo.PutUint32(data[1:], uint32(s.Range.Start))