Optimize zed.Value.Under (#4739)

Move the loop in Value.Under into a separate method, Value.under, so the
compiler will inline Under.

goos: darwin
goarch: arm64
pkg: github.com/brimdata/zed
                        |  before.txt  |              after.txt               |
                        |    sec/op    |    sec/op     vs base                |
ValueUnder/primitive-10   2.0560n ± 2%   0.9328n ± 0%  -54.63% (p=0.000 n=10)
ValueUnder/named-10        27.77n ± 0%    27.96n ± 1%   +0.72% (p=0.000 n=10)
geomean                    7.555n         5.107n       -32.40%

value.go

@@ -437,20 +437,25 @@ func (v *Value) MissingAsNull() *Value {
  return v
 }
 
+// Under resolves named types and untags unions repeatedly, returning a value
+// guaranteed to have neither a named type nor a union type.
 func (v *Value) Under() *Value {
- typ := v.Type
- if _, ok := typ.(*TypeNamed); !ok {
- if _, ok := typ.(*TypeUnion); !ok {
- // common fast path
- return v
- }
+ switch v.Type.(type) {
+ case *TypeUnion, *TypeNamed:
+ return v.under()
  }
- bytes := v.Bytes()
+ // This is the common case; make sure the compiler can inline it.
+ return v
+}
+
+// under contains logic for Under that the compiler won't inline.
+func (v *Value) under() *Value {
+ typ, bytes := v.Type, v.Bytes()
  for {
  typ = TypeUnder(typ)
  union, ok := typ.(*TypeUnion)
  if !ok {
- return NewValue(typ, bytes)
+ return &Value{typ, bytes}
  }
  typ, bytes = union.Untag(bytes)
  }

value_test.go

@@ -8,6 +8,24 @@ import (
  "github.com/stretchr/testify/assert"
 )
 
+func BenchmarkValueUnder(b *testing.B) {
+ b.Run("primitive", func(b *testing.B) {
+ val := zed.Null
+ b.ResetTimer()
+ for i := 0; i < b.N; i++ {
+ val.Under()
+ }
+ })
+ b.Run("named", func(b *testing.B) {
+ typ, _ := zed.NewContext().LookupTypeNamed("name", zed.TypeNull)
+ val := zed.NewValue(typ, nil)
+ b.ResetTimer()
+ for i := 0; i < b.N; i++ {
+ val.Under()
+ }
+ })
+}
+
 func TestValueValidate(t *testing.T) {
  recType := zed.NewTypeRecord(0, []zed.Field{
  zed.NewField("f", zed.NewTypeSet(0, zed.TypeString)),