internal/impl: add fast-path unmarshal

Benchmarks run with:
  go test ./benchmarks/ -bench=Wire  -benchtime=500ms -benchmem -count=8

Fast-path vs. parent commit:

  name                                      old time/op    new time/op    delta
  Wire/Unmarshal/google_message1_proto2-12    1.35µs ± 2%    0.45µs ± 4%  -67.01%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message1_proto3-12    1.07µs ± 1%    0.31µs ± 1%  -71.04%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message2-12            691µs ± 2%     188µs ± 2%  -72.78%  (p=0.000 n=7+8)

  name                                      old allocs/op  new allocs/op  delta
  Wire/Unmarshal/google_message1_proto2-12      60.0 ± 0%      25.0 ± 0%  -58.33%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message1_proto3-12      42.0 ± 0%       7.0 ± 0%  -83.33%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message2-12            28.6k ± 0%      8.5k ± 0%  -70.34%  (p=0.000 n=8+8)

Fast-path vs. -v1:

  name                                      old time/op    new time/op    delta
  Wire/Unmarshal/google_message1_proto2-12     702ns ± 1%     445ns ± 4%   -36.58%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message1_proto3-12     604ns ± 1%     311ns ± 1%   -48.54%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message2-12            179µs ± 3%     188µs ± 2%    +5.30%  (p=0.000 n=7+8)

  name                                      old allocs/op  new allocs/op  delta
  Wire/Unmarshal/google_message1_proto2-12      26.0 ± 0%      25.0 ± 0%    -3.85%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message1_proto3-12      8.00 ± 0%      7.00 ± 0%   -12.50%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message2-12            8.49k ± 0%     8.49k ± 0%    -0.01%  (p=0.000 n=8+8)

Change-Id: I6247ac3fd66a63d9acb902cbd192094ee3d151c3
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185147
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/impl/codec_extension.go b/internal/impl/codec_extension.go
index 577f1df..10a9fa2 100644
--- a/internal/impl/codec_extension.go
+++ b/internal/impl/codec_extension.go
@@ -13,9 +13,10 @@
 )
 
 type extensionFieldInfo struct {
-	wiretag uint64
-	tagsize int
-	funcs   ifaceCoderFuncs
+	wiretag             uint64
+	tagsize             int
+	unmarshalNeedsValue bool
+	funcs               ifaceCoderFuncs
 }
 
 func (mi *MessageInfo) extensionFieldInfo(xt pref.ExtensionType) *extensionFieldInfo {
@@ -34,7 +35,17 @@
 		tagsize: wire.SizeVarint(wiretag),
 		funcs:   encoderFuncsForValue(xt, xt.GoType()),
 	}
-
+	// Does the unmarshal function need a value passed to it?
+	// This is true for composite types, where we pass in a message, list, or map to fill in,
+	// and for enums, where we pass in a prototype value to specify the concrete enum type.
+	switch xt.Kind() {
+	case pref.MessageKind, pref.GroupKind, pref.EnumKind:
+		e.unmarshalNeedsValue = true
+	default:
+		if xt.Cardinality() == pref.Repeated {
+			e.unmarshalNeedsValue = true
+		}
+	}
 	mi.extensionFieldInfosMu.Lock()
 	if mi.extensionFieldInfos == nil {
 		mi.extensionFieldInfos = make(map[pref.ExtensionType]*extensionFieldInfo)