Add sparse enum decoding benchmarks (#984)

With upcoming change we'll improve decoding performance of enums, but
there will be a difference between "sparse" and "dense" enum decoding
performance, even though they'll both be faster.

To be able to measure the difference add a "sparse" enum type and a
benchmark for decoding it.

"Sparse" means the enum has large gaps between known enum values, or
negative enum values.

When decoding this kind of enums, the mapping from the wire `varint` to
the Dart value for the enum needs to be done by binary search, map
lookup, or similar.

For "dense" enums, we can have a list of enum values and index the list
directly with the `varint` value, after a range check.

These changes will be done in the follow-up PR(s).

Author: osa1

benchmarks/bin/binary_decode_packed.dart

@@ -181,6 +181,25 @@ class PackedEnumDecodingBenchmark extends BenchmarkBase {
   }
 }
 
+class PackedSparseEnumDecodingBenchmark extends BenchmarkBase {
+  late final Uint8List encoded;
+
+  PackedSparseEnumDecodingBenchmark() : super('PackedSparseEnumDecoding') {
+    final rand = Random(123);
+    final message = PackedFields();
+    final numEnums = SparseEnum.values.length;
+    for (var i = 0; i < 1000000; i += 1) {
+      message.sparseEnum.add(SparseEnum.values[rand.nextInt(numEnums)]);
+    }
+    encoded = message.writeToBuffer();
+  }
+
+  @override
+  void run() {
+    sink = PackedFields()..mergeFromBuffer(encoded);
+  }
+}
+
 void main() {
   PackedInt32DecodingBenchmark().report();
   PackedInt64DecodingBenchmark().report();
@@ -190,6 +209,7 @@ void main() {
   PackedSint64DecodingBenchmark().report();
   PackedBoolDecodingBenchmark().report();
   PackedEnumDecodingBenchmark().report();
+  PackedSparseEnumDecodingBenchmark().report();
 
   if (int.parse('1') == 0) print(sink);
 }

benchmarks/protos/packed_fields.proto

@@ -10,6 +10,7 @@ message PackedFields {
     repeated bool packedBool = 7 [packed = true];
     repeated Enum1 packedEnum1 = 8 [packed = true];
     repeated Enum2 packedEnum2 = 9 [packed = true];
+    repeated SparseEnum sparseEnum = 10 [packed = true];
 }
 
 enum Enum1 {
@@ -27,3 +28,23 @@ enum Enum2 {
   ENUM_2_4 = 4;
   ENUM_2_5 = 5;
 }
+
+// An enum with large gaps between the known values, and with negative values.
+//
+// This will be slower to decode as the varint to enum value mapping needs to be
+// done with binary search, or map lookup etc.
+enum SparseEnum {
+    ENUM_ZERO = 0;
+    ENUM_MIN_INT = -2147483648;
+    ENUM_1 = -1000000000;
+    ENUM_2 = -100000000;
+    ENUM_3 = -10000000;
+    ENUM_4 = -1000000;
+    ENUM_5 = -100000;
+    ENUM_6 = 100000;
+    ENUM_7 = 1000000;
+    ENUM_8 = 10000000;
+    ENUM_9 = 100000000;
+    ENUM_10 = 1000000000;
+    ENUM_MAX_INT = 2147483647;
+}