[chttp2] Enable new huffman decoder for debug builds (#30959)

bazel/experiments.bzl

@@ -18,11 +18,14 @@
 
 EXPERIMENTS = {
     "dbg": {
-    },
-    "off": {
         "core_end2end_tests": [
             "new_hpack_huffman_decoder",
         ],
+        "hpack_test": [
+            "new_hpack_huffman_decoder",
+        ],
+    },
+    "off": {
         "endpoint_test": [
             "tcp_frame_size_tuning",
             "tcp_rcv_lowat",
@@ -35,9 +38,6 @@ EXPERIMENTS = {
             "tcp_rcv_lowat",
             "tcp_read_chunks",
         ],
-        "hpack_test": [
-            "new_hpack_huffman_decoder",
-        ],
         "resource_quota_test": [
             "memory_pressure_controller",
             "periodic_resource_quota_reclamation",

src/core/lib/experiments/experiments.cc

@@ -44,6 +44,11 @@ const char* const description_unconstrained_max_quota_buffer_size =
 const char* const description_new_hpack_huffman_decoder =
     "New HPACK huffman decoder - should be much faster than the existing "
     "implementation.";
+#ifdef NDEBUG
+const bool kDefaultForDebugOnly = false;
+#else
+const bool kDefaultForDebugOnly = true;
+#endif
 }  // namespace
 
 namespace grpc_core {
@@ -60,7 +65,8 @@ const ExperimentMetadata g_experiment_metadata[] = {
      description_periodic_resource_quota_reclamation, false},
     {"unconstrained_max_quota_buffer_size",
      description_unconstrained_max_quota_buffer_size, false},
-    {"new_hpack_huffman_decoder", description_new_hpack_huffman_decoder, false},
+    {"new_hpack_huffman_decoder", description_new_hpack_huffman_decoder,
+     kDefaultForDebugOnly},
 };
 
 }  // namespace grpc_core

src/core/lib/experiments/experiments.yaml

@@ -98,7 +98,7 @@
 - name: new_hpack_huffman_decoder
   description:
     New HPACK huffman decoder - should be much faster than the existing implementation.
-  default: false
+  default: debug
   expiry: 2022/10/01
   owner: ctiller@google.com
   test_tags: ["core_end2end_tests", "hpack_test"]