Avoid unnecessary ref/unref calls to get mdelem from slices.

grpc_mdelem_from_slices() unref's the key and value. As a result,
in quite a few cases on the hot path, we first ref slice, so that
grpc_mdelem_from_slices() can unref them.

Add grpc_mdelem_from_slices_no_unref() which does not unref() the
input slices. This cuts 0.5% - 1.0% across app benchmarks.

9 files changed, 37 insertions, 32 deletions

diff --git a/include/grpc/grpc.h b/include/grpc/grpc.h
index a9beee1c9e..416958ed96 100644
--- a/include/grpc/grpc.h
+++ b/include/grpc/grpc.h
@@ -211,7 +211,7 @@ GRPCAPI int grpc_channel_support_connectivity_watcher(grpc_channel* channel);
     possible values). */
 GRPCAPI grpc_call* grpc_channel_create_call(
     grpc_channel* channel, grpc_call* parent_call, uint32_t propagation_mask,
-    grpc_completion_queue* completion_queue, grpc_slice method,
+    grpc_completion_queue* completion_queue, const grpc_slice& method,
     const grpc_slice* host, gpr_timespec deadline, void* reserved);
 
 /** Ping the channels peer (load balanced channels will select one sub-channel
diff --git a/src/core/ext/filters/client_channel/client_channel.cc b/src/core/ext/filters/client_channel/client_channel.cc
index 388736b60a..3df5f6f2a0 100644
--- a/src/core/ext/filters/client_channel/client_channel.cc
+++ b/src/core/ext/filters/client_channel/client_channel.cc
@@ -2211,7 +2211,7 @@ static void add_retriable_send_initial_metadata_op(
                                    .grpc_previous_rpc_attempts);
   }
   if (GPR_UNLIKELY(calld->num_attempts_completed > 0)) {
-    grpc_mdelem retry_md = grpc_mdelem_from_slices(
+    grpc_mdelem retry_md = grpc_mdelem_from_slices_no_unref(
         GRPC_MDSTR_GRPC_PREVIOUS_RPC_ATTEMPTS,
         *retry_count_strings[calld->num_attempts_completed - 1]);
     grpc_error* error = grpc_metadata_batch_add_tail(
diff --git a/src/core/ext/filters/http/client_authority_filter.cc b/src/core/ext/filters/http/client_authority_filter.cc
index 1ca20ebb26..40b3ea22fd 100644
--- a/src/core/ext/filters/http/client_authority_filter.cc
+++ b/src/core/ext/filters/http/client_authority_filter.cc
@@ -59,9 +59,8 @@ void authority_start_transport_stream_op_batch(
       initial_metadata->idx.named.authority == nullptr) {
     grpc_error* error = grpc_metadata_batch_add_head(
         initial_metadata, &calld->authority_storage,
-        grpc_mdelem_from_slices(
-            GRPC_MDSTR_AUTHORITY,
-            grpc_slice_ref_internal(chand->default_authority)));
+        grpc_mdelem_from_slices_no_unref(GRPC_MDSTR_AUTHORITY,
+                                         chand->default_authority));
     if (error != GRPC_ERROR_NONE) {
       grpc_transport_stream_op_batch_finish_with_failure(batch, error,
                                                          calld->call_combiner);
diff --git a/src/core/ext/transport/chttp2/transport/chttp2_transport.cc b/src/core/ext/transport/chttp2/transport/chttp2_transport.cc
index 2d4b4da4c6..77a0a69393 100644
--- a/src/core/ext/transport/chttp2/transport/chttp2_transport.cc
+++ b/src/core/ext/transport/chttp2/transport/chttp2_transport.cc
@@ -2127,9 +2127,8 @@ void grpc_chttp2_fake_status(grpc_chttp2_transport* t, grpc_chttp2_stream* s,
       GRPC_LOG_IF_ERROR(
           "add_status_message",
           grpc_chttp2_incoming_metadata_buffer_replace_or_add(
-              &s->metadata_buffer[1],
-              grpc_mdelem_from_slices(GRPC_MDSTR_GRPC_MESSAGE,
-                                      grpc_slice_ref_internal(slice))));
+              &s->metadata_buffer[1], grpc_mdelem_from_slices_no_unref(
+                                          GRPC_MDSTR_GRPC_MESSAGE, slice)));
     }
     s->published_metadata[1] = GRPC_METADATA_SYNTHESIZED_FROM_FAKE;
     grpc_chttp2_maybe_complete_recv_trailing_metadata(t, s);
diff --git a/src/core/lib/security/credentials/plugin/plugin_credentials.cc b/src/core/lib/security/credentials/plugin/plugin_credentials.cc
index 73946ce039..d97963d891 100644
--- a/src/core/lib/security/credentials/plugin/plugin_credentials.cc
+++ b/src/core/lib/security/credentials/plugin/plugin_credentials.cc
@@ -102,8 +102,7 @@ static grpc_error* process_plugin_result(
     } else {
       for (size_t i = 0; i < num_md; ++i) {
         grpc_mdelem mdelem =
-            grpc_mdelem_from_slices(grpc_slice_ref_internal(md[i].key),
-                                    grpc_slice_ref_internal(md[i].value));
+            grpc_mdelem_from_slices_no_unref(md[i].key, md[i].value);
         grpc_credentials_mdelem_array_add(r->md_array, mdelem);
         GRPC_MDELEM_UNREF(mdelem);
       }
diff --git a/src/core/lib/surface/channel.cc b/src/core/lib/surface/channel.cc
index 4294434504..cd0b3cb458 100644
--- a/src/core/lib/surface/channel.cc
+++ b/src/core/lib/surface/channel.cc
@@ -326,20 +326,18 @@ static grpc_call* grpc_channel_create_call_internal(
   return call;
 }
 
-grpc_call* grpc_channel_create_call(grpc_channel* channel,
-                                    grpc_call* parent_call,
-                                    uint32_t propagation_mask,
-                                    grpc_completion_queue* cq,
-                                    grpc_slice method, const grpc_slice* host,
-                                    gpr_timespec deadline, void* reserved) {
+grpc_call* grpc_channel_create_call(
+    grpc_channel* channel, grpc_call* parent_call, uint32_t propagation_mask,
+    grpc_completion_queue* cq, const grpc_slice& method, const grpc_slice* host,
+    gpr_timespec deadline, void* reserved) {
   GPR_ASSERT(!reserved);
   grpc_core::ExecCtx exec_ctx;
   grpc_call* call = grpc_channel_create_call_internal(
       channel, parent_call, propagation_mask, cq, nullptr,
-      grpc_mdelem_from_slices(GRPC_MDSTR_PATH, grpc_slice_ref_internal(method)),
-      host != nullptr ? grpc_mdelem_from_slices(GRPC_MDSTR_AUTHORITY,
-                                                grpc_slice_ref_internal(*host))
-                      : GRPC_MDNULL,
+      grpc_mdelem_from_slices_no_unref(GRPC_MDSTR_PATH, method),
+      host != nullptr
+          ? grpc_mdelem_from_slices_no_unref(GRPC_MDSTR_AUTHORITY, *host)
+          : GRPC_MDNULL,
       grpc_timespec_to_millis_round_up(deadline));
 
   return call;
@@ -347,15 +345,15 @@ grpc_call* grpc_channel_create_call(grpc_channel* channel,
 
 grpc_call* grpc_channel_create_pollset_set_call(
     grpc_channel* channel, grpc_call* parent_call, uint32_t propagation_mask,
-    grpc_pollset_set* pollset_set, grpc_slice method, const grpc_slice* host,
-    grpc_millis deadline, void* reserved) {
+    grpc_pollset_set* pollset_set, const grpc_slice& method,
+    const grpc_slice* host, grpc_millis deadline, void* reserved) {
   GPR_ASSERT(!reserved);
   return grpc_channel_create_call_internal(
       channel, parent_call, propagation_mask, nullptr, pollset_set,
-      grpc_mdelem_from_slices(GRPC_MDSTR_PATH, grpc_slice_ref_internal(method)),
-      host != nullptr ? grpc_mdelem_from_slices(GRPC_MDSTR_AUTHORITY,
-                                                grpc_slice_ref_internal(*host))
-                      : GRPC_MDNULL,
+      grpc_mdelem_from_slices_no_unref(GRPC_MDSTR_PATH, method),
+      host != nullptr
+          ? grpc_mdelem_from_slices_no_unref(GRPC_MDSTR_AUTHORITY, *host)
+          : GRPC_MDNULL,
       deadline);
 }
 
diff --git a/src/core/lib/surface/channel.h b/src/core/lib/surface/channel.h
index e5ff2c3596..4ac76b8a29 100644
--- a/src/core/lib/surface/channel.h
+++ b/src/core/lib/surface/channel.h
@@ -45,8 +45,8 @@ grpc_channel* grpc_channel_create_with_builder(
     value of \a propagation_mask (see propagation_bits.h for possible values) */
 grpc_call* grpc_channel_create_pollset_set_call(
     grpc_channel* channel, grpc_call* parent_call, uint32_t propagation_mask,
-    grpc_pollset_set* pollset_set, grpc_slice method, const grpc_slice* host,
-    grpc_millis deadline, void* reserved);
+    grpc_pollset_set* pollset_set, const grpc_slice& method,
+    const grpc_slice* host, grpc_millis deadline, void* reserved);
 
 /** Get a (borrowed) pointer to this channels underlying channel stack */
 grpc_channel_stack* grpc_channel_get_channel_stack(grpc_channel* channel);
diff --git a/src/core/lib/transport/metadata.cc b/src/core/lib/transport/metadata.cc
index d164502280..8f84e06fae 100644
--- a/src/core/lib/transport/metadata.cc
+++ b/src/core/lib/transport/metadata.cc
@@ -237,7 +237,7 @@ static void rehash_mdtab(mdtab_shard* shard) {
 }
 
 grpc_mdelem grpc_mdelem_create(
-    grpc_slice key, grpc_slice value,
+    const grpc_slice& key, const grpc_slice& value,
     grpc_mdelem_data* compatible_external_backing_store) {
   if (!grpc_slice_is_interned(key) || !grpc_slice_is_interned(value)) {
     if (compatible_external_backing_store != nullptr) {
@@ -324,13 +324,19 @@ grpc_mdelem grpc_mdelem_create(
   return GRPC_MAKE_MDELEM(md, GRPC_MDELEM_STORAGE_INTERNED);
 }
 
-grpc_mdelem grpc_mdelem_from_slices(grpc_slice key, grpc_slice value) {
+grpc_mdelem grpc_mdelem_from_slices(const grpc_slice& key,
+                                    const grpc_slice& value) {
   grpc_mdelem out = grpc_mdelem_create(key, value, nullptr);
   grpc_slice_unref_internal(key);
   grpc_slice_unref_internal(value);
   return out;
 }
 
+grpc_mdelem grpc_mdelem_from_slices_no_unref(const grpc_slice& key,
+                                             const grpc_slice& value) {
+  return grpc_mdelem_create(key, value, nullptr);
+}
+
 grpc_mdelem grpc_mdelem_from_grpc_metadata(grpc_metadata* metadata) {
   bool changed = false;
   grpc_slice key_slice =
diff --git a/src/core/lib/transport/metadata.h b/src/core/lib/transport/metadata.h
index 338082276c..22481d6824 100644
--- a/src/core/lib/transport/metadata.h
+++ b/src/core/lib/transport/metadata.h
@@ -109,7 +109,11 @@ struct grpc_mdelem {
                               (uintptr_t)GRPC_MDELEM_STORAGE_INTERNED_BIT))
 
 /* Unrefs the slices. */
-grpc_mdelem grpc_mdelem_from_slices(grpc_slice key, grpc_slice value);
+grpc_mdelem grpc_mdelem_from_slices(const grpc_slice& key,
+                                    const grpc_slice& value);
+/* Does not unref the slices. */
+grpc_mdelem grpc_mdelem_from_slices_no_unref(const grpc_slice& key,
+                                             const grpc_slice& value);
 
 /* Cheaply convert a grpc_metadata to a grpc_mdelem; may use the grpc_metadata
    object as backing storage (so lifetimes should align) */
@@ -120,7 +124,7 @@ grpc_mdelem grpc_mdelem_from_grpc_metadata(grpc_metadata* metadata);
    compatible_external_backing_store if it is non-NULL (in which case it's the
    users responsibility to ensure that it outlives usage) */
 grpc_mdelem grpc_mdelem_create(
-    grpc_slice key, grpc_slice value,
+    const grpc_slice& key, const grpc_slice& value,
     grpc_mdelem_data* compatible_external_backing_store);
 
 bool grpc_mdelem_eq(grpc_mdelem a, grpc_mdelem b);