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.

2 files changed, 14 insertions, 4 deletions

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);