Optimize error handling for special cases.

This commit contains a few improvements:
1. Using a consequetive range of [0..4], will allow us to merge all
   branches of error_is_special into one comparison.
2. With (1), we can remove the for loops to find entries in
   error_status_map with a single O(1) lookup.
3. grpc_error_is_special() code paths should be inlined for ref
   and unref to avoid callq for the majority of cases where speical
   error is used.
4. grpc_error_get_int() should never accept a nullptr argument to
   avoid an expensive branch in the hot path. Callers should all
   allocate a dummy int on the stack when calling.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/core/lib/transport/error_utils.cc b/src/core/lib/transport/error_utils.cc
index 2eff8b2916..558f1d494c 100644
--- a/src/core/lib/transport/error_utils.cc
+++ b/src/core/lib/transport/error_utils.cc
@@ -26,8 +26,9 @@
 
 static grpc_error* recursively_find_error_with_field(grpc_error* error,
                                                      grpc_error_ints which) {
+  intptr_t unused;
   // If the error itself has a status code, return it.
-  if (grpc_error_get_int(error, which, nullptr)) {
+  if (grpc_error_get_int(error, which, &unused)) {
     return error;
   }
   if (grpc_error_is_special(error)) return nullptr;
@@ -102,7 +103,8 @@ void grpc_error_get_status(grpc_error* error, grpc_millis deadline,
 }
 
 bool grpc_error_has_clear_grpc_status(grpc_error* error) {
-  if (grpc_error_get_int(error, GRPC_ERROR_INT_GRPC_STATUS, nullptr)) {
+  intptr_t unused;
+  if (grpc_error_get_int(error, GRPC_ERROR_INT_GRPC_STATUS, &unused)) {
     return true;
   }
   uint8_t slot = error->first_err;