[Libguestfs] [PATCH nbdkit] server: Use GCC hints to move debug and error handling code out of hot paths.

[Richard W.M. Jones]

For GCC only, define unlikely() macro.  Use it on error paths to move
code out of the hot path.

In the server only, use the debug() macro (don't call nbdkit_debug
directly).  This macro checks the verbose flag and moves the call to
nbdkit_debug out of the hot path.
---
 server/connections.c | 11 ++++++-----
 server/internal.h    | 17 ++++++++++++++++-
 server/plugins.c     |  2 +-
 server/protocol.c    |  4 ++--
 server/sockets.c     |  4 ++--
 5 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/server/connections.c b/server/connections.c
index c55d381..95d8296 100644
--- a/server/connections.c
+++ b/server/connections.c
@@ -97,7 +97,7 @@ connection_set_status (struct connection *conn, int value)
 
       assert (conn->status_pipe[1] >= 0);
       if (write (conn->status_pipe[1], &c, 1) != 1 && errno != EAGAIN)
-        nbdkit_debug ("failed to notify pipe-to-self: %m");
+        debug ("failed to notify pipe-to-self: %m");
     }
     conn->status = value;
   }
@@ -176,7 +176,7 @@ _handle_single_connection (int sockin, int sockout)
     debug ("handshake complete, processing requests with %d threads",
            nworkers);
     workers = calloc (nworkers, sizeof *workers);
-    if (!workers) {
+    if (unlikely (!workers)) {
       perror ("malloc");
       goto done;
     }
@@ -185,12 +185,13 @@ _handle_single_connection (int sockin, int sockout)
       struct worker_data *worker = malloc (sizeof *worker);
       int err;
 
-      if (!worker) {
+      if (unlikely (!worker)) {
         perror ("malloc");
         connection_set_status (conn, -1);
         goto wait;
       }
-      if (asprintf (&worker->name, "%s.%d", plugin_name, nworkers) < 0) {
+      if (unlikely (asprintf (&worker->name, "%s.%d", plugin_name, nworkers)
+                    < 0)) {
         perror ("asprintf");
         connection_set_status (conn, -1);
         free (worker);
@@ -199,7 +200,7 @@ _handle_single_connection (int sockin, int sockout)
       worker->conn = conn;
       err = pthread_create (&workers[nworkers], NULL, connection_worker,
                             worker);
-      if (err) {
+      if (unlikely (err)) {
         errno = err;
         perror ("pthread_create");
         connection_set_status (conn, -1);
diff --git a/server/internal.h b/server/internal.h
index 5e11e1a..7e0c375 100644
--- a/server/internal.h
+++ b/server/internal.h
@@ -45,6 +45,17 @@
 #include "cleanup.h"
 #include "nbd-protocol.h"
 
+/* Define unlikely macro, but only for GCC.  These are used to move
+ * debug and error handling code out of hot paths.
+ */
+#if defined(__GNUC__)
+#define unlikely(x) __builtin_expect (!!(x), 0)
+#define if_verbose if (unlikely (verbose))
+#else
+#define unlikely(x) (x)
+#define if_verbose if (verbose)
+#endif
+
 #ifdef __APPLE__
 #define UNIX_PATH_MAX 104
 #else
@@ -262,7 +273,11 @@ extern int crypto_negotiate_tls (struct connection *conn,
   __attribute__((__nonnull__ (1)));
 
 /* debug.c */
-#define debug nbdkit_debug
+#define debug(fs, ...)                                   \
+  do {                                                   \
+    if_verbose                                           \
+      nbdkit_debug ((fs), ##__VA_ARGS__);                \
+  } while (0)
 
 /* log-*.c */
 #if !HAVE_VFPRINTF_PERCENT_M
diff --git a/server/plugins.c b/server/plugins.c
index 87daaf2..65f6817 100644
--- a/server/plugins.c
+++ b/server/plugins.c
@@ -71,7 +71,7 @@ plugin_thread_model (struct backend *b)
 #if !(defined SOCK_CLOEXEC && defined HAVE_MKOSTEMP && defined HAVE_PIPE2 && \
       defined HAVE_ACCEPT4)
   if (thread_model > NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS) {
-    nbdkit_debug ("system lacks atomic CLOEXEC, serializing to avoid fd leaks");
+    debug ("system lacks atomic CLOEXEC, serializing to avoid fd leaks");
     thread_model = NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS;
   }
 #endif
diff --git a/server/protocol.c b/server/protocol.c
index 89fbdfa..f6ea35c 100644
--- a/server/protocol.c
+++ b/server/protocol.c
@@ -508,8 +508,8 @@ extents_to_block_descriptors (struct nbdkit_extents *extents,
 
 #if 0
   for (i = 0; i < *nr_blocks; ++i)
-    nbdkit_debug ("block status: sending block %" PRIu32 " type %" PRIu32,
-                  blocks[i].length, blocks[i].status_flags);
+    debug ("block status: sending block %" PRIu32 " type %" PRIu32,
+           blocks[i].length, blocks[i].status_flags);
 #endif
 
   /* Convert to big endian for the protocol. */
diff --git a/server/sockets.c b/server/sockets.c
index 1585a09..119cb99 100644
--- a/server/sockets.c
+++ b/server/sockets.c
@@ -365,7 +365,7 @@ accept_connection (int listen_sock)
   const int flag = 1;
 
   thread_data = malloc (sizeof *thread_data);
-  if (!thread_data) {
+  if (unlikely (!thread_data)) {
     perror ("malloc");
     return;
   }
@@ -409,7 +409,7 @@ accept_connection (int listen_sock)
   pthread_attr_setdetachstate (&attrs, PTHREAD_CREATE_DETACHED);
   err = pthread_create (&thread, &attrs, start_thread, thread_data);
   pthread_attr_destroy (&attrs);
-  if (err != 0) {
+  if (unlikely (err != 0)) {
     fprintf (stderr, "%s: pthread_create: %s\n", program_name, strerror (err));
     close (thread_data->sock);
     free (thread_data);
-- 
2.23.0