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

On 11/2/19 4:24 PM, Richard W.M. Jones wrote: >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. >--- >+++ 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)) Doesn't clang define __GNUC__, at which point all of our supported compilers (since we require __attribute__((cleanup)) support) also have __builtin_expect? >+#else >+#define unlikely(x) (x) >+#define if_verbose if (verbose) >+#endif Or put another way, this #else may be dead code, and the #if may be unnecessary compared to just unconditionally defining unlikely() and if_verbose.