Re: [PATCH] git grep: be careful to use mutices only when they are initialized

[Jeff King <peff@peff.net>]

On Wed, Oct 26, 2011 at 01:02:40PM -0700, Junio C Hamano wrote:

>  - Could we lose "#ifndef NO_PTHREADS" inside grep_sha1(), grep_file(),
>    and possibly cmd_grep() functions and let the compiler optimize things
>    away under NO_PTHREADS compilation?

I don't think so. If NO_PTHREADS is set, we might not have pthread
functions at all. Sure, many compilers will optimize:

  if (0)
          pthread_mutex_lock(...);

to remove the call completely. But would a compiler be wrong to complain
that pthread_mutex_lock is not defined, or to include reference to it
for the linker? gcc, both with and without optimizations, will complain
about:

  echo 'int main() { if (0) does_not_exist(); return 0; }' >foo.c
  gcc -Wall -c foo.c

though it does actually remove the dead code and link properly. I
wouldn't be surprised if some other compilers don't work, though (and of
course the warning is ugly).

I think you would have to do something like this in thread-utils.h:

  #ifndef NO_PTHREADS
  #include <pthread.h>
  #else
  #define pthread_mutex_t int
  #define pthread_mutex_init(m, a) do {} while(0)
  #define pthread_mutex_lock(m) do {} while(0)
  #define pthread_mutex_unlock(m) do {} while (0)
  /* and so forth for every pthread function */
  #endif

-Peff