The Boston Diaries

Sunday, July 11, 2004

“Hey! What's that code doing there?”

While the __builtin_expect() aspect of GCC didn't work, all the recent profiling I've done on mod_blog (which reminds me, I need to make the current codebase available) did however, bring my attention to BufferIOCtl(), which if you noticed, was one of the top four functions in term of CPU utilization.

int (BufferIOCtl)(const Buffer buf,int cmd, ... )
{
  va_list alist;
  int     rc;
  
  ddt(buf        != NULL);
  ddt(buf->ioreq != NULL);
  ddt(cmd         >   -1);
  
  if (buf == NULL)
    return(ErrorPush(CgiErr,BUFFERIOCTL,BUFERR_NULLPTR,"i",cmd));

  if (buf->ioreq == NULL)
    return(ErrorPush(CgiErr,BUFFERIOCTL,BUFERR_NULLHANDLER,"i",cmd));
  
  va_start(alist,cmd);
  rc = (*buf->ioreq)(buf,cmd,alist);
  va_end(alist);
  return(rc);
}

ddt() is similar to the ANSI C call assert(), which basically states a condition that should exist (and if that condition isn't met, the program aborts—this action can be turned off for production code; it's meant for debugging). But you'll notice that the code first checks to see if buf is not NULL within ddt(), then the first thing it does is check to see if buff is NULL.

It shouldn't be NULL to begin with.

The same for the tests of buf->ioreq. When I removed the extraneous code:

int (BufferIOCtl)(const Buffer buf,int cmd, ... )
{
  va_list alist;
  int     rc;
  
  ddt(buf        != NULL);
  ddt(buf->ioreq != NULL);
  ddt(cmd        >  -1);
  
  va_start(alist,cmd);
  rc = (*buf->ioreq)(buf,cmd,alist);
  va_end(alist);
  return(rc);
}

The runtime of BufferIOCtl() dropped to 1/3 the original time.

Not much in the grand scheme of things, but just goes to show you how expensive extraneous if statements can be. Especially if it's called 6,646,086 times.

Sunday, July 11, 2004

“Hey! What's that code doing there?”

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