* * * * *
“Hey! What's that code doing there?”
While the __builtin_expect() aspect of GCC didn't work, all the recent
profiling I've done [1] on mod_blog [2] (which reminds me, I need to make the
current codebase available) did however, bring my attention to BufferIOCtl(),
which if you noticed [3], was one of the top four functions in term of CPU
(Central Processing Unit) 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 (American National Standards Institute) 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.
[1]
gopher://gopher.conman.org/1Phlog:2004/07/10
[2]
https://boston.conman.org/mod_blog.tar.gz
[3]
gopher://gopher.conman.org/0Phlog:2004/07/10.1
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