C pre-processor

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I'm using perl's -P option so that I can prevent blocks of code from being compiled by perl when I'm not debugging.

It seems to be working very well, except that I need to extend the concept to some supporting modules, which have been created with 'h2xs -X' and are using autoloader.

Basically, I want my main program to be something like:

#!/usr/local/bin/perl -P
#define DEBUG 1

Then in my module methods, I want to be allowed to do this:

#ifdef DEBUG
  &big_expensive_function();
#endif

The problem is that the c pre-processor doesn't seem to work within the modules. And the code block is being executed every single time, regardless of how DEBUG is set.

I know I could just use a perl if(). But in the end product, I'd end up processing multiple if's on the order of millions of times, and efficency is very important.

Thanks

Comment on C pre-processor

Replies are listed 'Best First'.
Re: C pre-processor by chromatic (Archbishop) on May 01, 2001 at 23:20 UTC
Perl does some internal optimization. (Various meanings can be assigned to 'some', most of which are correct.) In particular, it can remove execution paths which will never be reached: `chromatic$ perl -MO=Deparse print "hi"; if (0) { print "hi" } - syntax OK print 'hi!'; '???';` [download] If you use a constant for DEBUG and set it to 0, I'm 99.8% convinced Perl will do the right thing and not even compile the debugging code. Update: I just ran a test with a real Perl package, and it works. You might want to put your constant declaration in a BEGIN{} block, just to be on the safe side.	[reply] [d/l]
Re (tilly) 2: C pre-processor by tilly (Archbishop) on May 01, 2001 at 23:31 UTC
Note. Perl's definition of "never be reached" is unaware of the possibility of being reached by goto. This is documented, so the following proves your assertion and is not a bug in Perl: `if (0) { FOO: { print "How many times will I print?\n"; } } goto FOO;` [download] Note, however, that Perl will parse the code in the commented out section. If this bothers you, you can do arbitrary pre-processing through Filter::Simple. (Note that the inability to parse arbitrary Perl makes this facility substantially less useful than it could be...)	[reply] [d/l]