code running very slowly after importing ansi c into iphone project

Question

I have an ANSI C code that is about 10,000 lines long that I am trying to use in an iPhone project. When I compile the code with gcc on the command line, I type the following:

gcc -o myprog -O3 myprog.c

This program reads in large jpeg files and does some fancy processing on them, so I call it with the following

./myprog mypic.jpg

and from the command line, this takes take about 0.1 seconds.

I'm trying to import this code into an iPhone project but I'm not entirely sure how. I was able to get it to compile and run successfully by renaming myprog.c to myprog.h and then calling the functions in the C code from within a generic NSObject class. I added the O3 optimization to the project's Other C Flags. However, when I do this, the code on the simulator takes about 2 seconds to run and on the iPhone about 7 seconds to run which renders an unacceptable user experience.

Any tips on on hoe to get this going would be much appreciated.

Answer 1

It's hard to say for sure where the slowness comes from, or if there is any way around it, but right off the bat you've done something wrong.

You shouldn't have renamed a .c file to a .h file and included it. You should have written a .h (header) file that had the function, variable, and type declarations declared:

myprog.h:

#ifndef MYPROG_H_
#define MYPROG_H_

struct thing {
     int a;
     int b;
};

extern int woof;

int foo(void * buf, int size);

#endif /* MYPROG_H_ */

Then you should compile the .c file to an object file (or library) and link the main program against that. If you were to have included the .h file that was really just a renamed .c file into more than one source code files it may have resulted in having multiple versions of some data and code in your program.

You'll probably also want to go through and separate out any code in myprog.c that you won't be using in your iPhone program. I'll bet that there is plenty.

As far as why the program is slowing down, this could have to do with myprog being written to make use of some resources that aren't available on the iPhone. The first thing that comes to mind is large amounts of RAM, since many desktop applications are written as though available RAM is infinite, and I could see how some .jpg manipulation code could be written this way. The way to get around this would be to try to rework the algorithm so that it did not load as much of the picture at one time while working on it.

The second thing that come is floating point code. Floating point operations are common in image manipulation code, but often either not available or severely limited in embedded systems. In the case of iPhones they are available, but according to something I heard, their performance is noticeably hampered if you compile your code to thumb rather than regular ARM code. (I've never developed for an iPhone or its particular processor so I don't know for sure, but it is worth looking into).

Another place where things could be slowing down would be if there were some sort of translation between Objective C objects and C structures that you have somehow introduced and is happening a lot more often than it should need to. There are probably other slow downs that could happen because of this, but you might be able to test this theory out by creating a objective C program for your desktop that uses the myprog.c code in a manner similar to the iPhone program's use of it.

Another thing you probably should look into is profiling your iPhone program. Profiling determines (or only helps to determine, in some cases) where the program is spending its time. Knowing this doesn't necessarily tell you that the code that runs the most is bad or that anything about it could be improved, but it does tell you where to look. And sometimes you may look at the results and immediately know that some function that you thought was only going to be called once at the beginning of the program is actually being called repeatedly, which highly suggests that some improvement can be made.

I'm sure that a little searching will turn up how to go about this.