Moving from C++ to C -> extremely slow

[fafner]

I've been working on a path tracer project for the few days, and since I ultimately want it to run on the GPU using OpenCL, I wrote most of it in C. Earlier today I decided to move it entirely to C, just to be sure that the code I write will require minimum effort to port to OpenCL, and in doing so, the only part of the path tracer code I even touched was the random generator, going from this:

typedef struct _mt_state {
    static unsigned long mt[N]; /* the array for the state vector  */
    static int mti;
} mt_state;

unsigned long _mt_state::mt[N];
int _mt_state::mti;

to this

typedef struct _mt_state {
    unsigned long mt[N]; /* the array for the state vector  */
    int mti;
} mt_state;

After that I compiled with a C compiler without problems, but when I run my path tracer now, it is radically slower than it was with the C++ version. I know this isn't much info to go on, but does anyone know why this would happen? Again, the path tracer code has remained untouched except for the above adjustments.


Fafner

[LB]

Why do you need to go to C at all? Why do you think C++ cannot interact with the GPU or OpenCL?

Your slowdown is probably because you changed static (global) variables into non-static ones and now have duplicates of the same data everywhere.

[fafner]

I wanted to do it so that when I move my rendering code over to an OpenCL kernel its all C. OpenCL still doesn't have native support for C++. I thought about the change of the static variables, but I only have one struct, passing pointers to it to other functions. Its not being passed by value anywhere, so there shouldn't be any copies of it, right?

EDIT: You were right, I had overlooked a couple of crucial functions to which the struct was being passed by value! It's all working like it did before now, cheers :)