Can I make #Pragma unroll accept macros/expressions rather than plain numbers?

Question

I am trying to tell my compiler to unroll a loop for me using #pragma unroll. However, the number of iterations is determined by a compile-time variable, so the loop needs to be unrolled that many times. Like this:

#define ITEMS 4

#pragma unroll (ITEMS + 1)
for (unsigned int ii = 0; ii <= ITEMS; ++ii) 
    /* do something */;

The compiler doesn't like this, though, as it gives me the following warning: warning: extra characters in the unroll pragma (expected a single positive integer), ignoring pragma for this loop. I understand what this means, of course: it wants a single integer rather than an expression. Is there a way to do this, though, without changing the unroll parameter every time I change ITEMS?

The compiler I am using is CUDA's NVCC compiler.

Answer 1

You could do it the other way around:

Note: Just noticed Daniel Fischer's comment, which suggests exactly the same, before me.

#define ITEMS_PLUS_ONE 5
#define ITEMS (ITEMS_PLUS_ONE - 1)

The issue is that the preprocessor doesn't do math. It only does copy&paste.
When you write #define ITEMS_PLUS_ONE (ITEMS + 1), unroll is replaced with (4 + 1), not with 5.
Once this reaches the compiler, it doesn't matter. Even without optimization, the calculation is done during compilation, and (4 + 1) is exactly the same as 5.
But in your compiler, #pragma unroll is processed before compilation, and wants the simple number.