Loop Unrolling -Microblaze C programming

Question

Can we have loop unrolling in Microblaze C programming using EDK?

This is required because I need more performance. Traditionally my C code will run serially, so having loop unrolling using some compiler directive can accelerate my application.
(e.g as we do using openMP).

#pragma Unroll 
for (i = 0; i < 100; i++ ) {
    a[i] = fetch_data(i);
}

Is this possible for Microblaze? If yes is there any example on the same?

Answer 1

No, there isn't any automatic loop unrolling like that. For tight loops like that the common recommendation on the Xilinx forums is to manually unroll 10-20 times and see if the performance is acceptable or write the looping code in assembly.

You are typically losing 3 or 4 clock cycles on every one of the loop branches so depending on how long fetch_data takes to execute you could figure out how much unrolling you want to do.

for (i = 0; i < 100; i+=10 ) { 
    a[i] = fetch_data(i); 
    a[i+1] = fetch_data(i+1); 
    a[i+2] = fetch_data(i+2); 
    a[i+3] = fetch_data(i+3); 
    a[i+4] = fetch_data(i+4); 
    a[i+5] = fetch_data(i+5); 
    a[i+6] = fetch_data(i+6); 
    a[i+7] = fetch_data(i+7); 
    a[i+8] = fetch_data(i+8); 
    a[i+9] = fetch_data(i+9); 
} 

Make sure to heed the standard loop unrolling caveats like watching for interval sizes that aren't a multiple of your increment steps.

Answer 2

I got this reply from Xilinx(though I have not yet verified this):

http://gcc.gnu.org/onlinedocs/gcc-3.3.6/gcc/Optimize-Options.html

-funroll-loops section

The different -O optimization switchs (available directly in the SDK GUI) may do loop unrolling because it enables -floop-optimize which states:

-floop-optimize Perform loop optimizations: move constant expressions out of loops, simplify exit test conditions and optionally do strength-reduction and loop unrolling as well.

Enabled at levels-O,-O2,-O3,-Os.