Optimizing ARM Cortex M3 code

Question

I have a C Function which tries to copy a framebuffer to FSMC RAM.

The functions eats the frame rate of the game loop to 10FPS. I would like to know how to analyze

Answer 1

Not exactly answering your question, but I see you aspire for fast execution of the loops.

Here are some tips from the book: 'ARM System Developer's Guide: Designing and Optimizing System Software (The Morgan Kaufmann Series in Computer Architecture and Design)' http://www.amazon.com/ARM-System-Developers-Guide-Architecture/dp/1558608745

Chapter 5 contains section named 'C looping structures'. Here is the summary of the section:

Writing Loops Efficiently

• Use loops that count down to zero. Then the compiler does not need to allocate a register to hold the termination value, and the comparison with zero is free.
• Use unsigned loop counters by default and the continuation condition i!=0 rather than i>0. This will ensure that the loop overhead is only two instructions.
• Use do-while loops rather than for loops when you know the loop will iterate at least once. This saves the compiler checking to see if the loop count is zero.
• Unroll important loops to reduce the loop overhead. Do not overunroll. If the loop overhead is small as a proportion of the total, then unrolling will increase code size and hurt the performance of the cache.
• Try to arrange that the number of elements in arrays are multiples of four or eight. You can then unroll loops easily by two, four, or eight times without worrying about the leftover array elements.

Based on the summary, your inner loop might look as below.

uinsigned int i = 240/4;  // Use unsigned loop counters by default
                          // and the continuation condition i!=0

do
{
    // Unroll important loops to reduce the loop overhead
    LCD_WriteData( (u16)frameBuffer[ (i--) + (j*fbWidth) ] );
    LCD_WriteData( (u16)frameBuffer[ (i--) + (j*fbWidth) ] );
    LCD_WriteData( (u16)frameBuffer[ (i--) + (j*fbWidth) ] );
    LCD_WriteData( (u16)frameBuffer[ (i--) + (j*fbWidth) ] );
}
while ( i != 0 )  // Use do-while loops rather than for
                  // loops when you know the loop will
                  // iterate at least once

You might want to experiment also with 'pragmas', e.g. :

#pragma Otime

http://www.keil.com/support/man/docs/armcc/armcc_chr1359124989673.htm

#pragma unroll(n)

http://www.keil.com/support/man/docs/armcc/armcc_chr1359124992247.htm

And as it is Cortex-M3 try to find out if MCU hardware gives you chance to arrange the code/data to take advantage of its Harvard architecture (I experienced 30% speed increase).

see here my other answer

Maybe not everything may be applicable in your application (filling a buffer in reverse order). I just wanted to draw your attention to the book and possible points for optimization.