Re: [avr-gcc-list] code optimisation

[Bruce D. Lightner]

Trevor White wrote:

> Many thanks for all your comments. I have been working on my first full 
> blown project with an AVR and GCC. Before this project I have worked in 
> assembler. C gives many great benefits and I have enjoyed using it. The 
> code I write is readable and my brain is not taken up with assembler 
> coding techniques. The structure of C seems to just allow me to get on 
> with writing code.
>
> The cost of this is that my code seems large and I dont really know how 
> long the code will take. I have to measure it on a scope and toggle port 
> pins. I have used 8k already and now seem to spend time optimizing so I 
> can carry on writing code. This is where I have found that re-writing 
> certain lines seems to help. For example...
>
>    temp = ~temp;
>    temp += 1024
>  
> I have not tried this but am using it off the top of my head to show 
> abstract ( to me ) code optimization.
>
>    temp = (~(temp) + 1024);
>
> I seem to find things like this above sometimes make better code. 
> Sometimes a switch..case flow control works better than If statements, 
> sometimes it is the other way round. So I was really wondering if there 
> were golden rules and if there were any specific to gcc for avr.

If you haven't done this already, try re-compiling everything with the 
"gcc" switch "-mcall-prologues" and then re-link.  If you have a lot of 
subroutines, this can save a surprising amount of code-space, and you 
don't have to change any C-code.

Best regards,

Bruce

--
 Bruce D. Lightner
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