AVR-Chat

Thomas Keller wrote:
> On Thu, 2005-08-18 at 08:33 +1000, Don Ingram wrote:
> 
>>  Also, code efficiency and optimization differ between compilers.  An
>>Atmel FAE told me that most folks say that  GCC-AVR is almost as good
>>as IAR (who is the best currently).
>>This was what I was lead to believe too, but an example shown here a
>>few weeks ago dispelled that myth. If I recall correctly ( & I may
>>not ;-)  the particular example was the overhead in entering / exiting
>>an interrupt ( fading memory.)... The particular example compared the
>>overhead between GCC & CVAVR, the difference was substantial. Someone
>>more knowledgeable may wish to clarify this.
> 
> 
>   On this specific issue I cannot comment, but I can say that avr-gcc is
> quite inefficient in its handling of variables.  Every time the compiler
> references the value of an identifier, it loads it into r24,r25 (for a
> 16 bit int), then transfers it to another register pair before
> mainpulation of the value.  Wastes execution time as well as memory
> space for unnecessary instructions.

   I noticed that too.  I mean, if I declare a variable volatile and 
make it global I WANT the compiler to treat it as special and not futz 
about in the ISR.  GCC generated what I thought was really sloppy ISR 
code.  But I'm used to PIC C (C18 from Microchip) which allowed you to 
make fantastically small ISR code.  I just want the compiler to save the 
CCR and current working register and assume that I've taken care of the 
rest.  What resulted was kind of ugly, I guess that I'll have to go in 
an hand tune that bit...

   Other that that, I find that GCC-AVR does a pretty good job of code 
density - I just don't like the variable referencing and sloppy (or 
perhaps overly hand-holding) job it does with ISR's.

DLC

-- 
---------------------------------------------------------------------
* Dennis Clark  dlc@frii.com http://www.techtoystoday.com             *
* "Programming and Customizing the OOPic Microcontroller" Mcgraw-Hill *
---------------------------------------------------------------------