[Icc-avr] Variables declared in main() and the stack

[David Brown]

Albert vanVeen wrote:
> How do you solve Fibernacci without recursion?
> 

By "solve", I presume you mean how do you calculate Fibernacci numbers? 
  You use a loop:

extern void output(uint16 x);

void fib(uint16 max) {
	uint16 a = 1;
	uint16 b = 1;
	uint16 c;

	while (a <= max) {
		output(a);
		c = a + b;
		a = b;
		b = c;
	}
}

If you just want to calculate a particular Fibernacci number, use an 
approximate power of phi.

There are functions that cannot be generated without recursion, but they 
are only of use in courses on computing theory, and you don't need to 
use recursive C functions to calculate them.  Recursion can certainly be 
useful in some situations (it is very common in some programming 
languages), but for C on a small micro, stick to loops.


> But Kevin set me wondering about vars 'local' in main(), usually only
> used for initial setups; will they then remain on the stack, forever
> unused? As I've never run out of memeory, I've never worried about these
> things.
> 

Put the initialisation parts in a separate "init" function, called by 
main before the main loop code - that way any local variable space will 
be reclaimed.

mvh.,

David


> Albert.
> 
>  
> 
> -----Original Message-----
> From: icc-avr-bounces at imagecraft.com
> [mailto:icc-avr-bounces at imagecraft.com] On Behalf Of David Brown
> Sent: Friday, November 30, 2007 09:52 AM
> To: Discussion list for ICCAVR and ICCtiny Users. You do NOT need
> tosubscribe to icc-announce if you are a member of this.
> Subject: Re: [Icc-avr] Variables declared in main() and the stack
> 
> Kevin Davis wrote:
>> David,
>>
>> Thanks for the good info ... wasn't aware of the register/efficiency 
>> issue.
>>
>> Regarding your statement on the (lack of a) difference between 
>> variables on the stack or main ram, I assume you mean then that the 
>> only thing you can control with a compiler option (unless you're using
> 
>> a custom config) is the size of the hardware stack containing function
> 
>> return addresses, so whether the variables are put on the bottom or 
>> top (software stack below the hardware stack) of the ram makes
> essentially no difference.
> 
> Pretty much correct.
> 
>> And, I guess what amounts to the same thing, unless you have a heavily
> 
>> recursive program, a stack overflow is primarily caused by the 
>> software stack encroaching on the heap.
>>
> 
> A program for an AVR should not be using recursion (at least, not until
> you are answering questions like these instead of asking them!).  Also,
> a program for the AVR should not be using the heap (again, unless you
> are an expert) - avoid using dynamic memory.
> 
> mvh.,
> 
> David
> 
> 
>> Kevin
>>
>> Kevin Davis
>> All Traffic Solutions
>> -----Original Message-----
>> From: icc-avr-bounces at imagecraft.com
>> [mailto:icc-avr-bounces at imagecraft.com] On Behalf Of David Brown
>> Sent: Wednesday, November 28, 2007 4:32 PM
>> To: Discussion list for ICCAVR and ICCtiny Users. You do NOT need 
>> tosubscribe to icc-announce if you are a member of this.
>> Subject: Re: [Icc-avr] Variables declared in main() and the stack
>>
>> Kevin Davis wrote:
>>> I'm learning more about pointers and their relation to the heap and 
>>> stack and was wondering:
>>>
>>> Does it make sense (if scope is not an issue) to make variables that 
>>> will only be used in main() global rather than use up stack space 
>>> with
>> them?
>>>  
>> In a word, no.  On a target like the AVR, it is *always* more 
>> efficient to use local variables rather than global ones.  For the 
>> most part, they
>>
>> exist only in registers - only if you have lots of them, or if they 
>> are aggregates (arrays or structs), do they spill out onto the stack.
> 
>> And even if they are allocated on the stack, there is no difference 
>> between taking up memory on the stack, and taking up memory in the
> main ram.
>>> Just curious as to any rules-of-thumb with regards to this (if there
>> are
>>> any). If this question makes no sense let me know as well but from
>> what
>>> I've learned the variables declared in main() are put on the stack 
>>> and
>>> would remain there for the program's duration.
>>>
>> Local variables in a function do not necessarily exist throughout the 
>> lifetime of the function.  They exist *theoretically* throughout their
> 
>> scope - but the scope can be any block within a function.  So if you 
>> have variables that are only used for part of a function, you can put 
>> them inside a block (within a {} pair) for efficiency.  The compiler 
>> can
>>
>> do this sort of lifetime analysis itself - it does not need to 
>> allocate a variable before it is actually used, and can remove it (and
> 
>> re-use its
>>
>> register or stack space) as soon as it is finished with.
>>
> 
> _______________________________________________
> Icc-avr mailing list
> Icc-avr at imagecraft.com
> http://dragonsgate.net/mailman/listinfo/icc-avr
> 
> Scanned by Bizo Email Filter
> 
> 
> _______________________________________________
> Icc-avr mailing list
> Icc-avr at imagecraft.com
> http://dragonsgate.net/mailman/listinfo/icc-avr