Re: [avr-gcc-list] Incorrect code by gcc?

[Andy H]

Ok you should not be using naked.

If function never returns (I mean never!) then it should be something 
like this :

void main(void) __attribute__((noreturn));

-fomit-frame-pointer  will work if gcc can really omit it.

Frame pointer will be used to store "local" variables that are to big to fit in 
registers and/or need to be saved around calls.
It may also use R2-R17 to save values. Many factors determine which it will 
use, such as loops - speed -size - overhead of saving registers etc.
It is not always perfect.

If you find situation where it could omit frame pointer - but does not, that 
might be a bug
(gcc is not perfect - I know several parts that could be better).

By all means post "bug" here first if you are not sure.

I know gcc bugs should have pre-processed sources (.i files) - but as first 
look it is easier to just look at c code.
I think Eric and others would have spotted it sooner.


best regards and hope tulips look good.

Andy




Ruud Vlaming wrote:
> On Thursday 03 April 2008 00:27, Andy H wrote:
>   
> > Not a bug
> >     
> Very good spotting. Andy! Thank you!
>
> Thats the reason i wanted it to post here before sending in a bug report.
> Unfortunately i already did so, so i marked the bug as INVALID now.
> The bugnumber was 35807 
>
>   
> > According to the sources you posted,
> > privQueuRequestBody
> > void privQueuRequestBody(uint8_t uiSlot, int8_t siFreeFilling, uint16_t 
> > uiTicksToWait) __attribute__ ( ( naked ) );
> >
> > The relevant part being "naked"
> >
> > That means gcc will omit prolog - which is where stack and frame pointer are 
> > setup.
> > R28/29 is the frame pointer. What you see in assembler is gcc saving the register around a call. 
> > Depending on optimization, it may choose to do this rather than use a register (as the register would need to be saved on the stack). 
> >
> > If you use naked, you must replace all of prolog/epilog by hand. (its normal 
> > use is for assembler only functions)
> >     
>
> OK, naked is a "dangerous" attribute. I must use it for i do a context save 
> just before the
> body function is called, but must preserve all registers so the function parameters are 
> left alone. 
>
> Is there a way to prohibit the compiler to do so? In other words can i somehow forbid 
> to make use of the frame pointer? -fomit-frame-pointer seems not to do the trick.
>
> Since i know on beforehand i will not return from privQueuRequestBody there is 
> no
> need to save the stack or so. 
>
> Ideas?
>
> Ruud.
>
>
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