Re: .text variable and optimization issue

[Václav Zeman <vhaisman at gmail dot com>]

On 07/04/2012 07:56 PM, Marcin S wrote:
> Hello,
>
> I'm trying to write a helper functions for arm microcontroller, that
> will help to determine if program is run for the first time after
> programming. I'm using STM32 chip, with ability to modify it's flash
> memory at runtime.
> The idea is to put const variable into flash memory (.text)  let's
> name it SIGNATURE, and then, during initialization sequence check for
> that variable value and eventually change it in flash under it's
> address (&SIGNATURE). When program will be launched next time, this
> variable will be already changed, letting you know that this is not a
> first run.
>
> Here is a simple  code snippet to do that
>
> const uint16_t CFlashSignature::SIGNATURE = 0xFFFF; //initial value in flash.
>
> bool CFlashSignature::isFristRun()
> {
> 	return *(uint16_t*)&SIGNATURE; //it must be written that way,
> otherwise program will not always notice change at SIGNATURE's address
> }
>
> void CFlashSignature::setSignature()
> {
> 	if(isFristRun())
> 	{
> 		FLASH_Unlock();
> 		FLASH_ProgramHalfWord((uint32)&SIGNATURE,0); //this function writes
> 0 at signature's address
> 		FLASH_Lock();
> 		
> 	}
> }
>
>
> and those functions are working as intended, but inly with NO
> optimization enabled, when i add any optimization (-Ox) isFirstRun()
> method always returns true! as if
>
> return *(uint16_t*)&SIGNATURE;
>
> was translated to
>
> return 0xffff;
>
> or something, i'm not exactly sure.
The compiler probably sees through your games. The language defines that
constants are really, constant, AFAIK. Try changing the 'const uint16_t
CFlashSignature::SIGNATURE' to 'const volatile uint16_t
CFlashSignature::SIGNATURE' or just drop the const entirely (and add
volatile).
>
> I could disable optimization for this translation unit as a
> workaround, but there is no guarantee it stays that way in future
> versions of gcc or something.
>
> So in general, I need 2 byte variable that will be always in .text and
> it's address should obtainable at any point. Any suggestions how to
> achieve it to be 100% it will be there?
>
-- 
VZ

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