[WinAVR-users] [ winavr-Bugs-2492893 ] WinAVR 20081205 - optimization bug

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Bugs item #2492893, was opened at 2009-01-07 17:47
Message generated for change (Comment added) made by nategoos
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Category: GCC
Group: None
Status: Open
Resolution: Fixed
Priority: 5
Private: No
Submitted By: Aleksey V (pro8229)
Assigned to: Nobody/Anonymous (nobody)
Summary: WinAVR  20081205 - optimization bug

Initial Comment:
// next line skipping, if I use optimization
posudomoyka.sost=WORK;	

while (newEvent!=TIME_END) // it's give forever loop
   wdt_reset();

//next line newer start
posudomoyka.sost=SLEEP;

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Comment By: Nathan Moore (nategoos)
Date: 2009-01-08 16:31

Message:
You should not be declaring variables in header files (*.h).  If you
include example.h within 2 different source files (*.c) each of these may
(and should) end up with their own copy of those variables.
Within the data.h files you should have stuff like:

extern bool savePoint;
extern volatile sPosudomoyka posudomoyka;
extern volatile signed char newEvent;

And in one (and only one) *.c file you should have stuff like:

bool savePoint=false;
volatile sPosudomoyka posudomoyka =
{0,0,0,0,0,SLEEP,SLEEP,NO_EVENT,NO_EVENT,false,false,NO_ERROR,true};
volatile signed char newEvent=-1;

Since you only had one *.c file here that's not your problem, but it's
going to bite you and it's basic C.
You should notice however that there's crazy issues with declaring
variables volatile that are bigger than what can be loaded in one cpu
operation (like your struct or even an int on AVR).



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Comment By: Aleksey V (pro8229)
Date: 2009-01-08 14:11

Message:
//next line does not help me
volatile signed char newEvent=-1;

//next line has helped me
volatile sPosudomoyka posudomoyka = {0,0,0,0,0, ...

it's not like  in FAQ, I think, isn't it?
FAQ #1:
http://www.nongnu.org/avr-libc/user-manual/FAQ.html#faq_volatile

thanks for help.

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Comment By: Eric Weddington (arcanum)
Date: 2009-01-07 18:31

Message:
You have your variable newEvent defined in data.h as this:
signed char newEvent=-1;

You are changing the value of newEvent in an interrupt service routine,
and you are reading it in the mainline code in main(). You need to declare
this variable as volatile. See the avr-libc user manual, FAQ #1:
http://www.nongnu.org/avr-libc/user-manual/FAQ.html#faq_volatile

If it is not declared as volatile, the compiler will never read the value
from memory and it will generate an infinite loop. Since it knows that it
is in an infinite loop, the compiler correctly determines that the line
"posudomoyka.sost=WORK;" can be optimized away as it does nothing and has
no valid side effects.


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