Re: [avr-gcc-list] Volatile bad compiled while

[max2009tiny]

OK
i can't drop register and i use on this code more register vars and all work
ok exept
while clause. That generate bad jump -4 .

My query is why not for removing register clause.
If memory store all code is +2 bytes for read and +2 any once write var and
this is over memory limit....


 

David Brown-4 wrote:
> 
> max2009tiny wrote:
>> Hi everybody. I make project on Tiny2313 and use 
>> 
>> volatile register unsigned char lastrec asm("r8")
>> 
>> and lastrec is modified on rs232 rx isr.
>> 
>> When compile little function
>> __attribute__ ((noinline)) void waitACK(u08 cmack)
>> {
>> while(cmack!=lastrec) ;      //wait for reply request
>> }
>> 
> 
> 
> Fixing a variable to a register in this way is almost never a good idea. 
>   It is /possible/, because there are rare occasions when you want to do 
> so, but such cases are very rare.  It is also very easy to break other 
> parts of your program by using register variables like this, due to 
> conflicts with register usage in the library or the compiler.  See
> 
> <http://www.nongnu.org/avr-libc/user-manual/FAQ.html#faq_regbind>
> 
> (Note that explicitly recommends /not/ using r8 for this use.)
> 
> 
> 
> On some versions of gcc, using a "volatile register" variable will give 
> the warning "volatile register variables don't work as you might wish" 
> if -Wall or -Wvolatile-register-var is enabled.  For some reason, this 
> warning does not seem to be triggered by avr-gcc while testing. 
> However, the explanation from the manual remains relevant:
> 
> """
> -Wvolatile-register-var
>      Warn if a register variable is declared volatile. The volatile 
> modifier does not inhibit all optimizations that may eliminate reads 
> and/or writes to register variables. This warning is enabled by -Wall.
> """
> 
> 
> Here are some other comments about global volatile register variables:
> 
> <http://gcc.gnu.org/ml/gcc-patches/2005-11/msg00657.html>
> <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34351>
> 
> 
> 
> In summary, you use "register" variables for data that is absolutely 
> speed-critical and used widely throughout your code, and for which 
> dedicating a register makes sense (i.e., almost never).  You use 
> "volatile" when you specifically want to limit the compiler's 
> optimisation of the variable.  "volatile register" is a conflict of 
> interests, and it does not work as you think it should - the compiler 
> cannot treat it as both a restricted volatile and a highly-optimised 
> register at the same time.
> 
> Drop the "register", and your code will be fine.
> 
> 
> 
> 
> 
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