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

From: David Brown
Subject: [avr-gcc-list] Re: Volatile bad compiled while
Date: Thu, 26 Nov 2009 13:38:54 +0100
User-agent: Thunderbird (Windows/20090812)

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


(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:

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:


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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