RE: [avr-gcc-list] Problem with delay loop

[John Regehr]

> However, David brings up a good point. A local variable is put on the stack,
> generally not the place for hardware to modify the variable. And generally,
> other parts of the program (such as ISRs) don't have access to the specific
> location of the variable on the stack. Both hardware and ISRs work with
> global variables. So *could* a compiler reason that local variables could
> never be volatile? Or are there truly situations where hardware or ISRs
> could modify local variables on the stack?

The common use of volatile automatic variables is in conjunction with 
setjmp/longjmp.  This idiom would break if volatile locals went into 
registers.

Also there's no reason (in principle) that interrupts cannot access stack 
frames of non-interrupt contexts.

So anyway, I would say that regardless of what the standard actually says, 
it *should* specify that volatile locals behave similarly to volatile 
globals: loads and stores in the abstract machine must correspond 
one-to-one with loads and stores (to RAM, not registers) in the physical 
machine.

John Regehr