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

From: David Brown
Subject: Re: [avr-gcc-list] Problem with delay loop
Date: Mon, 01 Oct 2007 16:37:53 +0200
User-agent: Thunderbird (Windows/20070728)

Paulo Marques wrote:
David Brown wrote:
it could perhaps reason that since there is no way for anything outside the program to find out where the local volatile variable resides, there is no way for anything else to influence or use the variable, and therefore the "volatile" qualifier can be ignored.

This sentence makes no sense at all. The "volatile" is precisely to warn the compiler that it should not "reason" anything about this variable.

I think the standards are pretty vague regarding exactly what "volatile" means. There is nothing (that I know of) in the standards saying where a volatile variable must be allocated.

If you give a volatile qualifier to a local variable, it's obvious that you want it to behave differently from regular local variables, so I think gcc is doing the best it can, from the weakly defined volatile semantics.

It may be obvious to *you*, as the author, that you mean "volatile" to work like this. It certainly works like that on avr-gcc at the moment. But what appears "obvious" to programmers (even expert experienced programmers), and what the standards say, what the compiler does, and how it all works on the target in question, can be very different things. When working with bigger cpus with caches and instruction re-ordering, for example, "volatile" is not nearly strong enough to give you the kind of guarantees we take for granted on avr-gcc.

This makes as much sense as saying that any volatile is futile, since you can compile a program with "-combine -whole-program" and so the compiler can always "reason" that any variable will not be accessed outside of its control.

No, it's not quite the same. In particular, if the variable's address is known outside the code (for example, if it is given a fixed address, such as by the definition of the port I/O registers), then there is no way the compiler could make guarantees about the safety of removing the "volatile". Similarly, if different globally accessible functions (such as interrupt functions) accessed the variable, it could not remove the "volatile". But for local variables within a function, it is much more straightforward to see how such variables could be accessed or addressed.

Note that I'm not disputing that the compiler could in theory use a register instead. I personally don't think that would be a good idea, but it might be allowed by the specs. What I'm disputing is that the compiler _can not_ ignore the volatile and optimize the loop away entirely. That would be a compiler bug, for sure.

It would certainly be a surprise to many (including me) if it *did* remove the volatile variable in practice. But I'm not 100% sure that the standards disallow such optimisations - that's all I'm saying.



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