[LLVMdev] LLVMdev Digest, Vol 76, Issue 43

[Kenneth Boyd]

On 10/25/2010 6:31 PM, Peter Lawrence wrote:
>> The above logic makes sense when you're talking about non-volatile
>> loads
>> and stores.  To me, it doesn't make sense for volatile loads and
>> stores.
>>
>> The whole point of volatile is to tell the compiler that its
>> assumptions
>> about how memory works doesn't apply to this load or store and it
>> should, therefore, leave it alone and not do any optimization.
>> Informally, it's the programmer's way of telling the compiler, "I know
>> what I'm doing and you don't, so don't touch that memory operation."
>>
>> What you and Duncan are saying is that volatile is volatile except
>> when
>> it isn't.  I think that's poor design.  At the very least, it is
>> confusing, and at worst, it prevents LLVM from handling C's "volatile"
>> keyword correctly.
>>
>> If it's decided that the current behavior is what LLVM will do, it
>> should at least be documented in the LLVM Language Reference Manual.
>> Right now, the current behavior directly contradicts the reference
>> manual, and that is definitely confusing.
>>
>> -- John T.
> John,
>              On the one hand "volatile" it is to inform the compiler
> that either the "memory" being
> referenced isn't really simple memory rather it is for example a
> device register with side-effects,
> or that it is simple memory but holds a thread-shared global
> variable.  On the other hand "volatile"
> means don't delete, duplicate, reorder, or otherwise optimize
> references to this memory
> location.  But the latter is merely a convenient way to implement the
> necessary restrictions
> required by the former, so who is to say that the implementation must
> dictate the definition ?
>
> The documentation is simply telling you what a lazy implementation
> might do, not what
> a system is required to do.
>
> Your objection is like saying that it isn't kosher to ignore the
> "register" keyword, because
> "I know what I am doing and you don't.....".
What isn't kosher, is making side effects disappear from a C program.

I'm not going to bother quoting C99 5.1.2.3/2,3 verbatim, as we already 
have Word of God (Duncan Sands) that the C99 (and C90) standard's 
requirements on volatile are intentionally being violated.

Kenneth