Re: Volatile qualification on pointer and data

[David Brown <david@westcontrol.com>]

On 21/09/2011 15:57, Ian Lance Taylor wrote:
> David Brown<david@westcontrol.com>  writes:
>
>> On 21/09/2011 10:21, Paulo J. Matos wrote:
>>> On 21/09/11 08:03, David Brown wrote:
>>>> Asking to read it by a volatile read does not
>>>> change the nature of "foo" - the compiler can still implement it as a
>>>> compile-time constant.
>>>
>>> But since I am accessing the data through the pointer and the pointer
>>> qualifies the data as volatile, shouldn't the compiler avoid this kind
>>> of optimization for reads through the pointer?
>>>
>>
>> My thought is that the nature of "foo" is independent of how it is
>> accessed.  On the other hand, some uses of a variable will affect its
>> implementation - if you take the address of "foo" and pass that on to
>> an external function or data, then the compiler would have to generate
>> "foo" in memory (but in read-only memory, and it can still assume its
>> value does not change).  So I am not sure what the "correct" behaviour
>> is here - I merely ask the question.
>>
>> Fortunately, this situation is not going to occur in real code.
>
> I think your description is supported by the standard.  However, I also
> think that gcc should endeavor to fully honor the volatile qualifier in
> all cases, because that is least surprising to the programmer.  This is
> not a case where we should let optimization override the programmer's
> desire; by using volatile, the programmer has explicitly told us that
> they do not want any optimization to occur.
>

That makes sense - the principle of least surprise.  And since this 
situation would not occur in real code (at least, not code that is 
expected to do something useful other than test the compiler's code 
generation), there is no harm in making sub-optimal object code.

Are there any warning flags for "programmer doing something technically 
legal but logically daft", that could be triggered by such cases? :-)