Re: Volatile qualification on pointer and data

[Georg-Johann Lay <avr@gjlay.de>]

David Brown schrieb:
> 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.

ACK.

> 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? :-)

The combination of const and volatile can be reasonable in real world code.

One example is a special function register (SFR) that is read-only but 
can be altered by hardware.

Second example is a lookup table that can be changed after building the 
software, e.g. you have some calibration data that has to be drawn from 
the environment (drift of sensors, inductivity of motor windings, offset 
of actors, etc).  In such a case you want to read the data from the 
lookup table in, say, .rodata.  By no means you want the compiler to 
insert/propagate known values from the lookup table to immediate 
operands in instructions.  That's exacly what "const volatile" does.

Johann