On Thu, 25 Oct 2007, Andi Kleen wrote:

quoted text
> On Thursday 25 October 2007 05:24, Nick Piggin wrote:
>
>> Basically, what the gcc developers are saying is that gcc is
>> free to load and store to any memory location, so long as it
>> behaves as if the instructions were executed in sequence.
>
> This case is clearly a bug, a very likely code pessimization.
> I guess it wasn't intentional, just an optimization that is useful
> for local register values doing too much.


I don't think it is a BUG, but one should certainly be able
to turn it off. Gcc is correct in that the 'C' language allows
a lot of implimentation details that are not covered by the
language. In other words, 'C' is not assembly-language.


quoted text
>
>> I guess that dynamically allocated memory and computed pointers
>> are more difficult for gcc to do anything unsafe with, because
>> it is harder to tell if a given function has deallocated the
>> memory.
>
> Often accesses happen without function calls inbetween.
> Also I think newer gcc (not 3.x) can determine if a pointer
> "escapes" or not so that might not protect against it.
>
>> Any thoughts?
>
> We don't have much choice: If such a case is found it has to be marked
> volatile or that particular compiler version be unsupported.
>
> It might be useful to come up with some kind of assembler pattern
> matcher to check if any such code is generated for the kernel
> and try it with different compiler versions.
>
> -Andi

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.59 BogoMips).
My book : http://www.AbominableFirebug.com/
_


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