While optimization and gshared data

[Alex Rønne Petersen]

On 06-12-2012 19:28, Timo Sintonen wrote:
> I have some memory mapped hardware in my arm controller.
> I define struct regs to describe the registers and __gshared regs* p to
> point to the actual address.
> I have a loop like: while (p.status==0) {...}
>
> The body of the loop does not touch status and so gdc optimizer does not
> evaluate status again, resulting to an endless loop. Gcc does the same,
> unless the variable is defined as volatile.
>
> This is ok in tls variables, but gshared variables are not thread safe.
> Actually I can see from assembler code that this occurs in all shared
> variables, like:
> __gshared int a=0;
> while (a==0) {}
> with nothing in the body generates just a jump to itself and the program
> stops.
>
> Another thread, interrupt or hardware can change a __gshared variable
> any time, so I think that the evalution should not be optimized away.

Yes, the compiler *must* assume that anything __gshared is implicitly 
volatile in the C sense.

Please file an issue: http://gdcproject.org/bugzilla/

>
>
> Another related question: is there a way to make the pointer p immutable
> but leave the data mutable? The pointer is always set to the fixed
> address of the hardware, but making the pointer const makes also the
> data immutable.

Not with D's type system, since type qualifiers are transitive.

-- 
Alex Rønne Petersen
alex at lycus.org
http://lycus.org