Re: [PATCH] Fix eisa_mmap evaluation, add memory existence check

[Christian Franke]

Marco Gerards wrote:
> Christian Franke <address@hidden> writes:
>   
> > ...
> > volatile is necessary here to tell the complier that the memory
> > address might not behave like regular memory. Otherwise, the optimizer
> > might legitimately remove memory accesses and then constant
> > propagation detects an unchanged value.
> >
> > gcc actually does a very good job here. Result with volatile removed:
> >     
>
> I think this is just normal memory, not memory mapped hardware.  Or
> perhaps I am misunderstanding something here.
>
>   

It is normal memory - but only if present :-) Like memory mapped 
hardware, missing memory violates the optimizers assumptions about 
memory accesses. Therefore, volatile is mandatory here.


> > $ gcc -S -O -fomit-frame-pointer init.c && cat init.s
> > ...
> > addr_is_valid:
> >    movl    $1, %eax
> >    ret
> > ...
> >
> >
> > aka:
> >
> > static int
> > addr_is_valid (grub_addr_t addr)
> > {
> >  return 1;
> > }
> >
> >
> > This is at least a proof that the original function returns the
> > correct result when real memory is present :-)
> >     
>
> :-)
>
>   
BTW: this also proves that the routine leaves memory unchanged.

Christian