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

[Christian Franke <Christian.Franke@t-online.de>]

Marco Gerards wrote:
> Christian Franke <Christian.Franke@t-online.de> 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