Re: volatile vs aliased

[Martin Krischik <krischik@users.sourceforge.net>]

Rolf wrote:

> Martin Krischik wrote:
>> Ludovic Brenta wrote:
>>
>> > pragma Volatile (Variable) says the compiler must not optimise away
>> > any reads or writes to that variable, and that it may not add extra
>> > reads or writes beyond those you explicitly request in your program
>> > text.  You want to use that for hardware registers, where a "read"
>> > operation may have a side effect such as changing the device's state.
>>
>> Interesting! So with  pragma Volatile (X) the following two statements
>> are not the same:
>>
>> Y := X * X;
>> Y := X ** 2;
> 
> Do we have a difference to C/C++ increment operator here? consider
> 
> pragma Volatile (x);
> x := x + 1;   -- (1)
>         vs.
> x++;          -- (2)
> 
> In the Ada case (1) we are forced to have "read from memory",
> "increment" and "write to memory" instructions, whereas in C/C++ (2)
> you can get a single "increment memory" instruction (presuming the
> mentioned assembler instructions exist on a given processor)

Well, all machine code I have seen so far only have a INC for registers. And
if you think for it for a sec.: It can't be another way, even if there is a
INC for memory the CPU still has to read the memory increment internally
write back.

The truth is that x++ is a cheat and has allways been. x++ only exists
because it needs less memory to implement then a peep hole optimiser and -
in the light of modern CPU with powerfull MMUs and triple cache - without
any effect anyway.

Martin
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