Peter Dimov <pdimov@mmltd.net> wrote:
Alexander Terekhov wrote:
Peter Dimov wrote: <snip>
The observable behavior is
write volatile @a 1
nop
read volatile @a x
nop
write volatile @a x-1
nop
exit(x-1)
push 0 call _exit
in the first case, and
Happy now (debugger notwithstanding)?
No. A conforming compiler is not allowed to do that.
C'mon, volatile is brain-dead.
Nobody's arguing otherwise. ;-) But a nop it isn't.
I think Alexander is arguing that without a clear definition of what it means for a memory access to be "observable", the fact that volatile memory accesses are "observable behaviour" doesn't prevent them from being optimised away under the as-if rule. I expect most implementations treat "observable" as "observable by a software or hardware debugger", but of course the standard doesn't and can't say that.