Re: Unreferenced lock variables

[Simon Wright <simon@pogner.demon.co.uk>]

Robert Dewar <robert_dewar@my-dejanews.com> writes:

> In article <x7vk8viqsla.fsf@pogner.moho>,
>   Simon Wright <simon@pogner.demon.co.uk> wrote:
> > Because I'm concerned that aggressive optimization (-O3)
> > will eliminate the lock variable altogether.
> >
> > The last time this happened was
> >
> >   Dummy : Foo := Some_Function_With_Side_Effects;
> >
> > and -O3 ended up without the side effects.
> 
> Well that would have course have been a significant bug,
> and we have never seen any such bug in -O2 mode (-O3
> is irrelevant here), nor had any such problem reported
> that I can remember. The backend may well remove the
> variable itself, but cannot remove the call.

In what way irrelevant? Am I supposed not to use -O3? or do you just
mean that there is no distinction here between 2 and 3 (I only have
the 3.10 version of the secret manual here, it indicates that -O3 only
adds automatic inlining).

I certainly saw this with GNAT 3.11p, the compiler warned that the
variable was unused, when I compiled -O3 there was a misbehaviour in
my code explainable as above, change the code to achieve the
side-effect via a procedure call in the body and the misbehaviour goes
away. I can try to reconstitute the problem and report it (if it is
indeed a bug to optimize away the initialization of an unused
variable? it struck me at the time that it was much more my fault!)