r224465 - Adding a -Wunused-value warning for expressions with side effects used in an unevaluated expression context, such as sizeof(), or decltype(). Also adds a similar warning when the expression passed to typeid() *is* evaluated, since it is equally likely that the user would expect the expression operand to be unevaluated in that case.

[Richard Smith]

On Fri, Jan 2, 2015 at 2:56 PM, Aaron Ballman <aaron at aaronballman.com>
wrote:

> On Tue, Dec 23, 2014 at 11:01 AM, Joerg Sonnenberger
> <joerg at britannica.bec.de> wrote:
> > On Wed, Dec 17, 2014 at 09:57:17PM -0000, Aaron Ballman wrote:
> >> Author: aaronballman
> >> Date: Wed Dec 17 15:57:17 2014
> >> New Revision: 224465
> >>
> >> URL: http://llvm.org/viewvc/llvm-project?rev=224465&view=rev
> >> Log:
> >> Adding a -Wunused-value warning for expressions with side effects used
> >> in an unevaluated expression context, such as sizeof(), or decltype().
> >
> > I think in the case of sizeof, it is too aggressive. It triggered in
> > NetBSD's mount on logic like the following:
> >
> >         char ** volatile argv;
> >
> >         argv = calloc(count, sizeof(*argv));
> >
> > because the volatile marker supposed makes the *argv have side effects.
> > It is present in this case, because the function later on uses vfork and
> > GCC complains about trashing local variables for a function that returns
> > twice. setjmp would be slightly less obscure.
>
> The original patch had volatile reads as not being side-effecting, but
> Richard desired the current behavior. The specifications are pretty
> clear in that reads of a volatile value *are* side-effecting, but I
> originally believed as you did, the above code is pretty idiomatic.
>
> > I think it should *not* trigger in this case for two important reasons:
> >
> > (1) The sizeof use is completely idiomatic.
>
> Agreed. However, the dereference of a volatile value is still
> side-effecting, and having a side-effecting operation appear in a
> context where side effects are not evaluated is what this patch was
> all about.
>
> > (2) The only workaround for the warning introduces possible maintainance
> > costs, as it would require duplicating the type of argv.
>
> That strikes me as the bigger reason why the warning should be
> silenced in this particular case -- it *is* idiomatic code, and the
> way to silence the warning isn't particularly ideal.
>
> > A C programmer should know and expect the memory access to not happen.
> > I would say this is different from the case Aaron gave on IRC about
> > sizeof(i++). That's a side effect most would expect to still happen.
> > To keep the number of exceptions small, I propose the relax the warning
> > to not trigger on dereference of volatile pointers.
>
> I'm not opposed to this, but would like Richard to weigh in.


Well, my position on this was "let's try warning on it and see what
happens" =)

But I don't buy the argument here: a C programmer should know and expect
side-effects inside sizeof to not happen, whether they're due to 'volatile'
or an increment  (this warning is *supposed* to have false-positives if the
expression always has a side-effect that the programmer doesn't expect to
actually happen...).

It seems like the issue is that 'volatile' is sometimes used for reasons
other than to ensure a side-effect (the example code above isn't a great
case for this, where it's apparently been used as an attempt to provide
atomicity/thread safety across a vfork, but there are other reasonable
cases where it's used as a compiler optimization barrier), so we shouldn't
assume a volatile load or store is *always* a side-effect. Also, in the
general case of an access through a volatile-qualified type, we don't
actually know whether the object itself is volatile-qualified, which
affects whether there is actually a side-effect. On that basis, I think we
should unconditionally treat volatile access as the maybe-side-effect case
rather than the always-side-effect case.
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