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.

[Aaron Ballman]

On Fri, Jan 2, 2015 at 6:14 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> 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.

I think we should always assume a volatile *store* is 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.

Okay, I will make that change shortly*.

Thanks!

~Aaron

* Shortly may mean Monday.