On Fri, Jan 2, 2015 at 2:56 PM, Aaron Ballman <[email protected]> wrote:
> On Tue, Dec 23, 2014 at 11:01 AM, Joerg Sonnenberger > <[email protected]> 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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