On Fri, Jan 2, 2015 at 6:14 PM, Richard Smith <[email protected]> wrote: > 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.
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. _______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
