------- Comment #15 from rguenther at suse dot de 2009-01-25 19:59 ------- Subject: Re: [4.4 regression] Compiler warns about uninitialized variable that is an object with a constructor
On Sun, 25 Jan 2009, mmitchel at gcc dot gnu dot org wrote: > ------- Comment #14 from mmitchel at gcc dot gnu dot org 2009-01-25 19:45 > ------- > Richard -- > > I don't agree that it's necessarily the FE's job to omit all stores to such > types. Our general theory is that FEs get to emit dumb code and the > optimizers > get to fix it up. Of course, I don't object to making the FE generate simpler > code, if that's easy to do; just that if our design relies on that, I think > that's a mistake. Oh, I agree. See my attempt to fix it during gimplification. > I can imagine ways this could come up in other languages as well. For > example, > copying a C structure with an anonymous bit-field, but no other content, or an > Ada record that uses Ada's layout directives to control size. > > Therefore, I don't think that the key here is "zero-size". Instead, it's the > fact that structure cannot be initialized. That's useful both for warnings > and > for optimization; it can't be initialized, so there's no point about warning > about uninitialized uses, and there's no reason to actually generate code for > the copies. Ok, I think mapping cannot be initialized to zero-size is ok, as that is the only thing we can currently query (and we even specialize this for C++ to deal with the 1 byte vs. empty case). > That leads to something I do think is something that the FEs could be asked to > do: set a bit on the type to indicate that it is uninitializable or, if you > like, logically empty. > > I also don't see this as a P1 defect. It's certainly annoying, but, > fundamentally, it limits the utility of a warning which has been known to give > false positives for a long time. Important to fix, yes -- but as important as > generating wrong code? It's a P1 defect as we didn't warn for uninitialized structure uses in any previous relelase. While we can argue that it is safe to downgrade this to P2 I think we should at least try to fix this issue for 4.4.0. Richard. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38851
