| Summary: | [3.3 Regression] bogus warning about passing non-PODs through ellipsis | ||
|---|---|---|---|
| Product: | gcc | Reporter: | Giovanni Bajo <giovannibajo> |
| Component: | c++ | Assignee: | Giovanni Bajo <giovannibajo> |
| Status: | RESOLVED FIXED | ||
| Severity: | minor | CC: | gcc-bugs |
| Priority: | P2 | Keywords: | diagnostic |
| Version: | 3.4.0 | ||
| Target Milestone: | 3.3.3 | ||
| Host: | Target: | ||
| Build: | Known to work: | 2.95.3 | |
| Known to fail: | 3.3.3 3.4.0 4.0.0 | Last reconfirmed: | 2004-01-14 21:40:39 |
I do not know but I will say the warning is partly correct even though it is not called at runtime. Confirmed. I'd say don't do that. I don't think this deserves a particularly high importance. Did you set the milestone because you wanted to fix it yourself? W. Subject: Re: [3.3/3.4 Regression] bogus warning about passing non-PODs through ellipsis
bangerth at dealii dot org wrote:
> Did you set the milestone because you wanted to fix it yourself?
I'll surely look into it, but it's also a regression even if it's very minor.
Giovanni Bajo
I believe that it's reasonable to issue a warning about the code in question, despite the fact that the code appears as the operand to a sizeof. Givoanni, if you disagree, please raise the issue with Jason. If you both agree that this really is a problem, reopen this PR. I spoke with Jason, and he agreed that we can at least try to tackle this issue. Checking skip_evaluation in in convert_arg_to_ellipsis might be enough. Yes, the question to me is not "can we change the compiler's behavior", but "should we?" Subject: Re: [3.3/3.4 Regression] bogus warning about passing non-PODs through ellipsis
mmitchel at gcc dot gnu dot org wrote:
> Yes, the question to me is not "can we change the compiler's behavior"
> , but "should we?"
Passing a non-POD object through ellipsis is undefined behaviour *if* the call
is done. In our situation, there is absolutely no call being performed (nor
code generated where we abort), so there is no undefined behaviour. We're not
warning of an existing potential issue, because the call will simply never
happen. We could even argue that the diagnostic is misleading - by saying "the
code will abort at runtime", we trick the user into believing that there is
indeed a function call.
Giovanni Bajo
Subject: Re: [3.3/3.4 Regression] bogus warning about passing non-PODs through ellipsis "giovannibajo at libero dot it" <gcc-bugzilla@gcc.gnu.org> writes: | mmitchel at gcc dot gnu dot org wrote: | | > Yes, the question to me is not "can we change the compiler's behavior" | > , but "should we?" | | Passing a non-POD object through ellipsis is undefined behaviour *if* the call | is done. In our situation, there is absolutely no call being performed (nor | code generated where we abort), so there is no undefined behaviour. This is a *warning* not an error, and warnings are just about that. If we knew it were actually undefined, we would probably be issueing an error. I can understand your request in the case of sizeof but I do not understand it as rephrased as above. How precisely do you define call being performed or code generated? if (0) { pass through ellipsis } else { do something else } Remember the "0" can come from macro expansion like "HAVE_FEATURE" and that people compile such codes so as to get as maximum of compiler check as they can. (And if you asked me, I consider SFINAE brittle code ;-) | We're not | warning of an existing potential issue, because the call will simply never | happen. We could even argue that the diagnostic is misleading - by saying "the | code will abort at runtime", we trick the user into believing that there is | indeed a function call. We can nitpick on how the actual wording should be, but the basic issue is not there. Subject: Re: [3.3/3.4 Regression] bogus warning about passing non-PODs through ellipsis gdr at integrable-solutions dot net wrote: >> Passing a non-POD object through ellipsis is undefined behaviour >> *if* the call is done. In our situation, there is absolutely no call being >> performed (nor code generated where we abort), so there is no >> undefined behaviour. > I can understand your request in the case of sizeof but I do not > understand it as rephrased as above. How precisely do you define call > being performed or code generated? I was thinking of something along the lines of "potentially evaluated", [basic.def.odr]/2. Anyway, if we both agree on my request for the sizeof() case, I'm happy with it, I'll let you pick my explanation of it which is more correct, or please do provide your own reasoning for this. Giovanni Bajo Subject: Re: [3.3/3.4 Regression] bogus warning about passing non-PODs through ellipsis "giovannibajo at libero dot it" <gcc-bugzilla@gcc.gnu.org> writes: | ------- Additional Comments From giovannibajo at libero dot it 2004-01-20 14:11 ------- | Subject: Re: [3.3/3.4 Regression] bogus warning about passing non-PODs through ellipsis | | gdr at integrable-solutions dot net wrote: | | >> Passing a non-POD object through ellipsis is undefined behaviour | >> *if* the call is done. In our situation, there is absolutely no call being | >> performed (nor code generated where we abort), so there is no | >> undefined behaviour. | | > I can understand your request in the case of sizeof but I do not | > understand it as rephrased as above. How precisely do you define call | > being performed or code generated? | | I was thinking of something along the lines of "potentially evaluated", | [basic.def.odr]/2. Oh yeah, go some paragraphs down, and you'll see that thw whole thing is a little bit confused. And remember, most people want intelligible diagnostics and are not well versed in "standardese" (i.e. common words with slightly different meaning). | Anyway, if we both agree on my request for the sizeof() | case, I'm happy with it, I'll let you pick my explanation of it which is more | correct, or please do provide your own reasoning for this. As I said earlier I understand the request for sizeof() as it is an *integral constant expression* whose value does not involve a runtime semantics and is well defined. -- Gaby Mine! Subject: Bug 13683 CVSROOT: /cvs/gcc Module name: gcc Changes by: giovannibajo@gcc.gnu.org 2004-01-30 15:08:37 Modified files: gcc/cp : ChangeLog call.c Log message: PR c++/13683 * call.c (convert_arg_to_ellipsis): Don't emit a warning if within a sizeof expression.block Patches: http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/cp/ChangeLog.diff?cvsroot=gcc&r1=1.3918&r2=1.3919 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/cp/call.c.diff?cvsroot=gcc&r1=1.455&r2=1.456 Subject: Bug 13683 CVSROOT: /cvs/gcc Module name: gcc Changes by: giovannibajo@gcc.gnu.org 2004-01-30 15:12:45 Modified files: gcc/testsuite : ChangeLog Added files: gcc/testsuite/g++.dg/template: sizeof6.C Log message: PR c++/13683 * g++.dg/template/sizeof6.C: New test. Patches: http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&r1=1.3443&r2=1.3444 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/g++.dg/template/sizeof6.C.diff?cvsroot=gcc&r1=NONE&r2=1.1 Patch here: http://gcc.gnu.org/ml/gcc-patches/2004-01/msg03395.html Committed to mainline, testing in progress for 3.4. Subject: Re: [3.3/3.4 Regression] bogus warning about passing non-PODs through ellipsis "giovannibajo at libero dot it" <gcc-bugzilla@gcc.gnu.org> writes: | ------- Additional Comments From giovannibajo at libero dot it 2004-01-30 15:14 ------- | Patch here: | http://gcc.gnu.org/ml/gcc-patches/2004-01/msg03395.html | | Committed to mainline, testing in progress for 3.4. OK for 3.3.3 -- Gaby Subject: Bug 13683 CVSROOT: /cvs/gcc Module name: gcc Branch: gcc-3_4-branch Changes by: giovannibajo@gcc.gnu.org 2004-01-30 16:21:26 Modified files: gcc/cp : ChangeLog call.c Log message: PR c++/13683 * call.c (convert_arg_to_ellipsis): Don't emit a warning if within a sizeof expression.block Patches: http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/cp/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=1.3892.2.22&r2=1.3892.2.23 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/cp/call.c.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=1.452.2.3&r2=1.452.2.4 Subject: Bug 13683 CVSROOT: /cvs/gcc Module name: gcc Branch: gcc-3_4-branch Changes by: giovannibajo@gcc.gnu.org 2004-01-30 16:24:00 Modified files: gcc/testsuite : ChangeLog Added files: gcc/testsuite/g++.dg/template: sizeof6.C Log message: PR c++/13683 * g++.dg/template/sizeof6.C: New test. Patches: http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=1.3389.2.33&r2=1.3389.2.34 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/g++.dg/template/sizeof6.C.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=NONE&r2=1.1.2.1 Now it's only a 3.3 regression. I'm testing the patch there and waiting for Gaby's approval. Subject: Re: [3.3 Regression] bogus warning about passing non-PODs through ellipsis "giovannibajo at libero dot it" <gcc-bugzilla@gcc.gnu.org> writes: | Now it's only a 3.3 regression. I'm testing the patch there and waiting for | Gaby's approval. I thought I OKed in a previous message. Thanks! -- Gaby Subject: Bug 13683 CVSROOT: /cvs/gcc Module name: gcc Branch: gcc-3_3-branch Changes by: giovannibajo@gcc.gnu.org 2004-01-30 18:20:55 Modified files: gcc/cp : ChangeLog call.c Log message: PR c++/13683 * call.c (convert_arg_to_ellipsis): Don't emit a warning if within a sizeof expression.block Patches: http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/cp/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-3_3-branch&r1=1.3076.2.245&r2=1.3076.2.246 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/cp/call.c.diff?cvsroot=gcc&only_with_tag=gcc-3_3-branch&r1=1.341.2.36&r2=1.341.2.37 Subject: Bug 13683 CVSROOT: /cvs/gcc Module name: gcc Branch: gcc-3_3-branch Changes by: giovannibajo@gcc.gnu.org 2004-01-30 18:31:08 Modified files: gcc/testsuite : ChangeLog Added files: gcc/testsuite/g++.dg/template: sizeof6.C Log message: PR c++/13683 * g++.dg/template/sizeof6.C: New test. Patches: http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-3_3-branch&r1=1.2261.2.353&r2=1.2261.2.354 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/g++.dg/template/sizeof6.C.diff?cvsroot=gcc&only_with_tag=gcc-3_3-branch&r1=NONE&r2=1.1.4.1 Fixed in 3.3.3 as well. Thank you! |
The following code emits a bogus warning: ----------------------------------------------------- struct B {}; struct NonPOD : B {}; struct A { static int check(...); static NonPOD GetNonPOD(void); enum { value = sizeof(A::check(A::GetNonPOD())) }; }; ----------------------------------------------------- bogus.cc:8: warning: cannot pass objects of non-POD type `struct NonPOD' through `...'; call will abort at runtime The problem is that GCC fails to realize that the function call will never happen because it's done in the context of a sizeof() expression. Thus, the warning should not be emitted. Notice that this warning is on by default, and I don't even know if there is a way to shut it off. Regression since 2.95 (where the warning probably didn't exist, but from an user standpoint it's a regression). BTW this was noticed with Boost code, so it's real world stuff. Luckily, the warning is not affected by -Werror. I rate this as a very minor regression, but it would be great if we could fix it someday.