Date: Wed, 6 Jan 2021 13:38:31 -0700 From: Warner Losh <imp@bsdimp.com> To: Ryan Libby <rlibby@freebsd.org> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: clang vs gcc warning flags Message-ID: <CANCZdfpHC6ovmP9syA=8V-92AjdYyRngO0o8m44JwqEtMkNzDA@mail.gmail.com> In-Reply-To: <CAHgpiFwK77o3J6Bm_3GAQRGAJz70=n8Z8bqqLirahL_gqXCM3w@mail.gmail.com> References: <CAHgpiFwK77o3J6Bm_3GAQRGAJz70=n8Z8bqqLirahL_gqXCM3w@mail.gmail.com>
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On Wed, Jan 6, 2021 at 12:53 PM Ryan Libby <rlibby@freebsd.org> wrote: > One of the more annoying things about keeping the gcc build going is the > set of warnings that gcc acts on but clang only recognizes for > compatibility. As a common example, -Wredundant-decls has no effect > in clang, but will break the gcc build. There are a couple dozen such > flags [1][2], and a few of them are in our default set of warnings, such > as in sys/conf/kern.mk, these ones in CWARNFLAGS: > > - Wredundant-decls > - Wnested-externs > - Wmissing-include-dirs > > additionally some warnings are explicitly disabled for clang, but not > for gcc in CWARNEXTRA: > > - Wempty-body > - Wunused-function > > Similarly, in share/mk/bsd.sys.mk: > > - Winline (although, Wno-error'd) > - Wnested-externs > - Wredundant-decls > - Wold-style-definition > > So I suggest we harmonize these somewhat. > > - Wnested-externs I just do not understand. We have specified this > warning flag for some 25 years but to me it seems completely without > value. I suggest we just delete it. > It's to prevent declaring functions inside of functions. why is that bad? It only has scope of the function, and we've decided that's not something we want in the tree... I'd say that the fact we don't have any in the tree shows that it's useful at its intended purpose. What code is causing issues here? > - Wredundant-decls, I'm not sure about. I have never seen this detect > anything that will cause misbehavior, but most of the time that it > fires it does indicate some kind of genuine--but harmless--mistake. > It also tends to prevent declarations that are the same on some platforms, but different on others. > - Wmissing-include-dirs doesn't seem to occur often and usually > indicates a genuine (but again harmless) mistake in a makefile. I > think we should keep it. > I do too. > - Wempty-body, Wunused-function. I'm not sure. These are proscriptive > about things that are not necessarily problems. We are apparently > already clean for them in the kernel gcc build, so perhaps we should > enable them for the kernel clang build. In any case, I think we > should bring these into agreement between clang and gcc, one way or > the other. > We do not remove it. We include the warning, but don't make it fatal for clang. Most of the clang neutering was due to bugs in the early days of clang. I'm not sure anybody has systematically reviewed them since then. Another sticking point may be contrib software. I think we generally > don't want to fail builds of contrib software for things that are > ultimately harmless. For bsd.sys.mk this could be accomplished by > enabling such warnings only at WARNS >= 6. For the kernel, we could > come up with some other mechanism. > The kernel is different. 'Ultimately harmless' is a value judgement. > I'll put up a review soon for deleting -Wnested-externs unless there are > objections. If there is agreement about the others, I'll include those > too. > > Ryan > > [1] https://clang.llvm.org/docs/DiagnosticsReference.html > [2] https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html > _______________________________________________ > freebsd-hackers@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-hackers > To unsubscribe, send any mail to "freebsd-hackers-unsubscribe@freebsd.org" >
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