On Tue, Jun 12, 2018 at 11:51 AM H. Peter Anvin <hpa@xxxxxxxxx> wrote: > > <caoj.fnst@xxxxxxxxxxxxxx>,Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>,jarkko.sakkinen@xxxxxxxxxxxxxxx,jgross@xxxxxxxx,Josh Poimboeuf <jpoimboe@xxxxxxxxxx>,Matthias Kaehlcke <mka@xxxxxxxxxxxx>,thomas.lendacky@xxxxxxx,Thiebaud Weksteen <tweek@xxxxxxxxxx>,mjg59@xxxxxxxxxx,joe@xxxxxxxxxxx > From: hpa@xxxxxxxxx > Message-ID: <191E4EBE-4CB2-4C8B-AB61-689A91FFE7A8@xxxxxxxxx> > > On June 12, 2018 11:33:14 AM PDT, Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote: > >On Fri, Jun 8, 2018 at 3:04 AM Sedat Dilek <sedat.dilek@xxxxxxxxx> > >wrote: > >> > >> On Fri, Jun 8, 2018 at 9:59 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote: > >> > On Thu, Jun 7, 2018 at 10:49 PM, Nick Desaulniers > >> > <ndesaulniers@xxxxxxxxxx> wrote: > >> >> Functions marked extern inline do not emit an externally visible > >> >> function when the gnu89 C standard is used. Some KBUILD Makefiles > >> >> overwrite KBUILD_CFLAGS. This is an issue for GCC 5.1+ users as > >without > >> >> an explicit C standard specified, the default is gnu11. Since c99, > >the > >> >> semantics of extern inline have changed such that an externally > >visible > >> >> function is always emitted. This can lead to multiple definition > >errors > >> >> of extern inline functions at link time of compilation units whose > >build > >> >> files have removed an explicit C standard compiler flag for users > >of GCC > >> >> 5.1+ or Clang. > >> >> > >> >> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > >> >> Suggested-by: H. Peter Anvin <hpa@xxxxxxxxx> > >> >> Suggested-by: Joe Perches <joe@xxxxxxxxxxx> > >> > > >> > I suspect this will break Geert's gcc-4.1.2, which I think doesn't > >have that > >> > attribute yet (4.1.3 or higher have it according to the > >documentation. > >> > > >> > It wouldn't be hard to work around that if we want to keep that > >version > >> > working, or we could decide that it's time to officially stop > >supporting > >> > that version, but we should probably decide on one or the other. > > > >Heh, so earlier we decided against compiler flags (-std=gnu89 or > >-fgnu89-inline) in preference to function attributes. The function > >attribute is preferable as some of the Makefiles [accidentally?] > >overwrite KBUILD_CFLAGS, which is problematic for gcc 5.1 users as the > >implicit c standard used was changed to gnu11 from gnu89. What's nice > >is that to support gcc 4.1 users, we simply don't need to add any > >attribute, as their implicit c standard is gnu89 which has the > >semantics for extern inline that we want. I have a simple change to > >this patch that can support users of various gcc versions, see below: > > > >> Good point. > >> What is the minimum requirement of GCC version currently? > >> AFAICS x86/asm-goto support requires GCC >= 4.5? > > > >Yes, but that's only for x86, IIUC. It seems the kernel may have > >different minimum required versions of GCC based on arch then? That > >may be ok, but I'm not sure that's easy to keep track of without > >having it explicitly stated somewhere like the docs perhaps? > > > >> Just FYI... > >> ...saw the last days in upstream commits that kbuild/kconfig for > >> 4.18-rc1 offers possibilities to check for cc-version dependencies. > > > >Those will be helpful. If we want to pursue compiler flags, which get > >set some Makefiles, then yes. But I think a simpler change to my > >patch would be as below. > > > >It seems gcc did not get __has_attribute [0] until 5.1, but will > >define __GNUC_GNU_INLINE__ if supported. [1] Unfortunately, Clang > >does not define __GNUC_GNU_INLINE__ [2]. So a proper feature test > >might look like: > > > >``` > >#ifndef __has_attribute > >#define __has_attribute(x) 0 > >#endif > > > >#if defined(__GNUC_GNU_INLINE__) || __has_attribute(gnu_inline) > >#define __gnu_inline __attribute__(gnu_inline) > >#endif > > > >#define inline inline __attribute__((always_inline, unused)) notrace > >__gnu_inline > >``` > > > >Thoughts on this approach? I can send a v5 tomorrow if there's no > >major issues. Feedback appreciated, as always. > > > >[0] https://clang.llvm.org/docs/LanguageExtensions.html#has-attribute > >[1] > >https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes > >[2] https://bugs.llvm.org/show_bug.cgi?id=37784 > > Please fix clang. It isn't all that hard to fix. > > However, __GCC_GNU_INLINE__ means you are in GNU mode by default, on gcc's new enough to have multiple misses. > > The right thing to look for is __GCC_STDC_INLINE__ in which case you need the attribute. Oh, by [0] __GCC_STDC_INLINE__ is indeed actually the correct symbol to check for. Clang does support that, so nothing to fix there. > By the way, you should check clang against gcc's predefined macros by doing: > > gcc [options] -x c -Wp,-dM -E /dev/null | sort > > Options can change the predefined macros substantially, especially the, -std=, arch and -O options. -x c can be replaced with e.g. -x c++, objective-c, assembler-with-cpp etc. Neat, I'll have to bookmark that incantation. I can s/gcc/clang/ to get a similar list (which is how I know it supports __GCC_STDC_INLINE__). Patch now becomes something like: #ifdef __GNUC_GNU_INLINE__ #define __gnu_inline __attribute__((gnu_inline)) #else #define __gnu_inline #endif #define inline inline __attribute__((always_inline, unused)) notrace __gnu_inline ... Issues with that approach? [0] https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html -- Thanks, ~Nick Desaulniers -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
