From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: [PATCH v4 1/3] compiler-gcc.h: add gnu_inline to all inline declarations Date: Tue, 12 Jun 2018 11:51:22 -0700 Message-ID: <201806121851.w5CIpTY7134381@mail.zytor.com> References: <20180607204927.219329-1-ndesaulniers@google.com> <20180607204927.219329-2-ndesaulniers@google.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Nick Desaulniers , sedat.dilek@gmail.com, Arnd Bergmann Cc: Kate Stewart , linux-efi@vger.kernel.org, brijesh.singh@amd.com, "J. Kiszka" , Will Deacon , virtualization@lists.linux-foundation.org, Masahiro Yamada , Manoj Gupta , boris.ostrovsky@oracle.com, mawilcox@microsoft.com, x86@kernel.org, akataria@vmware.com, Greg Hackmann , Cao.jin@zytor.com, mingo@redhat.com, geert@linux-m68k.org, David Rientjes , Andrey Ryabinin , Linux Kbuild mailing list , Kees Cook , rostedt@goodmis.org, acme@redhat.com, Michal Marek , Thomas Gleixner , Alistair Strachan , tstellar@redhat.com, Ard Biesheuvel L List-Id: linux-efi@vger.kernel.org ,Greg KH ,jarkko.sakkinen@linux.intel.com,jgross@suse.com,Josh Poimboeuf ,Matthias Kaehlcke ,thomas.lendacky@amd.com,Thiebaud Weksteen ,mjg59@google.com,joe@perches.com From: hpa@zytor.com Message-ID: <191E4EBE-4CB2-4C8B-AB61-689A91FFE7A8@zytor.com> On June 12, 2018 11:33:14 AM PDT, Nick Desaulniers wrote: >On Fri, Jun 8, 2018 at 3:04 AM Sedat Dilek >wrote: >> >> On Fri, Jun 8, 2018 at 9:59 AM, Arnd Bergmann wrote: >> > On Thu, Jun 7, 2018 at 10:49 PM, Nick Desaulniers >> > 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 >> >> Suggested-by: H. Peter Anvin >> >> Suggested-by: Joe Perches >> > >> > 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. 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. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.