Messages in this thread Patch in this message | | | From | Nick Desaulniers <> | Date | Tue, 23 Oct 2018 15:08:53 -0700 | Subject | Re: [PATCH RFC] x86: Don't include '-Wa,-' when building with Clang |
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On Tue, Oct 23, 2018 at 2:58 PM Nathan Chancellor <natechancellor@gmail.com> wrote: > > On Tue, Oct 23, 2018 at 01:01:22PM -0700, H. Peter Anvin wrote: > > On 10/23/18 11:40, Nick Desaulniers wrote: > > > On Mon, Oct 22, 2018 at 10:11 PM Nadav Amit <namit@vmware.com> wrote: > > >> > > >> at 5:37 PM, Nathan Chancellor <natechancellor@gmail.com> wrote: > > >> > > >> Commit 77b0bf55bc67 ("kbuild/Makefile: Prepare for using macros in > > >> inline assembly code to work around asm() related GCC inlining bugs") > > >> added this flag to KBUILD_CFLAGS, where it works perfectly fine with > > >> GCC. However, when building with Clang, all of the object files compile > > >> fine but the build hangs indefinitely at init/main.o, right before the > > >> linking stage. Don't include this flag when building with Clang. > > >> > > >> The kernel builds and boots to a shell in QEMU with both GCC and Clang > > >> with this patch applied. > > >> > > >> Link: https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FClangBuiltLinux%2Flinux%2Fissues%2F213&data=02%7C01%7Cnamit%40vmware.com%7C871daebc2ca44947d28d08d638811fb5%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C1%7C0%7C636758524579997650&sdata=shuxW81QRrO3TSqbgf462wgZYdLeAKeQEdGRxmnUX30%3D&reserved=0 > > >> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com> > > >> --- > > >> > > >> The reason this patch is labeled RFC is while I can verify that this > > >> fixes the issue, I'm not entirely sure why the '-Wa,-' works for GCC > > >> and not Clang. I looked into what the flag means and I couldn't really > > >> find anything so I just assume it's taking input from stdin? The issue > > >> could stem from how GCC forks gas versus how Clang does it. If this > > >> isn't of concern and the maintainers are happy with this patch as is, > > >> feel free to take it. > > >> > > > > Perhaps someone could actually, you know, time the build and see how > > much -pipe actually matters, if at all? > > > > -hpa > > > > Thank you for the suggestion! With the attached diff for removing > '-pipe' and 'make -j1' with defconfig (just to make sure any variance > would stand out), here are my results: > > -pipe (GCC): > > real 15m55.202s > user 14m17.748s > sys 1m47.496s > > No -pipe (GCC): > > real 16m4.430s > user 14m16.277s > sys 1m46.604s > > -pipe (Clang): > > real 21m26.016s > user 19m21.722s > sys 2m2.606s > > No -pipe (Clang): > > real 21m27.822s > user 19m22.092s > sys 2m4.151s
Looks like Clang eats `-pipe`: https://github.com/llvm-mirror/clang/blob/391667a023f79287f9c40868f34f08c161555556/lib/Driver/Driver.cpp#L962 commit r110007 has the log: Driver: Start ripping out support for -pipe, which is worthless and complicates too many other things.
> > Certainly seems like -pipe doesn't make a ton of difference. If this is > a better fix, I am happy to draft up a proper commit message and send > it out for review. > > ================================================== > > diff --git a/arch/x86/Makefile b/arch/x86/Makefile > index 73f4831283ac..672c689c1faa 100644 > --- a/arch/x86/Makefile > +++ b/arch/x86/Makefile > @@ -213,8 +213,6 @@ ifdef CONFIG_X86_64 > KBUILD_LDFLAGS += $(call ld-option, -z max-page-size=0x200000) > endif > > -# Speed up the build > -KBUILD_CFLAGS += -pipe > # Workaround for a gcc prelease that unfortunately was shipped in a suse release > KBUILD_CFLAGS += -Wno-sign-compare > # > @@ -239,7 +237,7 @@ archheaders: > archmacros: > $(Q)$(MAKE) $(build)=arch/x86/kernel arch/x86/kernel/macros.s > > -ASM_MACRO_FLAGS = -Wa,arch/x86/kernel/macros.s -Wa,- > +ASM_MACRO_FLAGS = -Wa,arch/x86/kernel/macros.s > export ASM_MACRO_FLAGS > KBUILD_CFLAGS += $(ASM_MACRO_FLAGS) >
-- Thanks, ~Nick Desaulniers
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