Re: [PATCH RFC] x86: Don't include '-Wa,-' when building with Clang

[Nick Desaulniers]

On Tue, Oct 23, 2018 at 3:44 PM Nathan Chancellor
<natechancellor@xxxxxxxxx> wrote:
>
> On Tue, Oct 23, 2018 at 03:08:53PM -0700, Nick Desaulniers wrote:
> > On Tue, Oct 23, 2018 at 2:58 PM Nathan Chancellor
> > <natechancellor@xxxxxxxxx> 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@xxxxxxxxxx> wrote:
> > > > >>
> > > > >> at 5:37 PM, Nathan Chancellor <natechancellor@xxxxxxxxx> 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&amp;data=02%7C01%7Cnamit%40vmware.com%7C871daebc2ca44947d28d08d638811fb5%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C1%7C0%7C636758524579997650&amp;sdata=shuxW81QRrO3TSqbgf462wgZYdLeAKeQEdGRxmnUX30%3D&amp;reserved=0
> > > > >> Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> > > > >> ---
> > > > >>
> > > > >> 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.
> >
>
> In that case, we can either keep this change (I'll resend with the
> explanation that Clang doesn't respect -pipe) or we can just rip out
> -pipe for GCC too. Here are three separate results for GCC with my
> normal jobs flag:
>
> -pipe (GCC):
>
> real    3m40.813s
> real    3m44.449s
> real    3m39.648s
>
> No -pipe (GCC):
>
> real    3m38.492s
> real    3m38.335s
> real    3m38.975s
>
> Practically no variance.

Thanks for these measurements.  With these in mind I agree with HPA
that `-pipe -Wa,-` doesn't buy us anything, and would be simpler to
remove it for compatibility with Clang.

>
> Thanks,
> Nathan
>
> > >
> > > 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



-- 
Thanks,
~Nick Desaulniers