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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 23 Oct 2018 16:23:07 -0700
Message-ID: <CAKwvOdnPbO-CVHCeoz9JWTAVtJOGGMzNv3kPUTYxeFs=uMzH4A@mail.gmail.com>
Subject: Re: [PATCH] x86: Remove -pipe from KBUILD_CFLAGS
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>, mingo@redhat.com,
        bp@alien8.de, hpa@zytor.com, x86@kernel.org,
        LKML <linux-kernel@vger.kernel.org>, namit@vmware.com,
        Kees Cook <keescook@chromium.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>
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On Tue, Oct 23, 2018 at 4:11 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 -Wa,- to KBUILD_CFLAGS, which breaks compiling with Clang (hangs
> indefinitely at compiling init/main.o). This happens because while Clang
> accepts -pipe (and has it documented in its list of supported flags), it
> silently ignores it after this 2010 commit (thanks to Nick Desaulniers
> for tracking this down), meaning that gas just infinitely waits for
> stdin and never receives it.
>
> https://github.com/llvm-mirror/clang/commit/c19a12dc3d441bec62eed55e312b76c12d6d9022
>
> Initially, I had suggested just add -Wa,- to KBUILD_CFLAGS when GCC was
> being used but that was before realizing it is because Clang doesn't do
> anything with -pipe. H. Peter Anvin suggested checking to see if -pipe
> gives us any gains out of GCC. Turns out it might actually be hurting:
>
> With -pipe:
>
> real    3m40.813s
> real    3m44.449s
> real    3m39.648s
>
> Without -pipe:
>
> real    3m38.492s
> real    3m38.335s
> real    3m38.975s
>
> The issue of -Wa,- being passed along to gas without -pipe being
> supported should still probably be fixed on the LLVM side (open issue:
> https://bugs.llvm.org/show_bug.cgi?id=39410) but this is not as much of
> a workaround anymore since it helps both GCC and Clang.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/213
> Suggested-by: H. Peter Anvin <hpa@zytor.com>
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

Thanks Nathan for this patch and the timings and HPA for the suggestions.
Tested-and-Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>  arch/x86/Makefile | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> 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)
>
> --
> 2.19.1
>


-- 
Thanks,
~Nick Desaulniers