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

From: Nathan Chancellor <natechancellor@gmail.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
	namit@vmware.com, Thomas Gleixner <tglx@linutronix.de>,
	mingo@redhat.com, bp@alien8.de, x86@kernel.org,
	LKML <linux-kernel@vger.kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Masahiro Yamada <yamada.masahiro@socionext.com>
Subject: Re: [PATCH RFC] x86: Don't include '-Wa,-' when building with Clang
Date: Tue, 23 Oct 2018 14:58:11 -0700	[thread overview]
Message-ID: <20181023215811.GA10771@flashbox> (raw)
In-Reply-To: <96ffedd2-a19d-95de-45fd-3a83025dd48a@zytor.com>

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&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@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

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)

