Messages in this thread | | | Date | Tue, 23 Oct 2018 15:04:37 -0700 | Subject | Re: [PATCH RFC] x86: Don't include '-Wa,-' when building with Clang | From | hpa@zytor ... |
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On October 23, 2018 2:58:11 PM PDT, 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 > >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)
make -j1 makes the test useless. You should aim for optimal performance for your machine. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
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