Messages in this thread | | | From | Nick Desaulniers <> | Date | Tue, 23 Oct 2018 11:40:37 -0700 | Subject | Re: [PATCH RFC] x86: Don't include '-Wa,-' when building with Clang |
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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. > > arch/x86/Makefile | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/Makefile b/arch/x86/Makefile > index 5b562e464009..4736dcc1caec 100644 > --- a/arch/x86/Makefile > +++ b/arch/x86/Makefile > @@ -239,7 +239,10 @@ archheaders: > archmacros: > $(Q)$(MAKE) $(build)=arch/x86/kernel arch/x86/kernel/macros.s > > -ASM_MACRO_FLAGS = -Wa,arch/x86/kernel/macros.s -Wa,- > +ASM_MACRO_FLAGS = -Wa,arch/x86/kernel/macros.s > +ifneq ($(cc-name),clang) > +ASM_MACRO_FLAGS += -Wa,- > +endif > export ASM_MACRO_FLAGS > KBUILD_CFLAGS += $(ASM_MACRO_FLAGS) > > > The '-Wa,-‘ was indeed used to take the input from stdin when the ‘-pipe’ > option is used in GCC. Perhaps clang ignores the ‘-pipe’ flag. >
Clang documents support for `-pipe`. https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-pipe
$ clang hello_world.c -no-integrated-as -Wa,-
is simple enough to reproduce the hang (notice no `-pipe`). `strace`'ing the above with and without `-Wa,-` shows clang hung in a `wait4` call after spawning as, while without `-Wa,-` shows it receiving a SIGCHLD.
I've filed a bug against Clang to investigate: https://bugs.llvm.org/show_bug.cgi?id=39410
In the meantime, I think Nathan's patch is the correct way to work around this, if it is indeed a bug in Clang.
Reviewed-and-Tested-by: Nick Desaulniers <ndesaulniers@google.com>
https://stackoverflow.com/questions/1512933/when-should-i-use-gccs-pipe-option has some interesting perspectives on the use of `-pipe`. Thanks for the patch Nathan, and more info Nadav. -- Thanks, ~Nick Desaulniers
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