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

[Nick Desaulniers <ndesaulniers@google.com>]

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.
>
> 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