Re: [PATCH] ARM: Emit __gnu_mcount_nc when using Clang 10.0.0 or newer

[Nick Desaulniers]

On Wed, Aug 28, 2019 at 11:27 PM Nathan Chancellor
<natechancellor@xxxxxxxxx> wrote:
>
> Currently, multi_v7_defconfig + CONFIG_FUNCTION_TRACER fails to build
> with clang:
>
> arm-linux-gnueabi-ld: kernel/softirq.o: in function `_local_bh_enable':
> softirq.c:(.text+0x504): undefined reference to `mcount'
> arm-linux-gnueabi-ld: kernel/softirq.o: in function `__local_bh_enable_ip':
> softirq.c:(.text+0x58c): undefined reference to `mcount'
> arm-linux-gnueabi-ld: kernel/softirq.o: in function `do_softirq':
> softirq.c:(.text+0x6c8): undefined reference to `mcount'
> arm-linux-gnueabi-ld: kernel/softirq.o: in function `irq_enter':
> softirq.c:(.text+0x75c): undefined reference to `mcount'
> arm-linux-gnueabi-ld: kernel/softirq.o: in function `irq_exit':
> softirq.c:(.text+0x840): undefined reference to `mcount'
> arm-linux-gnueabi-ld: kernel/softirq.o:softirq.c:(.text+0xa50): more undefined references to `mcount' follow
>
> clang can emit a working mcount symbol, __gnu_mcount_nc, when
> '-meabi gnu' is passed to it. Until r369147 in LLVM, this was
> broken and caused the kernel not to boot because the calling
> convention was not correct. Now that it is fixed, add this to
> the command line when clang is 10.0.0 or newer so everything
> works properly.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/35
> Link: https://bugs.llvm.org/show_bug.cgi?id=33845
> Link: https://github.com/llvm/llvm-project/commit/16fa8b09702378bacfa3d07081afe6b353b99e60
> Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> ---
>  arch/arm/Makefile | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> index c3624ca6c0bc..7b5a26a866fc 100644
> --- a/arch/arm/Makefile
> +++ b/arch/arm/Makefile
> @@ -112,6 +112,12 @@ ifeq ($(CONFIG_ARM_UNWIND),y)
>  CFLAGS_ABI     +=-funwind-tables
>  endif
>
> +ifeq ($(CONFIG_CC_IS_CLANG),y)
> +ifeq ($(shell test $(CONFIG_CLANG_VERSION) -ge 100000; echo $$?),0)
> +CFLAGS_ABI     +=-meabi gnu
> +endif
> +endif
> +

Thanks for the patch!  I think this is one of the final issues w/ 32b
ARM configs when building w/ Clang.

I'm not super enthused about the version check.  The flag is indeed
not recognized by GCC, but I think it would actually be more concise
with $(cc-option) and no compiler or version check.

Further, I think that the working __gnu_mcount_nc in Clang would
better be represented as marking the arch/arm/KConfig option for
CONFIG_FUNCTION_TRACER for dependent on a version of Clang greater
than or equal to Clang 10, not conditionally adding this flag. (We
should always add the flag when supported, IMO.  __gnu_mcount_nc's
calling convention being broken is orthogonal to the choice of
__gnu_mcount_nc vs mcount, and it's the former's that should be
checked, not the latter as in this patch.

>  # Accept old syntax despite ".syntax unified"
>  AFLAGS_NOWARN  :=$(call as-option,-Wa$(comma)-mno-warn-deprecated,-Wa$(comma)-W)
>
> --
> 2.23.0
>


-- 
Thanks,
~Nick Desaulniers