Re: [PATCH v2] ARM: Emit __gnu_mcount_nc when using Clang 10.0.0 or newer

[Matthias Kaehlcke]

On Fri, Aug 30, 2019 at 11:05:31PM -0700, Nathan Chancellor 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 with '-pg' because the
> calling convention was not correct. Always build with '-meabi gnu'
> when using clang but ensure that '-pg' (which is added with
> CONFIG_FUNCTION_TRACER and its prereq CONFIG_HAVE_FUNCTION_TRACER)
> cannot be added with it unless this is fixed (which means using
> clang 10.0.0 and newer).
> 
> 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
> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Reviewed-by: Stefan Agner <stefan@xxxxxxxx>
> Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>

Reviewed-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>