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

[Nathan Chancellor]

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>
---

v1 -> v2:

* Add Nick and Stefan's reviewed by tags
* Move version check from Makefile to Kconfig. This prevents '-pg` from
  ever being added if '-meabi gnu' would produce a non-booting kernel
  and it allows clang 9.0.0 and earlier to build and link all*config
  kernels because the function tracer can't be selected.

 arch/arm/Kconfig  | 2 +-
 arch/arm/Makefile | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index a98c7af50bf0..440ad41e77e4 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -83,7 +83,7 @@ config ARM
 	select HAVE_FAST_GUP if ARM_LPAE
 	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
 	select HAVE_FUNCTION_GRAPH_TRACER if !THUMB2_KERNEL && !CC_IS_CLANG
-	select HAVE_FUNCTION_TRACER if !XIP_KERNEL
+	select HAVE_FUNCTION_TRACER if !XIP_KERNEL && (CC_IS_GCC || CLANG_VERSION >= 100000)
 	select HAVE_GCC_PLUGINS
 	select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7)
 	select HAVE_IDE if PCI || ISA || PCMCIA
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index a43fc753aa53..aa7023db66c7 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -115,6 +115,10 @@ ifeq ($(CONFIG_ARM_UNWIND),y)
 CFLAGS_ABI	+=-funwind-tables
 endif
 
+ifeq ($(CONFIG_CC_IS_CLANG),y)
+CFLAGS_ABI	+= -meabi gnu
+endif
+
 # Accept old syntax despite ".syntax unified"
 AFLAGS_NOWARN	:=$(call as-option,-Wa$(comma)-mno-warn-deprecated,-Wa$(comma)-W)
 
-- 
2.23.0