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