LoongArch: Pass annotate-tablejump option if LTO is enabled

[ Upstream commit 5dfea66 ]

When compiling with LLVM and CONFIG_LTO_CLANG is set, there exist many
objtool warnings "sibling call from callable instruction with modified
stack frame".

For this special case, the related object file shows that there is no
generated relocation section '.rela.discard.tablejump_annotate' for the
table jump instruction jirl, thus objtool can not know that what is the
actual destination address.

It needs to do something on the LLVM side to make sure that there is the
relocation section '.rela.discard.tablejump_annotate' if LTO is enabled,
but in order to maintain compatibility for the current LLVM compiler,
this can be done in the kernel Makefile for now. Ensure it is aware of
linker with LTO, '--loongarch-annotate-tablejump' needs to be passed via
'-mllvm' to ld.lld.

Note that it should also pass the compiler option -mannotate-tablejump
rather than only pass '-mllvm --loongarch-annotate-tablejump' to ld.lld
if LTO is enabled, otherwise there are no jump info for some table jump
instructions.

Fixes: e20ab7d ("LoongArch: Enable jump table for objtool")
Closes: https://lore.kernel.org/loongarch/20250731175655.GA1455142@ax162/
Reported-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Co-developed-by: WANG Rui <wangrui@loongson.cn>
Signed-off-by: WANG Rui <wangrui@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: seehearfeel

arch/loongarch/Makefile

@@ -102,7 +102,13 @@ KBUILD_CFLAGS			+= $(call cc-option,-mthin-add-sub) $(call cc-option,-Wa$(comma)
 
 ifdef CONFIG_OBJTOOL
 ifdef CONFIG_CC_HAS_ANNOTATE_TABLEJUMP
+# The annotate-tablejump option can not be passed to LLVM backend when LTO is enabled.
+# Ensure it is aware of linker with LTO, '--loongarch-annotate-tablejump' also needs to
+# be passed via '-mllvm' to ld.lld.
 KBUILD_CFLAGS			+= -mannotate-tablejump
+ifdef CONFIG_LTO_CLANG
+KBUILD_LDFLAGS			+= -mllvm --loongarch-annotate-tablejump
+endif
 else
 KBUILD_CFLAGS			+= -fno-jump-tables # keep compatibility with older compilers
 endif