linux-kernel - Re: [PATCH v1 1/2] LoongArch: Make LTO case independent in Makefile

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Message-ID: <7eafa4a6-55bf-2fcb-19ef-b18bd51bfd39@loongson.cn>
Date: Sat, 20 Sep 2025 18:22:11 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>, Nathan Chancellor <nathan@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, WANG Rui <wangrui@...ngson.cn>,
 rust-for-linux@...r.kernel.org, loongarch@...ts.linux.dev,
 linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH v1 1/2] LoongArch: Make LTO case independent in Makefile

On 9/20/25 17:19, Tiezhu Yang wrote:
> On 9/20/25 16:23, Huacai Chen wrote:
>> On Sat, Sep 20, 2025 at 2:15 PM Nathan Chancellor <nathan@...nel.org> 
>> wrote:
>>>
>>> Hi Tiezhu,
>>>
>>> On Tue, Sep 09, 2025 at 05:27:06PM +0800, Tiezhu Yang wrote:
>>>> LTO is not only used for Clang, it maybe used for Rust, make LTO 
>>>> case out
>>>> of CONFIG_CC_HAS_ANNOTATE_TABLEJUMP in Makefile.
>>>>
>>>> This is preparation for later patch, no function changes.
>>>>
>>>> Suggested-by: WANG Rui <wangrui@...ngson.cn>
>>>> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
>>>> ---
>>>>   arch/loongarch/Makefile | 10 +++++-----
>>>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
>>>> index a3a9759414f4..9d80af7f75c8 100644
>>>> --- a/arch/loongarch/Makefile
>>>> +++ b/arch/loongarch/Makefile
>>>> @@ -102,16 +102,16 @@ KBUILD_CFLAGS                   += $(call 
>>>> cc-option,-mthin-add-sub) $(call cc-option,-Wa$(comma)
>>>>
>>>>   ifdef CONFIG_OBJTOOL
>>>>   ifdef CONFIG_CC_HAS_ANNOTATE_TABLEJUMP
>>>> +KBUILD_CFLAGS                        += -mannotate-tablejump
>>>> +else
>>>> +KBUILD_CFLAGS                        += -fno-jump-tables # keep 
>>>> compatibility with older compilers
>>>> +endif
>>>> +ifdef CONFIG_LTO_CLANG
>>>>   # 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
>>>>   endif
>>>>
>>>>   KBUILD_RUSTFLAGS             += 
>>>> --target=loongarch64-unknown-none-softfloat -Ccode-model=small
>>>> -- 
>>>> 2.42.0
>>>>
>>>
>>> This change is now in -next as commit b15212824a01 ("LoongArch: Make LTO
>>> case independent in Makefile"), where it breaks the build for clang-18,
>>> as '--loongarch-annotate-tablejump' is unimplemented there but there is
>>> no version check before using it.
>>>
>>>    $ make -skj"$(nproc)" ARCH=loongarch LLVM=1 mrproper defconfig
>>>
>>>    $ scripts/config -d LTO_NONE -e LTO_CLANG_THIN
>>>
>>>    $ make -skj"$(nproc)" ARCH=loongarch LLVM=1 olddefconfig vmlinuz.efi
>>>    ld.lld: error: -mllvm: ld.lld: Unknown command line argument 
>>> '--loongarch-annotate-tablejump'.
>>>    ...
>>>
>>>    $ scripts/config -s CC_HAS_ANNOTATE_TABLEJUMP
>>>    undef
>> Hmm, maybe we need this?
>> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
>> index ae419e32f22e..fcfa793f9bb0 100644
>> --- a/arch/loongarch/Makefile
>> +++ b/arch/loongarch/Makefile
>> @@ -115,7 +115,7 @@ ifdef CONFIG_LTO_CLANG
>>   # 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_LDFLAGS                 += -mllvm
>> --loongarch-annotate-tablejump
>> +KBUILD_LDFLAGS                 += $(call ld-option,-mllvm
>> --loongarch-annotate-tablejump)
>>   endif
>>   endif
> 
> We need to handle the other case to use -fno-jump-tables if llvm does
> not support -loongarch-annotate-tablejump.
> 
> So, it is better to update the minimal version of llvm to 20, which
> includes the following two important commits:
> 
> [LoongArch] Add options for annotate tablejump
> https://github.com/llvm/llvm-project/commit/4c2c17756739
> 
> [LoongArch] Avoid indirect branch jumps using the ra register
> https://github.com/llvm/llvm-project/commit/21ef17c62645
> 
> If you are OK, I will send a patch to modify the following file:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/min-tool-version.sh#n29 
> 
> 
> and keep arch/loongarch/Makefile as is.

The other way is to keep scripts/min-tool-version.sh as is, and only
modify arch/loongarch/Makefile, something like this (not tested):

config LLD_HAS_ANNOTATE_TABLEJUMP
	def_bool LLD_VERSION >= 200000

ifdef CONFIG_LTO_CLANG
ifdef CONFIG_LLD_HAS_ANNOTATE_TABLEJUMP
KBUILD_LDFLAGS			+= -mllvm --loongarch-annotate-tablejump
else
KBUILD_LDFLAGS			+= -mllvm --no-jump-tables
endif

Update the minimal llvm version or change Makefile to compat various
llvm versions, which one do you prefer? Pease let me know.

Thanks,
Tiezhu