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

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Message-ID: <c9c4c0e6-946d-95de-9c03-a4b8e1636b81@loongson.cn>
Date: Sat, 20 Sep 2025 17:19:00 +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 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.

Thanks,
Tiezhu