linux-kernel - Re: [PATCH v2 5/5] LoongArch: Enable jump table with GCC for objtool

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Message-ID: <62df4c24-68ed-fbfc-ed98-2df796697d89@loongson.cn>
Date: Wed, 6 Nov 2024 13:03:34 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Huacai Chen <chenhuacai@...nel.org>, Josh Poimboeuf
 <jpoimboe@...nel.org>, loongarch@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/5] LoongArch: Enable jump table with GCC for objtool

On 11/05/2024 10:15 PM, Peter Zijlstra wrote:
> On Tue, Nov 05, 2024 at 08:39:06PM +0800, Tiezhu Yang wrote:
>> For now, it is time to remove the compiler option -fno-jump-tables
>> to enable jump table for objtool if the compiler is GCC and it has
>> the compiler option -mannotate-tablejump, otherwise still keep the
>> compiler option -fno-jump-tables to maintain compatibility with the
>> older compilers.

...

>>
>>  ifdef CONFIG_OBJTOOL
>> +ifdef CONFIG_CC_IS_GCC
>> +ifdef CONFIG_CC_HAS_ANNOTATE_TABLEJUMP
>> +KBUILD_CFLAGS			+= $(call cc-option,-mannotate-tablejump)
>> +else
>>  KBUILD_CFLAGS			+= -fno-jump-tables
>>  endif
>> +endif
>> +ifdef CONFIG_CC_IS_CLANG
>> +KBUILD_CFLAGS			+= -fno-jump-tables
>> +endif
>> +endif
>
> This seems excessive. Why split between GCC and Clang, isn't
> CC_HAS_ANNOTATE_JUMPTABLE sufficient?

Thanks for your reply.

In theory, it is sufficient to only check CC_HAS_ANNOTATE_JUMPTABLE
to use -fno-jump-tables or not, and also this is my initial aim.

In fact, when compling with Clang on LoongArch, if the compiler has
the option -mannotate-tablejump and config CC_HAS_ANNOTATE_TABLEJUMP
is set, there still exists some objtool warnings if remove the option
-fno-jump-tables, this is because there are some special cases such
as different rodata relocation type and rodata entry size generated
by Clang, I am working in progress to address the corner issues, and
the final code looks something like this:

ifdef CONFIG_CC_HAS_ANNOTATE_TABLEJUMP
KBUILD_CFLAGS                   += $(call cc-option,-mannotate-tablejump)
else
KBUILD_CFLAGS                   += -fno-jump-tables
endif

Thanks,
Tiezhu