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

[Tiezhu Yang]

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