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

On Wed, Nov 13, 2024 at 1:11 PM Josh Poimboeuf <[email protected]> wrote:
>
> On Tue, Nov 12, 2024 at 08:26:56PM +0800, Tiezhu Yang wrote:
> > On 11/12/2024 11:15 AM, Xi Ruoyao wrote:
> > > On Wed, 2024-11-06 at 13:03 +0800, Tiezhu Yang wrote:
> > > > 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_CC_HAS_ANNOTATE_TABLEJUMP
> > > > KBUILD_CFLAGS                   += $(call cc-option,-mannotate-tablejump)
> > > > else
> > > > KBUILD_CFLAGS                   += -fno-jump-tables
> > > > endif
> > >
> > > Has -mannotate-tablejump been added to Clang?
> >
> > Yes.
> >
> > > IMO it's better to add it
> > > to Clang first, and add Clang & GCC support at once into objtool.
> >
> > Looks reasonable, the fact is that there are some corner issues
> > compiled with Clang due to different compiler behaviors, most of
> > the issues have been addressed and I need to do more test, I will
> > send v3 with about 10 patches after the coming merge window.
>
> Hm, I didn't know -mannotate-tablejump existed.  We really need
> something which supports all arches, not just loongarch.
>
> Others were looking at adding something similar (adding them to Cc).

Looks like this was added to clang in:
https://github.com/llvm/llvm-project/pull/102411

A comment in llvm/lib/Target/LoongArch/LoongArchAsmPrinter.cpp
describes the scheme:
+  // Emit an additional section to store the correlation info as pairs of
+  // addresses, each pair contains the address of a jump instruction (jr) and
+  // the address of the jump table.

Ard had a prototype in:
https://github.com/llvm/llvm-project/pull/112606
which used relocations rather than a discardable section.
-- 
Thanks,
~Nick Desaulniers