[RFC PATCH 12/17] gcc-plugins: objtool: Add plugin to detect switch table on arm64

[Josh Poimboeuf]

On Wed, Jan 27, 2021 at 02:15:57PM -0800, Nick Desaulniers wrote:
> > From: Raphael Gault <raphael.gault at arm.com>
> > 
> > This plugins comes into play before the final 2 RTL passes of GCC and
> > detects switch-tables that are to be outputed in the ELF and writes
> > information in an ".discard.switch_table_info" section which will be
> > used by objtool.
> > 
> > Signed-off-by: Raphael Gault <raphael.gault at arm.com>
> > [J.T.: Change section name to store switch table information,
> >        Make plugin Kconfig be selected rather than opt-in by user,
> >        Add a relocation in the switch_table_info that points to
> >        the jump operation itself]
> > Signed-off-by: Julien Thierry <jthierry at redhat.com>
> 
> Rather than tightly couple this feature to a particular toolchain via
> plugin, it might be nice to consider what features could be spec'ed out
> for toolchains to implement (perhaps via a -f flag).

The problem is being able to detect switch statement jump table vectors.

For a given indirect branch (due to a switch statement), what are all
the corresponding jump targets?

We would need the compiler to annotate that information somehow.

> Distributions (like Android, CrOS) wont be able to use such a feature as
> is.

Would a Clang plugin be out of the question?

-- 
Josh