Re: objtool warning in cfg80211_edmg_chandef_valid() with ThinLTO

Peter Zijlstra
2021-08-24T21:05:27Z

On Tue, Aug 24, 2021 at 01:08:58PM -0700, Nathan Chancellor wrote:

> The LLVM developers are under the impression that this is an issue with
> objtool; specifically quoting Eli Friedman:
>
> "The backend can, in general, create basic blocks that don't contain any
> instructions, and don't fall through to another block. A jump table entry
> can refer to such a block. I guess certain tools could be confused by this.
>
> If that's the issue, it should be possible to work around it using '-mllvm
> -trap-unreachable'."

So jump-tables are a weak point; ARM64 was having worse problems than
x86 there, they can't even locate them.

As to having a jump-table entry to an empty block and not falling
through; how are we supposed to know? Emitting them is a waste of space,
so I'd say it's a compiler bug :-))

It's been brought up before; but perhaps we should look at an 'informal'
ABI for jump-tables ?