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Date: Tue, 24 Aug 2021 23:05:07 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Nathan Chancellor <nathan@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Sami Tolvanen <samitolvanen@google.com>,
	Kees Cook <keescook@chromium.org>,
	clang-built-linux@googlegroups.com, llvm@lists.linux.dev,
	linux-kernel@vger.kernel.org
Subject: Re: objtool warning in cfg80211_edmg_chandef_valid() with ThinLTO
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References: <5913cdf4-9c8e-38f8-8914-d3b8a3565d73@kernel.org>
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In-Reply-To: <5913cdf4-9c8e-38f8-8914-d3b8a3565d73@kernel.org>

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 ?