linux-kernel - Re: odd objtool 'unreachable instruction' warning

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Message-ID: <20251029095638.06cce7c7@pumpkin>
Date: Wed, 29 Oct 2025 09:56:38 +0000
From: David Laight <david.laight.linux@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, Linux Kernel Mailing List
 <linux-kernel@...r.kernel.org>
Subject: Re: odd objtool 'unreachable instruction' warning

On Tue, 28 Oct 2025 12:29:11 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> Josh, Peter,
>  due to another entirely unrelated discussion, I ended up resurrecting
> my "make asm readable" patch that I have had in my local tree when I
> want to look at the actual generated code for user accesses.
> 
> That is a local hack that just removes the alternative noise for the
> common ops, so that I actually see the fences and clac/stac
> instructions as such, instead of seeing them as nops in the object
> file or as horrible noise in the assembler output.

I've toyed with using explicit nop sequences that would be identifiable
as stac, clac and lfence.

At least that would tell you which is which.

Since the flags can be trashed there are plenty to choose from.
(eg all the cmpb $n,%reg if you don't mind a false dependency.)

	David