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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 20 Jun 2020 10:50:30 -0700
Message-ID: <CAHk-=who=jSnB3RmZTUOn7aShx6rqHeS8E7tDv6f+kVUQUVBAw@mail.gmail.com>
Subject: Re: Commit 25f12ae45fc1 ("maccess: rename probe_kernel_address to
 get_kernel_nofault") causing several OOPSes
To:     "Kenneth R. Crudup" <kenny@panix.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Sat, Jun 20, 2020 at 9:49 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> I'm staring at that opatch and not seeing how it could _possibly_ make
> any difference in code generation.
>
> Which is the obvious next step: would you mind compiling that file
> with and without the patch and sending me the two object files?

Hmm. I did that here (well, I did the probe_roms.s files) and with gcc
I didn't see any difference in the generated code what-so-ever. The
assembler comments changed (because the line numbers changed), but the
code didn't.

With clang, the only difference is a slight code generation change in
the end condition of the top for-loop in 'probe_roms()'.

But that clang difference literally seems to be about instruction
scheduling, not about any semantic change, although I find the code
hard to read because clang has rotated the loop so it looks entirely
different.

Anyway, even after going through a compiler, the difference that
Christoph's patch could possibly make is not obvious.

So yeah, I'd like to see what the code generation difference is for
you, since it seems to matter to your install for some odd reason.

                      Linus