linux-kernel - Re: objtool warning "uses BP as a scratch register" with clang-9

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Message-ID: <CAMuHMdWvD37CQmDcDQAFtxSGv+vXw_dzPMOv_mLVpiQ3EniFtg@mail.gmail.com>
Date:   Wed, 4 Sep 2019 13:53:55 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Ilie Halip <ilie.halip@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: objtool warning "uses BP as a scratch register" with clang-9

Hi Linus,

On Thu, Aug 29, 2019 at 8:31 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Aug 29, 2019 at 10:35 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >
> > Peter suggested to try WRITE_ONCE for the two zero writes to see if that
> > "fixes" it.
>
> I'm sure it "fixes" it.
>
> .. and then where else will we hit this?
>
> It's one thing to turn a structure zeroing into "memset()", but some
> places really can't do it.
>
> We use "-ffreestanding" in some places to make sure that gcc doesn't
> start calling random libc routines. I wonder if we need to make it a
> general rule that it's done unconditionally.
>
> Sadly, I think that ends up also disabling things like
> "__builtin_memcpy()" and friends. Which we _do_ want to have access
> to, because then gcc can inline the memcpy() when we _do_ use
> memcpy().
>
> We used to do all of those heuristics by hand, but wanted to let the
> compiler do them for us.
>
> So:
>
>  - we do want "memcpy()" to become "__builtin_memcpy()" which can then
> be optimized to either individual inlined assignments _or_ to an
> out-of-line call to memcpy().

You can do

    #define memcpy(d, s, n) __builtin_memcpy(d, s, n)

Alpha, m68k, sparc, and x86 already do.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds