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

[Geert Uytterhoeven]

Hi Linus,

On Thu, Aug 29, 2019 at 8:31 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Aug 29, 2019 at 10:35 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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@xxxxxxxxxxxxxx

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