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

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Date:   Wed, 28 Aug 2019 19:28:20 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Ilie Halip <ilie.halip@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Eli Friedman <efriedma@...cinc.com>
Subject: Re: objtool warning "uses BP as a scratch register" with clang-9

On Wed, Aug 28, 2019 at 03:13:21PM -0700, Nick Desaulniers wrote:
> On Tue, Aug 27, 2019 at 12:22 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >
> > On Tue, Aug 27, 2019 at 09:00:52PM +0200, Arnd Bergmann wrote:
> > > On Tue, Aug 27, 2019 at 5:00 PM Ilie Halip <ilie.halip@...il.com> wrote:
> > > >
> > > > > > $ clang-9 -c  crc32.i  -O2   ; objtool check  crc32.o
> > > > > > crc32.o: warning: objtool: fn1 uses BP as a scratch register
> > > >
> > > > Yes, I see it too. https://godbolt.org/z/N56HW1
> > > >
> > > > > Do you still see this warning with -fno-omit-frame-pointer (assuming
> > > > > clang has that option)?
> > > >
> > > > Using this makes the warning go away. Running objtool with --no-fp
> > > > also gets rid of it.
> > >
> > > I still see the warning after adding back the -fno-omit-frame-pointer
> > > in my reduced test case:
> > >
> > > $ clang-9 -c  crc32.i -Werror -Wno-address-of-packed-member -Wall
> > > -Wno-pointer-sign -Wno-unused-value -Wno-constant-logical-operand -O2
> > > -Wno-unused -fno-omit-frame-pointer
> > > $ objtool check  crc32.o
> > > crc32.o: warning: objtool: fn1 uses BP as a scratch register
> >
> > This warning most likely means that clang is clobbering RBP in leaf
> > functions.  With -fno-omit-frame-pointer, leaf functions don't need to
> > set up the frame pointer, but they at least need to leave RBP untouched,
> > so that an interrupts/exceptions can unwind through the function.
> 
> It sounds like clang has `-mno-omit-leaf-frame-pointer` (via
> https://bugs.llvm.org/show_bug.cgi?id=43128#c6).  Arnd, can you give
> that a shot?

I think Arnd already confirmed that -mno-omit-leaf-frame-pointer fixed
it in an earlier email.

-- 
Josh