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

[Josh Poimboeuf <jpoimboe@redhat.com>]

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@gmail.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.

-- 
Josh