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

[Nick Desaulniers]

On Tue, Aug 27, 2019 at 12:22 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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@xxxxxxxxx> 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?
-- 
Thanks,
~Nick Desaulniers