Re: [PATCH] gas: Improve documentation for cfi_remember/restore_state

[Martin Galvan <martin dot galvan at tallertechnologies dot com>]

On Thu, Apr 14, 2016 at 8:16 PM, Alan Modra <amodra@gmail.com> wrote:
>
> On Thu, Apr 14, 2016 at 11:23:08AM -0300, Martin Galvan wrote:
> > You mean something like:
> >
> >     je label
> >     popq %rbx
> >     .cfi_remember_state
> >     .cfi_restore %rbx
> >     popq %rbp
> >     .cfi_restore %rbp
> >     popq %r12
> >     .cfi_restore %r12
> >     ret
> >
> > label:
> >     .cfi_restore_state
> >     /* Do something else */
> >
> > In that case we're using .cfi_restore_state to save us having to use
> > multiple CFI directives to recreate the original save locations.
>
> Yes, exactly.  However the above example shows a gcc bug!

If you're referring to the last example I sent (with the three pops),
I wrote that manually. So it's a programmer bug, not gcc's :)

> Hmm, seems like current mainline gcc is buggy in this area on x86_64.
> I see this sort of thing around a tail call:
>         je      .L4
>         popq    %rbp
>         .cfi_remember_state
>         .cfi_def_cfa 7, 8
>         movl    $1, %edi
>         jmp     *%rax
> .L4:
>         .cfi_restore_state
> So the cfa is set back to rsp on popping rbp, but there ought to be a
> ".cfi_restore 6".  Otherwise when an async interrupt hits after the
> pop of rbp, the unwinder will load rbp from the stack, which has just
> been trashed by the interrupt handler..

That's probably true, though. I can look into it a bit more if you
want. I know next to nothing about gcc internals, but a couple guys at
the office can give me a hand with it.

> It might be better to choose an example from gcc -fomit-frame-pointer
> -fasynchronous-unwind-tables code.

Could we keep my 3-pop example if I added the required CFA adjustment?
I'd like to keep the example as simple as possible for the
documentation.