Re: [PATCH 3/4] split-stack for powerpc64

[David Edelsohn <dje dot gcc at gmail dot com>]

On Fri, May 29, 2015 at 10:47 AM, Alan Modra <amodra@gmail.com> wrote:
> Two fixes for -fsplit-stack on powerpc64.  I goofed on the block
> scanned for uses of r12.  ENTRY_BLOCK_PTR_FOR_FN is the fake one.  The
> next block is the first one having insns.
>
> The second change emits an error if people use a global register
> variable r29 with -fsplit-stack.  For example:
> register struct important_stuff *quick_access __asm__ ("r29");
>
> Such code does exist in the wild, but probably doesn't currently use
> -fsplit-stack.
>
> The problem is that r29 is saved by morestack then used to pass the
> old stack pointer to the normal function prologue.  So on entry to a
> function, r29 will be modified.  It would be possible to restore r29
> from the morestack frame in order to let the function body see the
> original r29, I wrote such a patch, but that trick isn't safe if
> someone is using r29 in a signal handler.
>
> I suppose we could restore r29 and downgrade the error to a warning
> that using a global register asm r29 isn't safe in signal handlers
> with -fsplit-stack.  What do you think, David?
>
>         * config/rs6000/rs6000.c (split_stack_arg_pointer_used_p): Scan
>         correct block for use of r12.
>         (rs6000_expand_split_stack_prologue): Error on r29 asm global reg.

Okay.

I doubt that many developers explicitly use r29 as a global register.
I agree with you that it probably is better to error out completely
instead of likely generating buggy code for this particular
combination of r29 asm and split stack.

Thanks, David