Re: Add unwind information to mips epilogues

[Richard Henderson <rth at redhat dot com>]

On 09/05/2011 01:36 PM, Richard Sandiford wrote:
> Richard Henderson <rth@redhat.com> writes:
>> On 09/01/2011 12:13 AM, Richard Sandiford wrote:
>>> Also, for the frame_pointer_required case, it looks like there's a
>>> window between the restoration of the frame pointer and the deallocation
>>> of the stack in which the CFA is still defined in terms of the frame
>>> pointer register.  Is that significant?  If not (e.g. because we
>>> should never need to unwind at that point) then why do we still
>>> update the CFA here:
>>
>> What window are you referring to?
>>
>> Best I can tell from looking at the patch we restore the CFA to the
>> stack pointer before anything else.
> 
> Well, I'm probably showing my ignorance here, but what I meant was:
> we attach the DEF_CFA and the CFA_RESTORE notes to the final stack
> deallocation.  I was just worried that, immediately before that
> deallocation, the CFA is still defined in terms of the frame pointer,
> even though the frame pointer has already been restored.  So it seemed
> like someone trying to unwind at that point would get the wrong CFA.

I don't see that.  I'm pasting from mainline but not the patch, but here:

>   /* Copy TARGET into the stack pointer.  */
>   if (target != stack_pointer_rtx)
>     mips_emit_move (stack_pointer_rtx, target);

We restore the stack pointer from the frame pointer before restoring
the frame pointer.  The patch changes this spot with DEF_CFA.  So the
problem you're suggesting ought not be true.


r~