Frame pointer optimization issues

[Richard Henderson]

On 08/20/2014 08:22 AM, Wilco Dijkstra wrote:
> 2. Change the mid-end to call <arch>_frame_pointer_required even when
> !flag_omit_frame_pointer.

Um, it does that already.  At least as far as I can see from
ira_setup_eliminable_regset and update_eliminables.

It turns out to be much easier to re-enable a frame pointer for a given
function than to disable a frame pointer.  Thus I believe that you should
approach -momit_leaf_frame_pointer as setting flag_omit_frame_pointer, and then
re-enabling it in frame_pointer_required.  This requires more than one line in
common/config/arch/arch.c, but it shouldn't be much more than ten.

> A second issue with frame pointers is that update_eliminables() in reload1.c might set
> frame_pointer_needed to false without any checks.

How?  I don't see that path, since the very first thing update_eliminables does
is call frame_pointer_required -- even before it calls can_eliminate.

Incidentally, I was working on exactly this (plus improving the unwind info)
before I left on vacation a couple weeks ago.  Note that you'll also need to
remove x29 from the fixed registers before eliminating the frame pointer does
any real good.


r~