Issue with frame pointer elimination

[Segher Boessenkool]

Hi!

On Thu, Feb 04, 2021 at 11:54:09AM +0100, Henri Cloetens wrote:
> I am working on a custom gcc.9.2.0 back end, and have an issue with 
> frame pointer elimination.

It is a much better idea to use a more current GCC version, and it is
easier to upgrade now than to upgrade later.  Just a hint, do with it
what you want :-)

> - the result is OK till step '276 : ira (register allocation)
> - the result is NG from step '277 onward (reload)

Those numbers are useless to us: they depend on your configuration, and
on the exact GCC version (it often changes within one major+minor+
patchlevel as well).

The LRA pass (you do use LRA I hope?) sets up the hard frame pointer.
If you look at that dump file you can see what happened and what went
wrong (you'll probably need to look at the lra* files in the GCC source
to figure out what happened exactly, what everything means).

> What it did: It used R19 for the frame pointer. This register is 
> call-clobbered, but is not used for
>   argument passing. Some way, it assumed to be free, but it is not.

Is R19 the FRAME_POINTER_REGNUM or the HARD_FRAME_POINTER_REGNUM?  You
didn't show how you have anything of this set up, so it is hard to give
more detailed advice.

> So, the frame pointer is put in it, it is call-clobbered, and the result 
> is incorrect.
> 
> If the compile flag -fomit-frame-pointer is used, the issue goes away.
> 
> Has anybody any idea where to look ?.

You need to save the register in your prologue code if
frame_pointer_needed, etc.


Segher