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From: "jskumari at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/109009] Shrink Wrap missed opportunity
Date: Tue, 27 Jun 2023 13:18:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109009

--- Comment #11 from Surya Kumari Jangala <jskumari at gcc dot gnu.org> ---
(In reply to Peter Bergner from comment #9)
> (In reply to Surya Kumari Jangala from comment #8)
> > However, while computing the save/restore cost, we are considering only=
 the
> > memory move cost but not the BB frequency. I think it is important to
> > consider the frequency too.=20
>=20
> Yes, you'll need to factor in the BB frequency.  Since the save/restore c=
ode
> will go into (at this point modulo shrink-wrapping later) the prologue and
> epilogue, you'll want something like: <prologue/epilogue freq> * 2 *
> "ira_memory_move_cost".

Thanks for confirming that we have to factor in the BB frequency.=20

> I think the issue here, is that the "frequency" of the entry block isn't
> '1', but some larger value.  I'm not 100% sure, but maybe you can use:
>   REG_FREQ_FROM_BB (ENTRY_BLOCK_PTR_FOR_FN (cfun))

This works. It gives the frequency of the entry block.

> for the BB frequency of the prologue/epilogue?
>=20
>=20
> > We factor in the frequency when we compute the
> > savings on removed copies in allocno_copy_cost_saving(). In this routin=
e, we
> > multiply the frequency with ira_register_move_cost. So why not factor i=
n the
> > frequency for memory move cost?
>=20
> allocno_copy_cost_saving() is dealing with an actual copy instruction(s),=
 so
> a real instruction in a specific BB, so it knows the frequency of the
> copy(ies).  The ira_memory_move_cost is more of a HW cost of a generic
> load/store and it isn't tied to a specific instruction, so there is no
> frequency to scale by, so you'll need to do that manually here.=