ipa-cp: Separate and increase the large-unit parameter

ipa-cp: Separate and increase the large-unit parameter

A previous patch in the series has taught IPA-CP to identify the
important cloning opportunities in 548.exchange2_r as worthwhile on
their own, but the optimization is still prevented from taking place
because of the overall unit-growh limit.  This patches raises that
limit so that it takes place and the benchmark runs 30% faster (on AMD
Zen2 CPU at least).

Before this patch, IPA-CP uses the following formulae to arrive at the
overall_size limit:

base = MAX(orig_size, param_large_unit_insns)
unit_growth_limit = base + base * param_ipa_cp_unit_growth / 100

since param_ipa_cp_unit_growth has default 10, param_large_unit_insns
has default value 10000.

The problem with exchange2 (at least on zen2 but I have had a quick
look on aarch64 too) is that the original estimated unit size is 10513
and so param_large_unit_insns does not apply and the default limit is
therefore 11564 which is good enough only for one of the ideal 8
clonings, we need the limit to be at least 16291.

I would like to raise param_ipa_cp_unit_growth a little bit more soon
too, but most certainly not to 55.  Therefore, the large_unit must be
increased.  In this patch, I decided to decouple the inlining and
ipa-cp large-unit parameters.  It also makes sense because IPA-CP uses
it only at -O3 while inlining also at -O2 (IIUC).  But if we agree we
can try raising param_large_unit_insns to 13-14 thousand
"instructions," perhaps it is not necessary.  But then again, it may
make sense to actually increase the IPA-CP limit further.

I plan to experiment with IPA-CP tuning on a larger set of programs.
Meanwhile, mainly to address the 548.exchange2_r regression, I'm
suggesting this simple change.

gcc/ChangeLog:

2020-09-07  Martin Jambor  <mjambor@suse.cz>

	* params.opt (ipa-cp-large-unit-insns): New parameter.
	* ipa-cp.c (get_max_overall_size): Use the new parameter.