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From: "olegendo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/54089] [SH] Refactor shift patterns
Date: Sat, 03 Jun 2023 16:09:01 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D54089

--- Comment #57 from Oleg Endo <olegendo at gcc dot gnu.org> ---
(In reply to Alexander Klepikov from comment #56)
>=20
> > In that test you can see the unnecessary push/pop of PR.  This is becau=
se
> > initially it wanted to expand as a library call, but then your patterns
> > decided to change the insns. This can or can't be avoided, depending on=
 the
> > case.
>=20
> That's an valuable observation, thank you! I found why. Sometimes 'collap=
sed
> libcall' instruction is emitted in combine pass and 'clobber PR' is set
> then. I commented this out and checked that file and it seems to be OK. I
> will rerun testsuite again to be sure.

Another thing ... the reason why it's desirable to expand into the libcall
earlier is to allow hoisting the function call address outside of loops and
things like that.  That happens only once at the RTL level during compilati=
on
and that's before the combine pass.  Since there are no loops around
optimization passes in the compiler, it's always going to be a compromise. =
 But
it might be OK to repeat a particular optimization pass only for SH, if it
helps anything.

>=20
> Now regarding tests that fail on vanilla GCC and pass with patch. It looks
> like something was changed in the middle of GCC and it started to produce
> other instruction patterns that cannot be catched by existing isns and th=
us
> expanded to library calls. The patch simply fixed it.

Yes, that happens every now and then, as folks work on target independent
optimizations in the middle-end.  SH backend hasn't been keeping up for a w=
hile
in this regard.=