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Re: [RFC][PATCH LRA] WIP patch to fix one part of PR87507
- From: Segher Boessenkool <segher at kernel dot crashing dot org>
- To: Jeff Law <law at redhat dot com>
- Cc: Peter Bergner <bergner at linux dot ibm dot com>, Vladimir N Makarov <vmakarov at redhat dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Tue, 6 Nov 2018 18:14:02 -0600
- Subject: Re: [RFC][PATCH LRA] WIP patch to fix one part of PR87507
- References: <2c69adfc-b9db-687d-3a32-08f2efdde647@linux.ibm.com> <ca2d2a92-e8bc-4237-15cb-a3d06617b53b@linux.ibm.com> <5391dd89-e511-76e8-a22a-0d3692b44d19@redhat.com> <d944b375-bae4-819f-5043-54ad085bebb0@linux.ibm.com> <927deb43-5e00-5b9c-91f6-3bc17dd613ef@linux.ibm.com> <8e712264-7ffd-5715-9d4a-35be74344e5a@linux.ibm.com> <2c42a37e-3f96-02e4-1c72-4c3712872f1a@redhat.com>
On Tue, Nov 06, 2018 at 01:27:58PM -0700, Jeff Law wrote:
> So the one worry I have/had in this code is nested subregs. My
> recollection is they do happen in rare cases. But I can also find a
> reference where Jim W. has indicated they're invalid (and I absolutely
> trust Jim on this kind of historical RTL stuff).
>
> https://gcc.gnu.org/ml/gcc/2005-04/msg01173.html
rtl.texi says
@code{subreg}s of @code{subreg}s are not supported. Using
@code{simplify_gen_subreg} is the recommended way to avoid this problem.
(since r133982, from 2008).
> So, after all that, I think we're OK. It might make sense to verify we
> don't have nested subregs in the IL verifiers. Bonus points if you add
> that checking.
Or more general, that what is inside the subreg is a reg, because the
code does rely on that.
Segher