[RFC] PR69195, Reload confused by invalid reg equivs

[Alan Modra]

On Fri, Mar 04, 2016 at 06:39:54PM +0100, Bernd Schmidt wrote:
> I've managed to reproduce this, and I think your analysis of the problem is
> correct. So the patch is probably ok from the point of you of "will it
> work". I can probably be convinced to approve it as-is, but I wonder if
> you'd be prepared to try something else first.
> 
> This whole area in IRA is turning into spaghetti a little bit. I would
> prefer that we schedule a new small optimization pass beforehand (either as
> a real pass or just a function call) that does only the label-substituting
> trick from update_equiv_regs, then removes dead code as necessary. When we
> then get to IRA and setting up equiv regs, we should no longer get to a
> point where we need to reverify equivalences or update regstat.

I had the same thought myself, but wondered what I'd be getting into
so didn't mention the idea.  ;-)  I'll have a go.  (Not sure if I'd
trust me to get it right though, for stage 4.)

Something for later:  It seems to my untrained eye that more than just
the label substitution should be separated out.  All of the latter
half of update_equiv_regs doing global combine and moving insns could
be better separated from the rest of ira.  It is to some extent, by
use of reg_equiv[] vs. ira_reg_equiv[], but shares the REG_EQUIV
notes.  That means the REG_EQUIV notes created by ira need to take
into account ira, lra and reload behaviour and are thus rather more
restrictive than needed for the mini-combine pass.  eg. See the
comment about calls in validate_equiv_mem.

Incidentally, an all-langs bootstrap and regression test of gcc
triggers delete_unreachable_blocks just once for x86_64 and never for
powerpc64le.  The file is testsuite/gfortran.dg/pr46755.f, which of
course isn't that surprising since the pr46755 fix added the
delete_unreachable_blocks call.

-- 
Alan Modra
Australia Development Lab, IBM