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From: "amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/102844] [9/10/11/12 Regression] DOM jump
 threading not copying block that became non-empty
Date: Wed, 20 Oct 2021 12:49:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102844

--- Comment #22 from Andrew Macleod <amacleod at redhat dot com> ---
(In reply to Richard Biener from comment #20)
> (In reply to Aldy Hernandez from comment #18)
> > (In reply to rguenther@suse.de from comment #17)
> > > On Wed, 20 Oct 2021, aldyh at gcc dot gnu.org wrote:
> >=20
> > > > Silly question, why is the SSA form invalid on entry to VRP2?  That=
's just
> > > > asking for trouble.  Is this related to how asserts work?
> > >=20
> > > Well, DOM threading creates invalid SSA (definition not dominating us=
e).
> > > Doesn't have to do anything with VRP or asserts.
> >=20
> > Ah, I see.
> >=20
> > BTW, if this is still the case in mainline, this is bound to be a probl=
em
> > for the ranger.  Andrew, won't we get an UNDEFINED / unreachable if we =
query
> > the non dominating use at this point?
>=20
> Well, invalid IL is invalid - there's no need to "cope" with it.  We have=
 to
> fix the (forward) threading code.

Indeed, invalid is invalid.

And it depends. Any on-entry range is the union of all incoming blocks. If =
we
query a use that has no def anywhere in the dominator tree , then you will =
get
UNDEFINED. If the def is on at least one incoming path, then you will get t=
he
def or some derivative of it.=