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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109002] -O1 -ftree-pre -ftree-partial-pre
 results in stall value
Date: Fri, 03 Mar 2023 09:38:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109002

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
When doing partial PRE we somehow lose the effect of

  g =3D 1;

we also generate weird PHIs:

<bb 2>
  pretmp_20 =3D h;
  pretmp_22 =3D g;=20


<bb 3>
  # prephitmp_21 =3D PHI <pretmp_20(2), prephitmp_25(10)>
  # prephitmp_23 =3D PHI <pretmp_22(2), prephitmp_24(10)>
  # prephitmp_24 =3D PHI <pretmp_22(2), prephitmp_24(10)>
  # prephitmp_25 =3D PHI <pretmp_20(2), prephitmp_25(10)>

those are all unnecessary ...

There's one old detail I never understood in partial ANTIC compute and that
gets us weird expressions in here, which is that we do not PHI translate
ANTIC_IN when computing PA_OUT.  Fixing that fixes this bug.

I have no idea how to create a testcase for the testsuite though besides
verifying we're not inserting any PHI - but we also fail to partial-PRE the
load
of h here, likely because of the iteration order imposed by the infinite
loop which is probably also key to the miscompilation.

Anyway, I have a fix, but I'm not 100% sure it fixes the actual cause.=