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From: "amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/104038] ranger infinite loop on a PHI
 statement while building Clang
Date: Sun, 16 Jan 2022 15:57:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104038

--- Comment #8 from Andrew Macleod <amacleod at redhat dot com> ---
The issue is transitive relations are being registered in an unbounded way.
SymCount_369 =3D SymCount_368 + 1;
SymCount_370 =3D SymCount_369 + 1;
SymCount_371 =3D SymCount_370 + 1;


_370 > _369, and _369 is > _368, so it registers a transitive relation betw=
een
_370 > _368
Then we register _371 > 370 and transitive relations_371 > 369 as well as _=
371
> 368

This cascades as the chain gets longer and would be quadratic in nature.
This is then compounded by the currently linear nature of looking up a rela=
tion
within the current basic block.

On monday, I will introduce a capping heuristic to transitivity and/or limi=
ting
the number of relations we can add to a block, preventing either situation =
from
being an issue going forward.=