RFR: 8294217: Assertion failure: parsing found no loops but there are some

[Tobias Hartmann]

On Mon, 31 Oct 2022 07:39:54 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

>> This was reported on 11 and is not reproducible with the current
>> jdk. The reason is that the PhaseIdealLoop invocation before EA was
>> changed from LoopOptsNone to LoopOptsMaxUnroll. In the absence of
>> loops, LoopOptsMaxUnroll exits earlier than LoopOptsNone. That wasn't
>> intended and this patch makes sure they behave the same. Once that's
>> changed, the crash reproduces with the current jdk.
>> 
>> The assert fires because PhaseIdealLoop::only_has_infinite_loops()
>> returns false even though the IR only has infinite loops. There's a
>> single loop nest and the inner most loop is an infinite loop. The
>> current logic only looks at loops that are direct children of the root
>> of the loop tree. It's not the first bug where
>> PhaseIdealLoop::only_has_infinite_loops() fails to catch an infinite
>> loop (8257574 was the previous one) and it's proving challenging to
>> have PhaseIdealLoop::only_has_infinite_loops() handle corner cases
>> robustly. I reworked PhaseIdealLoop::only_has_infinite_loops() once
>> more. This time it goes over all children of the root of the loop
>> tree, collects all controls for the loop and its inner loop. It then
>> checks whether any control is a branch out of the loop and if it is
>> whether it's not a NeverBranch.
>
> src/hotspot/share/opto/loopnode.cpp line 4183:
> 
>> 4181: 
>> 4182: #ifdef ASSERT
>> 4183: bool PhaseIdealLoop::only_has_infinite_loops() {
> 
>> This time it goes over all children of the root of the loop
> tree, collects all controls for the loop and its inner loop. It then
> checks whether any control is a branch out of the loop and if it is
> whether it's not a NeverBranch.
> 
> Maybe you can add this summary as a comment here.

+1

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PR: https://git.openjdk.org/jdk/pull/10904