RFR: 8257575: C2: "failed: only phis" assert failure in loop strip mining verification

[Roland Westrelin]

On Fri, 4 Dec 2020 08:38:56 GMT, Nils Eliasson <neliasso at openjdk.org> wrote:

>> The loop in the test is a counted loop with a load pinned on the
>> backedge. The loop is strip mined. When the outer loop is expanded, it
>> ends up with a load on the backedge too as it must mirror the inner
>> loop. The load is input to a Phi. After expansion, the load on the
>> backedge of the outer loop is found to be rendundant with a dominating
>> load. That causes the Phi to be eliminated. The load on the backedge
>> of the inner loop is not eliminated. As a result, verification code
>> finds the inner and outer loops to have a different number of Phis
>> which is unexpected.
>> 
>> The load on the backedge of the inner loop could be eliminated. It's
>> not because it's not enqueued to be processed by the IGVN after loop
>> strip mining expansion. But beyond this immediate problem, because
>> IGVN processes each load separately, I think it's possible that it
>> succeeds in eliminating one of the loads but not the other (the graph
>> could have changed too much in the meantime). Because of that concern
>> and because it's a corner case, I think the most robust fix is to
>> relax the assert.
>
> Marked as reviewed by neliasso (Reviewer).

@neliasso @TobiHartmann thanks for the reviews

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PR: https://git.openjdk.java.net/jdk/pull/1555