RFR: 8281429: PhiNode::Value() is too conservative for tripcount of CountedLoop [v4]

[Roland Westrelin]

On Tue, 22 Mar 2022 12:58:14 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> src/hotspot/share/opto/loopPredicate.cpp line 1069:
>> 
>>> 1067:                 inner_loop = inner_loop->_child;
>>> 1068:                 inner_head = inner_loop->_head->as_Loop();
>>> 1069:                 inner_head->verify_strip_mined(1);
>> 
>> Why did you remove the verification code?
>
> I hit a failure in that code for an inner loop that no longer has the shape of a counted loop. Let me double check if I can still reproduce it.

In the update I just pushed, I reverted that change, added a test that causes the failure that this was supposed to fix. The new fix is the new cast in in PhaseIdealLoop::do_unroll(). Without it, c2 can't remove the back branch of a main loop and the graph contains a counted loop that doesn't have the right shape anymore (the exit test doesn't depend on the iv phi after transformation but doesn't constant fold).

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PR: https://git.openjdk.java.net/jdk/pull/7823