RFR: 8273115: CountedLoopEndNode::stride_con crash in debug build with -XX:+TraceLoopOpts

[Roland Westrelin]

On Mon, 11 Apr 2022 15:59:20 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> The crash occurs because a counted loop has an unexpected shape: the
>> exit test doesn't depend on a trip count phi. It's similar to a crash
>> I encountered in (not yet integrated) PR
>> https://github.com/openjdk/jdk/pull/7823 and fixed with an extra
>> CastII:
>> https://github.com/openjdk/jdk/pull/7823/files#diff-6a59f91cb710d682247df87c75faf602f0ff9f87e2855ead1b80719704fbedff
>> 
>> That fix is not sufficient here, though. But the fix I proposed here
>> works for both.
>> 
>> After the counted loop is created initially, the bounds of the loop
>> are captured in the iv Phi. Pre/main/post loops are created and the
>> main loop is unrolled once. CCP next runs and in the process, the type
>> of the iv Phi of the main loop becomes a constant. The reason is that
>> as types propagate, the type captured by the iv Phi and the improved
>> type that's computed by CCP for the Phi are joined and the end result
>> is a constant. Next the iv Phi constant folds but the exit test
>> doesn't. This results in a badly shaped counted loop. This happens
>> because on first unroll, an Opaque2 node is added that hides the type
>> of the loop limit. I propose adding a CastII to make sure the type of
>> the new limit (which cannot exceed the initial limit) is not lost.
>
> Good.

@vnkozlov @TobiHartmann thanks for the reviews

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PR: https://git.openjdk.java.net/jdk/pull/8178