RFR: 8307683: Loop Predication wrongly hoists IfNodes without a range check pattern as range check [v2]

[Christian Hagedorn]

On Fri, 26 May 2023 23:34:26 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

>> [JDK-4809552](https://bugs.openjdk.org/browse/JDK-4809552) allowed Loop Predication to be applied to `IfNodes` that have a positive value instead of a `LoadRangeNode`:
>> https://github.com/openjdk/jdk/blob/48d21bd089a3f344ee5407926f8ed2af3734d2b0/src/hotspot/share/opto/loopPredicate.cpp#L854-L862
>> 
>> This, however, is only correct if we have an actual `RangeCheckNode` for an array. The reason for that is that if we hoist a real range check and create a Hoisted Predicate for it, we only need to check the lower and upper bound of all array accesses (i.e. the array access of the first and the last loop iteration). All array accesses in between are implicitly covered and do not need to be checked again. 
>> 
>> But if we face an `IfNode` without a `LoadRangeNode`, we could be comparing anything. We do not have any guarantee that if the first and last loop iteration check succeed that the other loop iteration checks will also succeed. An example of this is shown in the test case `test()`. We wrongly create a Hoisted Range Check Predicate where the lower and upper bound are always true, but for some values of the loop induction variable, the hoisted check would actually fail. We then crash because an added Assertion Predicate exactly performs this failing check (crash with halt). Without any loop splitting (i.e. no Assertion Predicates), we have a wrong execution due to never executing the branch where we increment `iFld2` because we removed it together with the check.
>> 
>> Thanks,
>> Christian
>
> Christian Hagedorn has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - fix assertion
>  - new fix with bailout for "if iv <u limit then trap"

In the failing test case (see newly added `testRangeCheckNode()`), we have a `RangeCheckNode` with an unloaded trap on the true/success projection because we are always taking the false/exception path. Loop Predication wrongly detects this pattern as valid range check and tries to create a Hoisted Predicate for that `RangeCheckNode` by flipping the boolean condition. This is wrong due to the same reasons explained above for the `IfNode`. The failing assertion should have made sure that we will never have a `RangeCheckNode` there.

I'm therefore suggesting to remove the entire negation of range checks which seems to be wrong. We should bail out in `is_range_check_if()` if we have the false projection as success projection. I've pushed an update accordingly.

I will run some testing again.

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PR Comment: https://git.openjdk.org/jdk/pull/14156#issuecomment-1568184225