RFR: 8307683: Loop Predication should not hoist range checks with trap on success projection by negating their condition [v3]

[Tobias Hartmann]

On Tue, 30 May 2023 10:30:34 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 one additional commit since the last revision:
> 
>   remove negation

Nice analysis. Looks good to me. As we discussed offline, let's run some sanity performance testing (we can already integrate this).

src/hotspot/share/opto/loopPredicate.cpp line 870:

> 868:     // Example:
> 869:     // Loop: "for (int i = -1; i < 1000; i++)"
> 870:     // init = "scale*iv + offset" in first loop iteration = 1*-1 + 0 = -1

Suggestion:

    // init = "scale*iv + offset" in the first loop iteration = 1*-1 + 0 = -1

test/hotspot/jtreg/compiler/predicates/TestHoistedPredicateForNonRangeCheck.java line 143:

> 141:                 Math.ceil(34); // Never taken and unloaded -> trap
> 142:             } catch (Exception e) {
> 143:                 // False Proj of RangeCheckNod

Suggestion:

                // False Proj of RangeCheckNode

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Marked as reviewed by thartmann (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/14156#pullrequestreview-1453133922
PR Review Comment: https://git.openjdk.org/jdk/pull/14156#discussion_r1211674413
PR Review Comment: https://git.openjdk.org/jdk/pull/14156#discussion_r1211670902