RFR: 8307683: Loop Predication is wrongly applied to non-RangeCheckNodes without a LoadRangeNode

[Roland Westrelin]

On Thu, 25 May 2023 16:48:35 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

Looks good to me. Thanks for the quick fix.
Do we want to file a bug to revisit this in a correct way?

Do we want to file a bug to investigate whether this can be done in a correct way?

src/hotspot/share/opto/loopPredicate.cpp line 856:

> 854:   if (range->Opcode() != Op_LoadRange) {
> 855:     const TypeInteger* tinteger = phase->_igvn.type(range)->isa_integer(bt);
> 856:     if (!iff->is_RangeCheck() || tinteger == nullptr || tinteger->empty() || tinteger->lo_as_long() < 0) {

Can the problem not happen with a LoadRange as second input? Couldn't the LoadRange constant fold and then the predicates could constant fold as well?

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Marked as reviewed by roland (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/14156#pullrequestreview-1445589807
PR Comment: https://git.openjdk.org/jdk/pull/14156#issuecomment-1564007121
PR Review Comment: https://git.openjdk.org/jdk/pull/14156#discussion_r1206417031