RFR: 8296318: use-def assert: special case undetected loops nested in infinite loops [v3]

[Vladimir Kozlov]

On Wed, 14 Dec 2022 05:59:38 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> `PhaseCFG::verify` checks that the nodes scheduled into the blocks have "use-after-def".
>> https://github.com/openjdk/jdk/blob/8c472e481676ed0ef475c4989477d5714880c59e/src/hotspot/share/opto/block.cpp#L1376
>> 
>> As long as the control flow has no loops, this should always hold.
>> We make sure to not check the assert if the block-head is a `LoopNode`, and the use `n` is a `Phi`, since we may have inputs from the backedge, which would be "use-before-def" in the block-scheduling order, but that is to be expected.
>> https://github.com/openjdk/jdk/blob/8c472e481676ed0ef475c4989477d5714880c59e/src/hotspot/share/opto/block.cpp#L1364
>> 
>> **Problem**
>> This assumes that all loops are properly detected, if a loop was not detected the block-head may only be a `RegionNode`, and the assert can fail (see regression test). Why are not all loops detected?
>> During `PhaseIdealLoop::build_loop_tree`, we detect all loops, but not always attach infinite loops with their sub-loops to the loop tree, and then the loop-head `Regions` are not converted to `LoopNodes` in `PhaseIdealLoop::beautify_loops`.
>> This behaviour is expected, see also https://github.com/openjdk/jdk/pull/11473.
>> Thus, the assert fires, but it should not.
>> 
>> **Solution**
>> Add special casing to assert: also accept if the `Region` is in an infinite subgraph, and we are looking at a `Phi` as use (the def could be values from the backedge).
>
> Emanuel Peter has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
> 
>  - infinite_subgraph check made ASSERT only
>  - Merge branch 'master' into JDK-8296318
>  - review suggestions
>  - made assert more precise
>  - 8296318: use-def assert: special case undetected loops nested in infinite loops

Good.

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Marked as reviewed by kvn (Reviewer).

PR: https://git.openjdk.org/jdk/pull/11642