Integrated: 8296318: use-def assert: special case undetected loops nested in infinite loops

[Emanuel Peter]

On Tue, 13 Dec 2022 07:49:47 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).

This pull request has now been integrated.

Changeset: 736fcd49
Author:    Emanuel Peter <epeter at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/736fcd49f7cd3aa6f226b2e088415eaf05f97ee8
Stats:     138 lines in 5 files changed: 113 ins; 23 del; 2 mod

8296318: use-def assert: special case undetected loops nested in infinite loops

Reviewed-by: chagedorn, kvn

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