[jdk17] RFR: 8268884: C2: Change to meet_speculative in phase CCP assert

[Nils Eliasson]

On Tue, 29 Jun 2021 09:25:21 GMT, Nils Eliasson <neliasso at openjdk.org> wrote:

> Hi,
> 
> The test fails on "assert(t->meet(t0) == t) failed: Not monotonic" in PhaseCCP for a CheckCastPPNode. The test is really simple. The CheckCastPP is hanging of the ParmNode for the 'this' pointer. The user of this is a call, with an infinite empty loop, that gets inlined. The loop will have a safepoint that keeps the JVM State - the CheckCastPP is kept alive by that SafePointNode.
> 
> The assert triggers because the type for the CheckCastPP has a speculative part. In the assert "assert(t->meet(t0) == t) failed: Not monotonic" - t is the type evaluated for the CheckCastPP, t0 is the previous type (from _types map), which has not been updated yet and is still Type::Top. 
> 
> t->meet(t0) will drop the speculative part and fail the comparison.
> 
> Suggested fix - change meet to meet_specualtive that will keep the speculative part.
> 
> Please review,
> Best regards,
> Nils Eliasson

The new suggested fix changes so that Compile::remove_speculative_types() iterates the nodes from top to bottom. With this change all nodes that are reachable from the top will be visited.

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PR: https://git.openjdk.java.net/jdk17/pull/170