Integrated: 8257197: Add additional verification code to PhaseCCP

[Emanuel Peter]

On Tue, 6 Dec 2022 08:02:12 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> We have had many bugs that could be tracked back to missing optimizations during PhaseCCP.
> Often the problem is that a node `x` has an input (of input of input) `y` which is modified, but `y` does not notify `x` (does not push it to the worklist). For one this is a missed opportunity to optimize, but it can also lead to failed assumptions later: often we assume that all that can be optimize is already optimized.
> This verification helps us debug faster, and can also help when `Value` optimizations suddenly do further-reaching traversals, which would require further-reaching notifications.
> 
> Sadly, the verification is not total: we have some exceptions. Especially for `LoadNode`, which perform a walk up the memory inputs, which can go arbitrarily far, to look for relates `StoreNode`s. Notification would thus have to put very many nodes on the worklist. The question is if the potential additional optimization is worth the compile-time. If we decided yes, then one we might want to implement a listener-style notification: when a node visits inputs during `Value`, it could subscribe to all (or the relevant) visited input-nodes for future updates. Currently, we just mostly do fixed 1-hop or 2-hop notification of output nodes.
> 
> FYI: I plan to do a similar verification, and a refactoring of `PhaseCCP::push_child_nodes_to_worklist` and `PhaseIterGVN::add_users_to_worklist` in [JDK-8298094](https://bugs.openjdk.org/browse/JDK-8298094).

This pull request has now been integrated.

Changeset: 11aece21
Author:    Emanuel Peter <epeter at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/11aece21f4eb5b18af357b265bc27b80bcdbfbcb
Stats:     52 lines in 2 files changed: 52 ins; 0 del; 0 mod

8257197: Add additional verification code to PhaseCCP

Reviewed-by: chagedorn, kvn, thartmann

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PR: https://git.openjdk.org/jdk/pull/11529