RFR: 8173709: Fix VerifyLoopOptimizations - step 1

[Emanuel Peter]

On Tue, 28 Mar 2023 23:11:09 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> I am reviving `-XX:+VerifyLoopOptimizations` after many years of abandonment. There were many bugs filed, but so far it has not been addressed.
>> 
>> The hope is that this work will allow us to catch ctrl / idom / loop body bugs quicker, and fix many of the existing ones along the way.
>> 
>> **The Idea of VerifyLoopOptimizations**
>> Before loop-opts, we build many data-structures for dominance (idom), control, and loop membership. Then, loop-opts use this data to transform the graph. At the same time, they must maintain the correctness of the data-structures, so that other optimizations can be made, without needing to re-compute the data-structures every time.
>> `VerifyLoopOptimizations` was implemented to verify correctness of the data-structures. After some loop-opts, we re-compute a verification data-structure and compare it to the one we created before the loop-opts and maintained during loopopts.
>> 
>> **My Approach**
>> I soon realized that there were many reasons why `VerifyLoopOptimizations` was broken. It seemed infeasible to fix all of them at once. I decided to first remove any part that was failing, until I have a minimal set that is working. I will leave many parts commented out. In follow-up RFE's, I will then iteratively improve the verification by re-enabling some verification and fixing the corresponding bugs.
>> 
>> **What I fixed**
>> 
>>  - `verify_compare`
>>    - Renamed it to `verify_nodes`, since it does verification node-by-node (vs `verify_tree`, which verifies the loop-tree).
>>    - Previously, it was implemented as a BFS with recursion, which lead to stack-overflow. I flattened the BFS into a loop.
>>    - The BFS calls `verify_node` on every node. I refactored `verify_node` a bit, so that it is more readable.
>>    - Rather than having a thread-unsafe static variable `fail`, I now made it a reference argument.
>>  - `verify_tree`
>>    - I corrected the style and improved comments.
>>    - I removed the broken verification for `Opaque` nodes. I added some rudamentary verification for `CountedLoop`. I leave more of this work for follow-up RFE's.
>> 
>> **Disabled Verifications**
>> I commented out the following verifications:
>> 
>> (A) data nodes should have same ctrl
>> (B) ctrl node should belong to same loop
>> (C) ctrl node should have same idom
>> (D) loop should have same tail
>> (E) loop should have same body (list of nodes)
>> (F) broken verification in PhaseIdealLoop::build_loop_late_post, because ctrl was set wrong
>> 
>> 
>> Note: verifying `idom`, `ctrl` and `_body` is the central goal of `VerifyLoopOptimizations`. But all of them are broken in many parts of the VM, as we have now not verified them for many years.
>> 
>> **Follow-Up Work**
>> 
>> I filed a first follow-up RFE [JDK-8305073](https://bugs.openjdk.org/browse/JDK-8305073). The following tasks should be addressed in it, or in subsequent follow-up RFE's.
>> 
>> I propose the following order:
>> 
>>  - idom (C): The dominance structure is at the base of everything else.
>>  - ctrl / loop (A, B): Once dominance is fixed, we can ensure every node is assigned to the correct ctrl/loop.
>>  - tail (D): ensure the tail of a loop is updated correctly
>>  - body (E): nodes are assigned to the `_body` of a loop, according to the node ctrl.
>>  - other issues like (F)
>>  - Add more verification to IdealLoopTree::verify_tree. For example zero-trip-guard, etc.
>>  - Evaluate from where else we should call `PhaseIdealLoop::verify`. Maybe we are missing some cases.
>> 
>> **Testing**
>> I am running `tier1-tier6` and stress testing.
>> Preliminary results are all good.
>> 
>> **Conclusion**
>> With this fix, I have the basic infrastructure of the verification working.
>> However, all of the substantial verification are now still disabled, because there are too many places in the VM that do not maintain the data-structures properly.
>> Follow-up RFE's will have to address these one-by-one.
>
> src/hotspot/share/opto/loopnode.cpp line 4814:
> 
>> 4812:     IdealLoopTree **pp = &loop->_parent->_child;
>> 4813:     while (*pp != loop) {
>> 4814:       pp = &((*pp)->_next);
> 
> May be instead of this convoluted search, swap and verification code we can simply reorder whole siblings list first before verification? You don't need to swap - just create local ordered list.

Ok, I will refactor this code. I will have two local lists, sort them by `_head->_idx`. Then I can also verify that the lists are the same.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13207#discussion_r1151554026