RFR: 8294356: IGV: scheduled graphs contain duplicated elements [v2]

[Roberto Castañeda Lozano]

On Thu, 29 Sep 2022 07:55:31 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

>> This changeset removes duplicated nodes and edges from graph dumps that include a control-flow graph:
>> ![cfg-before-after](https://user-images.githubusercontent.com/8792647/192294554-73ca3927-dab3-4d8f-9503-0904a4da7434.png)
>> This is achieved by ensuring that HotSpot only visits each node once when dumping IGV graphs.
>> 
>> #### Testing
>> - Tested that tens of thousands of graphs do not contain duplicated nodes or edges by instrumenting IGV and running `java -Xcomp -XX:PrintIdealGraphLevel=4`.
>> - Tested manually that unscheduled graphs are not affected by this changeset.
>> - Tested that running compiler tests with `-XX:PrintIdealGraphLevel=3` does not trigger any failure.
>
> Roberto Castañeda Lozano has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use nullptr everywhere, remove empty line

May I get a second review for this one? The changes are small and debug-only. Thanks!

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PR: https://git.openjdk.org/jdk/pull/10423