RFR: 8252219: C2: Randomize IGVN worklist for stress testing

[Roberto Castañeda Lozano]

On Fri, 18 Sep 2020 10:28:28 GMT, Roberto Castañeda Lozano <github.com+8792647+robcasloz at openjdk.org> wrote:

> Add `StressIGVN` option to let C2 randomize IGVN worklist order. When enabled, the worklist is shuffled before each
> main run of the IGVN loop. Also add `GenerateStressSeed' and 'StressSeed=N` options to randomly generate or specify the
> seed. In either case, the seed is logged if `LogCompilation` is enabled.  The generation or specification of seeds also
> affects the randomization triggered by `StressLCM` and `StressGCM`. The new options are declared as
> production+diagnostic for consistency with these existing options.

Add 'StressIGVN' option to let C2 randomize IGVN worklist order. When enabled,
the worklist is shuffled before each main run of the IGVN loop. Also add
'GenerateStressSeed' and 'StressSeed=N' options to randomly generate or specify
the seed. In either case, the seed is logged if 'LogCompilation' is enabled.
    
The generation or specification of seeds also affects the randomization
triggered by 'StressLCM' and 'StressGCM'. The new options are declared as
production+diagnostic for consistency with these existing options.

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PR: https://git.openjdk.java.net/jdk/pull/242