RFR: 8344251: C2: remove blackholes with dead control input

[Vladimir Ivanov]

On Tue, 15 Apr 2025 17:30:09 GMT, Marc Chevalier <mchevalier at openjdk.org> wrote:

>> Last time I had this problem, I did this:
>> https://github.com/openjdk/jdk/blob/cec48ed270d3bdf704c389a091b42a32c2ed6440/test/hotspot/jtreg/compiler/c2/irTests/gc/ReferenceClearTests.java#L48-L49
>> 
>> https://github.com/openjdk/jdk/blob/cec48ed270d3bdf704c389a091b42a32c2ed6440/test/hotspot/jtreg/testlibrary/ctw/src/sun/hotspot/tools/ctw/CtwRunner.java#L303-L307
>
> That looks exactly like what I'm trying to achieve. I'll re-test with that. Thanks!

> -Xcomp is likely too heavy-weight here. 

FTR another alternative is to keep `-Xcomp`, but limit compilation to test methods (`-XX:CompileCommand=compileonly,<Test::test>`).

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PR Review Comment: https://git.openjdk.org/jdk/pull/24663#discussion_r2045166262