RFR: 8266542: Remove broken -XX:-UseLoopSafepoints flag

[Vladimir Kozlov]

On Wed, 5 May 2021 07:20:50 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> Simply running `java -XX:-UseLoopSafepoints` already asserts and more sophisticated testing triggers all kinds of asserts and crashes in various places. [JDK-6232281](https://bugs.openjdk.java.net/browse/JDK-6232281) made an attempt of fixing a common case. I had a quick look on what it would take to fix all cases and it's definitely non trivial. Also, we would need to make sure that new optimizations work without safepoints in loops. I don't see much value in keeping the flag for debugging either because disabling safepoints in loops has massive side-effects on other optimizations.
> 
> I therefore propose to remove the flag.
> 
> Thanks,
> Tobias

Note, `UseLoopSafepoints` is debug flag so we don't need CSR for removing it.

Consider removing `OptoRemoveUseless` debug flag too because it is related to safepoints and we never used it AFAIK:
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/parse1.cpp#L2291

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Marked as reviewed by kvn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/3871