RFR: 8308504: C2: "malformed control flow" after JDK-8303466

[Roland Westrelin]

On Thu, 22 Jun 2023 12:18:21 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> > It doesn't seem to be true that the loop incr never overflows in the general case. See this example:
> 
> But don't we check that the limit is small enough at runtime, such that there cannot be an overflow? We do that with `check_stride_overflow` and `insert_loop_limit_check_predicate`. And if it does overflow, we do not go into the counted-loop, but we uncommon trap, I think. Or are you sure that we actually enter the counted-loop with your example? Or just some peeled version?

Ran with:

 -XX:-BackgroundCompilation -XX:-UseOnStackReplacement -XX:-TieredCompilation -XX:CompileOnly=TestOverflowCountedLoopIncr::test -XX:CompileCommand=quiet -XX:LoopMaxUnroll=0 -XX:+UseCountedLoopSafepoints 

I see a single counted loop, no uncommon trap in the IR.

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PR Comment: https://git.openjdk.org/jdk/pull/14331#issuecomment-1602548414