RFR: 8263075: C2: simplify anti-dependence check in PhaseCFG::implicit_null_check() [v2]

[Vladimir Kozlov]

On Tue, 17 May 2022 20:19:14 GMT, Brian J. Stafford <duke at openjdk.java.net> wrote:

>> The reporter for this issue (https://bugs.openjdk.java.net/browse/JDK-8263075) indicated that there's an assumption that we can rely on that the while loop in question will run exactly one time. Based on this, I've done the following:
>> 
>> - Asserted the condition that makes sure the code runs at least once
>> - Asserted the condition that makes sure the code runs only once
>> - Removed the `while` loop
>> - Changed a couple of `break` statements into `continue` statements. They no longer need to break out of the `while` loop, now that it's gone. However, they were early exits from the `while` loop that ended up resulting in `continue` statements for the larger enclosing loop. Thus we can just call `continue` directly.
>> - Removed the local variable `b`, as we no longer need to traverse the node hierarchy. We can use `mb` directly.
>> 
>> Passes jdk, langtools, and hotspot Tier 1 tests on Linux (x64 and ARM64) and macOS (x64 and ARM64). Most Tier 1 tests pass on Windows (x64 and ARM64), but there are a handful of failures unrelated to this change.
>
> Brian J. Stafford has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Removed whitespace
>  - Added braces for if statements

Good.
Tobias is running testing. Please wait results.

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Marked as reviewed by kvn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/8684