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

[Vladimir Kozlov]

On Thu, 12 May 2022 16:48:40 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.

src/hotspot/share/opto/lcm.cpp line 344:

> 342:        }
> 343:        if (k < num_nodes)
> 344:          continue;             // Found anti-dependent load

Our code style requires to use {} for `if() {}`. the same for `if` at line 340.
When we fixing code we fix its style too if it was wrong.

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