[jdk20] RFR: 8298176: remove OpaqueZeroTripGuardPostLoop once main-loop disappears [v2]

[Vladimir Kozlov]

On Wed, 14 Dec 2022 06:21:47 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> We recently removed `Opaque2` nodes in [JDK-8294540](https://bugs.openjdk.org/browse/JDK-8294540). `Opaque2` nodes prevented some optimizations during loop-opts. The original idea was to prevent the use of both the un-incremented and incremented value of a loop phi after the loop, to reduce register pressure. But `Opaque2` also had the effect that the limit of the loop would not be optimized, which meant that the iv-value (entry value of phi) in post loop would never collapse (either to constant or TOP), but always remain a range. Now that `Opaque2` is gone, it can happen that when the main-loop disappears, the limit collapses. The zero-trip guard of the post-loop would be false, but does not collapse because of the `OpaqueZeroTripGuard`. The post-loop can half-collapse, leaving an inconsistent graph below the zero-trip guard if.
>> 
>> **Solution**
>> Have `OpaqueZeroTripGuardMainLoop` for main loop zero-trip guard, and `OpaqueZeroTripGuardPostLoop` for post-loop zero trip guard. Let `OpaqueZeroTripGuardPostLoop` remove itself once it cannot find the main-loop above it. We have these opaque nodes there to prevent collapsing of the zero-trip guards as long as the limits may still change, but after the main-loop is removed, no unrolling is done anymore, so the limit of the post-loop cannot change anymore, hence it is safe to remove the opaque node there.
>> 
>> An alternative approach was to let the main-loop remove the opaque node of the post-loop's zero-trip guard. But that does not work reliably, as the main-loop may get removed during PhaseCCP, and the main-loop is simply removed as "useless". Hence the LoopNode of the main-loop does not have a chance to detect its death during IGVN.
>
> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   remove swap files, how did I ever commit them?

Based on comment and original changes it checks only iv from main loop exit and not Phi as you pointed:

  Node *incr  = main_end ->incr();
...
  // Step A2: Build a zero-trip guard for the post-loop.  After leaving the
  // main-loop, the post-loop may not execute at all.  We 'opaque' the incr
  // (the main-loop trip-counter exit value) because we will be changing
  // the exit value (via unrolling) so we cannot constant-fold away the zero
  // trip guard until all unrolling is done.

  Node *zer_opaq = new (2) OpaqueNode(incr);


It seems incorrect based on what you said and I think you can test your suggestion to completely remove it.

Saying that, the current code looks the same. Where Phi comes from?

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PR: https://git.openjdk.org/jdk20/pull/22