RFR: 8254317: C2: Resource consumption of ConvI2LNode::Ideal() grows exponentially [v4]

[Roberto Castañeda Lozano]

> Prevent exponential number of calls to `ConvI2LNode::Ideal()` when AddIs are used multiple times by other AddIs in the optimization ConvI2L(AddI(x, y)) -> AddL(ConvI2L(x), ConvI2L(y)). This is achieved by (1) reusing existing ConvI2Ls if possible rather than eagerly creating new ones and (2) postponing the optimization of newly created ConvI2Ls. Remove hook node solution introduced in [8217359](https://github.com/openjdk/jdk/commit/cf554816d1952f722143e9d03ec669e80f955adf), since this is subsumed by (2). Use `phase->is_IterGVN()` rather than `can_reshape` to check if `ConvI2LNode::Ideal()` is called within iterative GVN, for clarity. Add regression tests that cover different shapes and sizes of AddI subgraphs, implicitly checking (by not timing out) that there is no combinatorial explosion.

Roberto Castañeda Lozano has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits:

 - Use 'hash_find_insert' to look for existing ConvI2L nodes
 - Merge master
 - Update tests
   
   Simplify JVM arguments and run each test case 100000 times to still trigger
   C2. Use randomization to avoid constant propagation in C2. Increase the load of
   the stress tests and their timeout to 30s to further reduce the risk of false
   positives.
 - Merge master
 - Generalize the fix to handle any input where AddIs are used multiple times by
   other AddIs, which could also lead to an exponential number of calls to
   ConvI2LNode::Ideal(). This is achieved by (1) reusing existing ConvI2Ls if
   possible rather than eagerly creating new ones and (2) postponing the
   optimization of newly created ConvI2Ls. Remove "hook" node solution introduced
   in JDK-8217359 since this is subsumed by (2). Test that ConvI2LNode::Ideal() is
   called within iterative GVN using phase->is_IterGVN() rather than can_reshape,
   for clarity.
   
   Merge all tests into a single class. Reimplement the microbenchmark as a test
   case that should time out in case of a combinatorial explosion. Add a second
   similar microbenchmark that demonstrates the need for this generalization.
 - Merge master
 - 8254317: C2: Resource consumption of ConvI2LNode::Ideal() grows exponentially
   
   In the optimization ConvI2L(AddI(x, y)) -> AddL(ConvI2L(x), ConvI2L(y)) within
   ConvI2LNode::Ideal(), handle the special case x = y by feeding both inputs of
   AddL from a single ConvI2L node rather than creating two semantically equivalent
   ConvI2L nodes. This avoids an exponential number of calls to
   ConvI2LNode::Ideal() when dealing with long chains of AddI nodes. Disable the
   optimization for the pattern ConvI2L(SubI(x, x)), which is simplified to zero
   during parsing anyway. Add a set of regression tests for the transformation that
   cover different shapes of AddI subgraphs. Also add a microbenchmark that
   exercises the special case, for performance regression testing.

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Changes: https://git.openjdk.java.net/jdk/pull/727/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=727&range=03
  Stats: 190 lines in 2 files changed: 180 ins; 4 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/727.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/727/head:pull/727

PR: https://git.openjdk.java.net/jdk/pull/727