8308105: Multi-array allocations are very slow

[bulasevich]

Multidimensional array allocations such as new int[n1][n2] are very slow because HotSpot routes them through a runtime call (multianewarray) that allocates the outer array and each subarray sequentially.

For constant-size cases like new int[2][2], C2 already optimizes during the parse phase by inlining a fixed number of AllocateArray nodes. This change extends that optimization to the variable-size 2D case. When ndimensions == 2, Parse::do_multianewarray now calls Parse::init_array2d, which builds an equivalent C2 IR graph:

  // before
  T[][] array = new T[size1][size2]; // runtime call

  // after - compiled directly by C2, no runtime calls
  T[][] array = new T[size1][];
  for (int i = 0; i < size1; i++) {
      array[i] = new T[size2];
  }

The performance result is like this:

  Benchmark                      (size)  (size2)  Mode  Cnt   Score    Error   Units
  ArrayAllocation.full (before)     128        2  avgt    4  540.707 ± 12.154  ns/op
  ArrayAllocation.full (after)      128        2  avgt    4   12.713 ±  0.435  ns/op
  ArrayAllocation.piecemeal         128        2  avgt    4   12.966 ±  0.297  ns/op

Status:

• jtreg tier1-3 passed with -Xcomp -XX:-TieredCompilation options. Further testing is in progress

---

Progress

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Issue

• JDK-8308105: Multi-array allocations are very slow (Enhancement - P4)

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[liach]

Dead branch now.

[openjdk]

@bulasevich hotspot has been added to this pull request based on files touched in new commit(s).

[openjdk]

@bulasevich this pull request can not be integrated into master due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout multiarray3
git fetch https://git.openjdk.org/jdk.git master
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge master"
git push