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 a C2 IR graph equivalent to:

  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.