JDK-8370964 : C2: Investigate effects of ClearArrayNode on allocation elimination
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: compiler
  • Affected Version: 26
  • Priority: P4
  • Status: Closed
  • Resolution: Not an Issue
  • Submitted: 2025-10-30
  • Updated: 2025-11-03
  • Resolved: 2025-11-03
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tbdResolved
Related Reports
Relates :  
JDK-8370405 - C2: mismatched store from MergeStores wrongly scalarized in allocation elimination
Description
ClearArrayNode is used to initialize new allocation by widening stores to reduce number of instructions for small objects/arrays.
JDK-8370405 shows that mismatch between new array element type and stores may prevent EA elimination of array allocation.

ClearArrayNode could be created before EA. Idealizing it could also happened before. May be we should delay it until after last macro nodes elimination.
Comments
[~rcastanedalo] could be interested in this EA case.
03-11-2025
Allocation was eliminated after replacing loop with set of stores. Looks like max unroll loop optimization happens after EA. And presence of ClearArray did not affect elimination. I am closing this issue then.
03-11-2025
It could be a different reason for NSR but I see ClearArray node in OptoAssembly output. JavaObject(4) NoEscape(NoEscape) NSR [ 257F 255F 99F 81F 213F 92F 110F [ 54 59 ]] 42 AllocateArray === 5 6 7 8 1 (31 23 28 22 38 10 1 1 1 ) [[ 43 44 45 52 53 54 ]] rawptr:NotNull ( int:>=0, java/lang/Object:NotNull *, bool, int, bool ) allocationKlass:[B TestClearArray::test @ bci:2 (line 3) !jvms: TestClearArray::test @ bci:2 (line 3)
03-11-2025
public class TestClearArray { public static int test(int c) { byte ar[] = new byte[16]; if ((c & 1) == 0) { ar[4] = (byte)(c * 2); ar[12] = (byte)(c * 4); } else { ar[0] = (byte)(c * 1); ar[8] = (byte)(c * 3); } int sum = 0; for (int i = 0; i < 16; i += 4) { sum += ar[i]; } return sum; } public static void main(String[] strArr) { for (int i = 0; i < 11000; i++) { test(i); } } }
03-11-2025
[~kvn] Do you have an example where this happens? Otherwise this task is a bit had to pick up for someone.
03-11-2025