RFR: 8327963: C2: fix construction of memory graph around Initialize node to prevent incorrect execution if allocation is removed [v5]

[Roberto Castañeda Lozano]

On Tue, 20 May 2025 14:50:36 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> > I still think it would be good to include test cases to confirm that these are not only theoretical concerns, but that should not block the progress of this PR.
> 
> Here is a test case:
> 
> ```
> import java.util.Arrays;
> 
> public class TestAllocNoUseBadMemoryState {
>     private static volatile int volatileField;
> 
>     public static void main(String[] args) {
>         boolean[] allTrue = new boolean[3];
>         Arrays.fill(allTrue, true);
>         A a = new A();
>         boolean[] allFalse = new boolean[3];
>         for (int i = 0; i < 20_000; i++) {
>             a.field1 = 0;
>             test1(a, allTrue);
>             test1(a, allFalse);
>             if (a.field1 != 42) {
>                 throw new RuntimeException("Lost Store");
>             }
>         }
>     }
> 
>     private static void test1(A otherA, boolean[] flags) {
>         if (flags == null) {
>         }
>         otherA.field1 = 42;
>         for (int i = 0; i < 3; i++) {
>             A a = new A();
>             if (flags[i]) {
>                 break;
>             }
>         }
>     }
> 
>     private static class A {
>         int field1;
>     }
> }
> ```
> 
> where all the damage is done early on when EA runs. A pass of loop opts before EA fully unrolls the loop and creates memory `Phi`s with incorrect `adr_type` (raw memory). Then EA removes the allocation. All that keeps the `Store` to `field1` alive then is uncommon traps from template predicates. Once they are removed, the `Store` goes away (first round of loop opts after EA).
> 
> I'll add that test case to the PR.

Thanks Roland for taking the time to research this, this failure really illustrates why the general solution proposed by this PR is needed.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/24570#issuecomment-2897374573