RFR: 8290432: C2 compilation fails with assert(node->_last_del == _last) failed: must have deleted the edge just produced

[Vladimir Kozlov]

On Sun, 31 Jul 2022 09:28:59 GMT, Yi Yang <yyang at openjdk.org> wrote:

> Hi, can I have a review for this patch? [JDK-8273585](https://bugs.openjdk.org/browse/JDK-8273585) recognized the form of `Phi->CastII->AddI` as additional parallel induction variables. In the following program:
> 
> class Test {
>     static int dontInline() {
>         return 0;
>     }
> 
>     static long test(int val, boolean b) {
>         long ret = 0;
>         long dArr[] = new long[100];
>         for (int i = 15; 293 > i; ++i) {
>             ret = val;
>             int j = 1;
>             while (++j < 6) {
>                 int k = (val--);
>                 for (long l = i; 1 > l; ) {
>                     if (k != 0) {
>                         ret += dontInline();
>                     }
>                 }
>                 if (b) {
>                     break;
>                 }
>             }
>         }
>         return ret;
>     }
> 
>     public static void main(String[] args) {
>         for (int i = 0; i < 1000; i++) {
>             test(0, false);
>         }
>     }
> }
> 
> `val` is incorrectly matched with the new parallel IV form:
> ![image](https://user-images.githubusercontent.com/5010047/182059398-fc5204bc-8d95-4e3e-8c66-15776af457b8.png)
> And C2 further replaces it with newly added nodes, which finally leads the crash:
> ![image](https://user-images.githubusercontent.com/5010047/182059498-13148d46-b10f-4e18-b84a-f6b9f626ac7b.png)
> 
> I think we can add more constraints to the new form. The form of `Phi->CastXX->AddX` appears when using Preconditions.checkIndex, and it would be recognized as additional IV when 1) Phi != phi2, 2) CastXX is controlled by RangeCheck(to reflect changes in Preconditions checkindex intrinsic)

>From your test I assume internal loop `for (long l=` is processed. But `val` is updated outside it. Its `AddI` node should not be part of loop's body. Could you explain?

src/hotspot/share/opto/loopnode.cpp line 3679:

> 3677:         incr2->Opcode() != Op_AddI ||
> 3678:         !incr2->in(2)->is_Con())
> 3679:       continue;

Since you touch near code, please update style in these lines too to use `{}`.

src/hotspot/share/opto/loopnode.cpp line 3682:

> 3680: 
> 3681:     if (incr2->in(1)->is_ConstraintCast()) {
> 3682:       if (!incr2->in(1)->in(0)->is_RangeCheck()) {

You can use `&&` instead of nested condition.

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

PR: https://git.openjdk.org/jdk/pull/9695