RFR: 8290432: C2 compilation fails with assert(node->_last_del == _last) failed: must have deleted the edge just produced [v2]

[Vladimir Kozlov]

On Fri, 5 Aug 2022 09:23:28 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)
>
> Yi Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   format

src/hotspot/share/opto/loopnode.cpp line 3689:

> 3687:       // The form of Phi->CastXX->AddX appears when using Preconditions.checkIndex, and it would
> 3688:       // be recognized as additional IV when 1) Phi != phi2, 2) CastXX is controlled by RangeCheck
> 3689:       // (to reflect changes in LibraryCallKit::inline_preconditions_checkIndex)

Thank you for updating comment. But it still do not say which issue you are trying to avoid. Yes, this pattern can be generated by inline_preconditions_checkIndex.
Currently your check skip search for parallel IV if CastII is NOT controlled by RangeCheck!
But the comment in the BODY of check talks about using parallel IV.
It is contradicting.
Aslo `1) Phi != phi2` says nothing to me - your check does not have such condition.

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