[jdk19] RFR: 8289954: C2: Assert failed in PhaseCFG::verify() after JDK-8183390

[Vladimir Kozlov]

On Mon, 11 Jul 2022 23:44:25 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Fuzzer tests report an assertion failure issue in C2 global code motion
>> phase. Git bisection shows the problem starts after our fix of post loop
>> vectorization (JDK-8183390). After some narrowing down work, we find it
>> is caused by below change in that patch.
>> 
>> 
>> @@ -422,14 +404,7 @@
>>        cl->mark_passed_slp();
>>      }
>>      cl->mark_was_slp();
>> -    if (cl->is_main_loop()) {
>> -      cl->set_slp_max_unroll(local_loop_unroll_factor);
>> -    } else if (post_loop_allowed) {
>> -      if (!small_basic_type) {
>> -        // avoid replication context for small basic types in programmable masked loops
>> -        cl->set_slp_max_unroll(local_loop_unroll_factor);
>> -      }
>> -    }
>> +    cl->set_slp_max_unroll(local_loop_unroll_factor);
>>    }
>>  }
>> 
>> 
>> This change is in function `SuperWord::unrolling_analysis()`. AFAIK, it
>> helps find a loop's max unroll count via some analysis. In the original
>> code, we have loop type checks and the slp max unroll value is set for
>> only some types of loops. But in JDK-8183390, the check was removed by
>> mistake. In my current understanding, the slp max unroll value applies
>> to slp candidate loops only - either main loops or RCE'd post loops -
>> so that check shouldn't be removed. After restoring it we don't see the
>> assertion failure any more.
>> 
>> The new jtreg created in this patch can reproduce the failed assertion,
>> which checks `def_block->dominates(block)` - the domination relationship
>> of two blocks. But in the case, I found the blocks are in an unreachable
>> inner loop, which I think ought to be optimized away in some previous C2
>> phases. As I'm not quite familiar with the C2's global code motion, so
>> far I still don't understand how slp max unroll count eventually causes
>> that problem. This patch just restores the if condition which I removed
>> incorrectly in JDK-8183390. But I still suspect that there is another
>> hidden bug exists in C2. I would be glad if any reviewers can give me
>> some guidance or suggestions.
>> 
>> Tested hotspot::hotspot_all_no_apps, jdk::tier1~3 and langtools::tier1.
>
> Testing passed.

> @vnkozlov Thanks for looking at this. I think a 2nd review is required, right?

yes

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PR: https://git.openjdk.org/jdk19/pull/130