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

[Vladimir Kozlov]

On Mon, 11 Jul 2022 08:41:21 GMT, Pengfei Li <pli 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.

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

Marked as reviewed by kvn (Reviewer).

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