RFR: 8263227: C2: inconsistent spilling due to dead nodes in exception block

[Roberto Castañeda Lozano]

On Tue, 6 Apr 2021 17:55:35 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> This change eliminates dead multi-nodes created by call-catch cleanup after GCM. Eliminating all dead code created by call-catch cleanup avoids potential issues when splitting the live range of call result values, see the analysis in the [bug report](https://bugs.openjdk.java.net/browse/JDK-8263227) for details. This solution is the least invasive of the three alternatives proposed in the bug report (the other two are constraining global code motion and extending live-range splitting).
>> 
>> The change also extends the control-flow graph verification pass to catch similar live-range splitting issues earlier (with `+VerifyRegisterAllocator`).
>> 
>> Tested on:
>> - original bug reproducer
>> - hs-tier1-5 (windows-x64, linux-x64, linux-aarch64, and macosx-x64) with `+VerifyRegisterAllocator`
>> - hs-tier1-3 (windows-x64, linux-x64, linux-aarch64, and macosx-x64) with `+VerifyRegisterAllocator` and `+StressGCM` (5 repetitions)
>
> src/hotspot/share/opto/lcm.cpp line 1419:
> 
>> 1417:                  "dead projection should correspond to current node");
>> 1418:           sb->get_node(k)->disconnect_inputs(C);
>> 1419:           sb->remove_node(k);
> 
> If you remove node here then `j` could be incorrect in `sb->remove_node(j)` at line #1424

`k` should always be greater than `j`, as `sb->get_node(k)` is a projection of `sb->get_node(j)`. Shouldn't this make the removal safe?

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

PR: https://git.openjdk.java.net/jdk/pull/3303