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

[Vladimir Kozlov]

On Thu, 1 Apr 2021 10:02:39 GMT, Roberto Castañeda Lozano <rcastanedalo 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 1415:

> 1413:       if (dead) {
> 1414:         // Remove projections if n is a dead multi-node.
> 1415:         for (uint k = j + n->outcnt(); sb->get_node(k)->is_Proj(); k--) {

I don't get this logic. The loop is not executed if sb->get_node(j + n->outcnt()) is not Proj node.

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

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