RFR: 8287432: C2: assert(tn->in(0) != __null) failed: must have live top node [v2]

[Christian Hagedorn]

> When intrisifying `java.lang.Thread::currentThread()`, we are creating an `AddP` node that has the `top` node as base to indicate that we do not have an oop (using `NULL` instead leads to crashes as it does not seem to be expected to have a `NULL` base):
> https://github.com/openjdk/jdk/blob/6ff2d89ea11934bb13c8a419e7bad4fd40f76759/src/hotspot/share/opto/library_call.cpp#L904
> 
> This node is used on a chain of data nodes into two `MemBarAcquire` nodes as precedence edge in the test case:
> ![Screenshot from 2022-06-07 11-12-38](https://user-images.githubusercontent.com/17833009/172344751-5338b72f-baa5-4e9e-a44c-6d970798d9f2.png)
> 
> Later, in `final_graph_reshaping_impl()`, we are removing the precedence edge of both `MemBarAcquire` nodes and clean up all now dead nodes as a result of the removal:
> https://github.com/openjdk/jdk/blob/6ff2d89ea11934bb13c8a419e7bad4fd40f76759/src/hotspot/share/opto/compile.cpp#L3655-L3679
> 
> We iteratively call `disconnect_inputs()` for all nodes that have no output anymore (i.e. dead nodes). This code, however, also treats the `top` node as dead since `outcnt()` of `top` is always zero:
> https://github.com/openjdk/jdk/blob/6ff2d89ea11934bb13c8a419e7bad4fd40f76759/src/hotspot/share/opto/node.hpp#L495-L500
> 
> And we end up disconnecting `top` which results in the assertion failure.
> 
> The code misses a check for `top()`. I suggest to add this check before processing a node for which `outcnt()` is zero. This is a pattern which can also be found in other places in the code. I've checked all other usages of `oucnt() == 0` and could not find a case where this additional `top()` check is missing. Maybe we should refactor these two checks into a single method at some point to not need to worry about `top` anymore in the future when checking if a node is dead based on the outputs.
> 
> Thanks,
> Christian

Christian Hagedorn has updated the pull request incrementally with one additional commit since the last revision:

  Update test/hotspot/jtreg/compiler/c2/TestRemoveMemBarPrecEdge.java
  
  Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/9060/files
  - new: https://git.openjdk.java.net/jdk/pull/9060/files/3f351d40..46a1ef11

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=9060&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=9060&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9060.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9060/head:pull/9060

PR: https://git.openjdk.java.net/jdk/pull/9060