RFR: 8264649: runtime/InternalApi/ThreadCpuTimesDeadlock.java crash in fastdebug C2 with -XX:-UseTLAB [v2]

[Vladimir Kozlov]

On Wed, 7 Apr 2021 12:00:10 GMT, Hui Shi <hshi at openjdk.org> wrote:

>> src/hotspot/share/opto/phaseX.cpp line 1481:
>> 
>>> 1479:   // Smash all inputs to 'old', isolating him completely
>>> 1480:   Node *temp = new Node(1);
>>> 1481:   temp->init_req(0,nn);     // Add a use to nn to prevent him from dying
>> 
>> Just wondering, why do we even need this? Without that code, `remove_dead_node(old)` would kill `nn` as well if it becomes dead.
>
> Thanks for your review!
> 
> Checking code history,  this code is quite old. From comments around, it doesn't want nn removed directly in PhaseIterGVN::subsume_node and leaves optimization to next GVN iteration.
> 
> In my understanding, it might save callers to insert codes checking if "nn" is removed/dead at every subsume_node/replace_node callsite, simplifies implementation.
> 
> 8153779a hotspot/src/share/vm/opto/phaseX.cpp (J. Duke                  2007-12-01 00:00:00 +0000 1479)   // Smash all inputs to 'old', isolating him completely
> 2a0815a5 hotspot/src/share/vm/opto/phaseX.cpp (Tobias Hartmann          2014-06-02 08:07:29 +0200 1480)   Node *temp = new Node(1);
> 8153779a hotspot/src/share/vm/opto/phaseX.cpp (J. Duke                  2007-12-01 00:00:00 +0000 1481)   temp->init_req(0,nn);     // Add a use to nn to prevent him from dying
> 8153779a hotspot/src/share/vm/opto/phaseX.cpp (J. Duke                  2007-12-01 00:00:00 +0000 1482)   remove_dead_node( old );
> 8153779a hotspot/src/share/vm/opto/phaseX.cpp (J. Duke                  2007-12-01 00:00:00 +0000 1483)   temp->del_req(0);         // Yank bogus edge

Note `nn` could be new node created during parsing which does not have users yet. that is why we need to keep it

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PR: https://git.openjdk.java.net/jdk/pull/3336