RFR: 8286179: Node::find(int) should not traverse from new to old nodes [v2]

[Emanuel Peter]

On Mon, 9 May 2022 13:04:03 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

>> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   added nullprt check for old_root
>
> src/hotspot/share/opto/node.cpp line 1617:
> 
>> 1615: Node* find_old_node(const int idx) {
>> 1616:   Node* root = old_root();
>> 1617:   assert(root != nullptr, "must have old_root() to find old nodes");
> 
> I think it's better to avoid assertions here and below and do nothing instead (you already print a warning in `old_root()` which is fine I think) since it would crash and stop the current debugging session.

@chhagedorn I added this `nullptr` because @vnkozlov asked for one. https://github.com/openjdk/jdk/pull/8567#discussion_r866966586
Is there an alternative to an assert?
I realize, that in `rr` the debuggin session is not stopped for me - it just unwinds - and I can continue debugging as if nothing happened. However in `gdb` this does crash the debugging session - at least by default. You can change that behavior with `set unwindonsignal on`.

There are other asserts that can be triggered. For example if we call `find_node(1)` from a non-compiler thread, we get an assert when internally `Compile::current()` comes across this

https://github.com/openjdk/jdk/blob/b849efdf154552903faaddd69cac1fe5f1ddf18a/src/hotspot/share/compiler/compilerThread.hpp#L63-L64

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PR: https://git.openjdk.java.net/jdk/pull/8567