IGVN worklist ordering

[Krystal Mok]

Hi compiler team,

I’d like to ask about the IGVN worklist ordering:

1. Is there a rule of thumb of which nodes should call record_for_igvn()?
If so, in what order (e.g. data nodes vs. their control dependence)?

Apparently, nodes that get a record_for_igvn() call right after their
creation usually wants to delay the call to transform(), perhaps due to
potential optimizations to their control input, or because they're created
after parsing and might affect other nodes.

But what would be a good list of guidelines about exactly what kind of
nodes, or what kind of patterns in the IR, that should be considered as
candidates to put onto the IGVN worklist, and vice versa?

2. Since it’s problematic for IGVN to process a node whose control path is
already dead (but not yet collapsed to TOP), why isn’t there a mechanism
built-in to IGVN’s worklist or PhaseIterGVN::transform_old() so that dead
control paths always collapses before IGVN decides to process a node?


I’ve had a doubt on this topic for quite a while now, so I’m seeking advice
from all of you who have had to deal with bugs in this area.
There have been a lot of bugs related to the IGVN worklist ordering in the
past, the most of the fixes does either:
a) add missing record_for_igvn() calls for problematic nodes
b) switch the order of adjacent record_for_igvn() calls on related nodes
Is it possible or a good idea to fix it down at the core of IGVN?

Happy holidays, guys!

Best regards,
Kris
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