RFR: 8263577: C2: reachable nodes shouldn't have dead uses at the end of optimizations

[Roland Westrelin]

On Mon, 15 Mar 2021 12:33:19 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> It's not uncommon for c2 to leave dead uses hanging from reachable
> nodes. It's usually harmless but can cause confusion (such as in
> valhalla JDK-8262289, not reproducible with non valhalla code AFAICT).
> 
> Most of the time, these are caused by calls to set_req() during igvn
> that should be set_req_X() calls. I added a verification pass at the
> end of optimization and fixed all the ones I found.

> hi, @rwestrel
> 
> Is Node::set_req_X() same as PhaseIterGVN::replace_input_of()?
> This even supports delayed hash. can I use them interchangeably?

They are not quite the same as replace_input_of() has the rehash_node_delayed() call. When modifying an existing node, it's a requirement to call has_delete() which rehash_node_delayed() does for you. If you're in Ideal() modifying the "this" node, the hash_delete() call happened before the ideal call.

So I guess roughly: in Ideal modifying this's input -> set_req_X(), otherwise replace_input_of()

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

PR: https://git.openjdk.java.net/jdk/pull/3012