RFR: 8308103: Massive (up to ~30x) increase in C2 compilation time since JDK 17

[Vladimir Kozlov]

On Fri, 30 Jun 2023 13:23:38 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> A long chain of nodes are sunk out of a loop. Every time a node is
> moved out of the loop, a cast is created to pin the node out of the
> loop. When its input is next sunk, the cast is removed (the cast is
> replaced by its input) and a new cast is created. Some nodes on the
> chain have 2 other nodes in the chain as uses. When such a node is
> sunk, 2 cast nodes are created, one for each use. So as the compiler
> moves forward in the chain, the number of cast to remove grows. From
> some profiling, removing those casts is what takes a lot of time.
> 
> The fix I propose is, when a node is processed, to check whether a
> cast at the out of loop control was already created for that node and
> to reuse it.
> 
> The test case takes 6 minutes when I run it without the fix and 3
> seconds with it.

src/hotspot/share/opto/loopopts.cpp line 1704:

> 1702:                   cast = prev;
> 1703:                 } else {
> 1704:                   register_new_node(cast, x_ctrl);

Can you move creation of `cast` here so you don't need to destroy it in case of previous cast existance?
Or it is possible that `ConstraintCastNode::make_cast_for_type() can return `null`?

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PR Review Comment: https://git.openjdk.org/jdk/pull/14732#discussion_r1248114592