RFR: 8254317: C2: Resource consumption of ConvI2LNode::Ideal() grows exponentially

[Roberto Castañeda Lozano]

On Mon, 19 Oct 2020 15:56:57 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> 2 concerns with the proposed fix on my side:
> 
>     * I’m not persuaded that covering “x == y” is enough to completely eliminates the issue;
> 
>     * The issue demonstrates there’s still a chance to introduce very deep recursion involving `Compile::constrained_convI2L`
>       and `PhaseIterGVN` which can cause a crash.
> 
> 
> IMO the root cause is an eager transformation happening top-down on `ConvI2L` nodes and it defeats memoization GVN
> naturally provides, so it causes a combinatorial explosion. If subsequent `Compile::constrained_convI2L()` calls could
> share the same `ConvI2L` node for the same input, it would be a more reliable fix for the problem.  Otherwise, the
> transformation may be extracted from GVN and turned into a separate pass (take a look at
> `Compile::optimize_logic_cones` as an example).  Some comments on the tests: (1) please, group the individual test
> cases into a single test class; and (2) I suggest to turn the benchmark into a test case which fails with timeout when
> fix is absent.

Thanks Vladimir for the thorough review! I will explore your suggestions to generalize the fix and see what can be done.

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PR: https://git.openjdk.java.net/jdk/pull/727