RFR: 8291466: C2: assert(false) failed: infinite loop in PhaseIterGVN::transform_old with -XX:+StressIGVN

[Pengfei Li]

On Wed, 10 Aug 2022 09:33:58 GMT, Pengfei Li <pli at openjdk.org> wrote:

> Recently, an igvn infinite loop issue was reported. Git bisection shows
> it appears after our JDK-8289996 patch. But after that patch is backed
> out, we find similar test still fails. We have attached a jtreg case to
> reproduce this issue.
> 
> The ideal graph of the problematic method has a `MulINode` multiplying a
> `PhiNode` by a `ConINode`. For better optimizations, `MulINode::Ideal()`
> moves the constant input to the right hand side. And `Ideal()` function
> in its parent class `MulNode` has the similar logic. In some code paths,
> `MulINode::Ideal()` calls `MulNode::Ideal()`. The problem here is that,
> `MulINode` and `MulNode` use different ways to check constant. One calls
> `type->singleton()` and the other calls `node->find_int_con(val)` which
> accepts constant in `PhiNode`. So in some corner cases where a `PhiNode`
> can be evaluated to a constant, the two inputs of the `MulNode` will be
> swapped back and forth in `Ideal()` calls. It eventually causes the igvn
> infinite loop issue.
> 
> This patch removes the `swap_edges()` logic in `MulINode` and `MulLNode`
> because it's enough to do this by calling `MulNode::Ideal()`. We also do
> some code cleanup in this patch as we have done in JDK-8289996.
> 
> Tested hotspot::hotspot_all_no_apps, jdk::tier1~3 and langtools::tier1.

PING. Any reviewer can help look at this? @shipilev

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PR: https://git.openjdk.org/jdk/pull/9820