RFR: 8291466: C2: assert(false) failed: infinite loop in PhaseIterGVN::transform_old with -XX:+StressIGVN [v2]

[Christian Hagedorn]

On Thu, 25 Aug 2022 09:44:47 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.
>
> Pengfei Li has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Reword some comments

src/hotspot/share/opto/mulnode.cpp line 252:

> 250:   // As the if block above tests the result of find_int_con(0), which
> 251:   // is zero if no constant is found, we cannot have "con == 0" here.
> 252:   if (con == 1) return NULL;   // By one is handled by Identity call

While at it, you could add braces here. Same further down for `MulL`.

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