RFR: 8359602: VerifyIterativeGVN fails with assert(!VerifyHashTableKeys || _hash_lock == 0) failed: remove node from hash table before modifying it
Tobias Hartmann
thartmann at openjdk.org
Mon Jun 30 12:06:39 UTC 2025
On Fri, 27 Jun 2025 10:59:57 GMT, Benoît Maillard <bmaillard at openjdk.org> wrote:
> This PR prevents some missed ideal optimizations in IGVN by notifying users of type refinements made during CCP, addressing a missed optimization that caused a verification failure with `-XX:VerifyIterativeGVN=1110`.
>
> ### Context
> During the compilation of the input program (obtained from the fuzzer, then simplified and added as a test) by C2, we end up with node `591 ModI` that takes `138 Phi` as its divisor input. An existing `Ideal` optimization is to get rid of the control input of a `ModINode` when we can prove that the divisor is never `0`.
>
> In this specific case, the type of the `PhiNode` gets refined during CCP, but the refinement fails to propagate to its users for the IGVN phase and the ideal optimization for the `ModINode` never happens. This results in a missed optimization and hits an assert in the verification phase of IGVN (when using `-XX:VerifyIterativeGVN=1110`).
>
> 
>
> ### Detailed Analysis
>
> In `PhaseCCP::analyze`, we call `Value` for the `PhiNode`, which
> results in a type refinement: the range gets restricted to `int:-13957..-1191`.
>
> ```c++
> // Pull from worklist; compute new value; push changes out.
> // This loop is the meat of CCP.
> while (worklist.size() != 0) {
> Node* n = fetch_next_node(worklist);
> DEBUG_ONLY(worklist_verify.push(n);)
> if (n->is_SafePoint()) {
> // Make sure safepoints are processed by PhaseCCP::transform even if they are
> // not reachable from the bottom. Otherwise, infinite loops would be removed.
> _root_and_safepoints.push(n);
> }
> const Type* new_type = n->Value(this);
> if (new_type != type(n)) {
> DEBUG_ONLY(verify_type(n, new_type, type(n));)
> dump_type_and_node(n, new_type);
> set_type(n, new_type);
> push_child_nodes_to_worklist(worklist, n);
> }
> if (KillPathsReachableByDeadTypeNode && n->is_Type() && new_type == Type::TOP) {
> // Keep track of Type nodes to kill CFG paths that use Type
> // nodes that become dead.
> _maybe_top_type_nodes.push(n);
> }
> }
> DEBUG_ONLY(verify_analyze(worklist_verify);)
>
>
> At the end of `PhaseCCP::analyze`, we obtain the following types in the side table:
> - `int` for node `591` (`ModINode`)
> - `int:-13957..-1191` for node `138` (`PhiNode`)
>
> If we call `find_node(138)->bottom_type()`, we get:
> - `int` for both nodes
>
> There is no progress on the type of `ModINode` during CCP, because `ModINode::Value`
> is not able to...
I agree with Marc, really nice write-up! The fix looks good to me.
Did you check if this allows enabling any of the other disabled verifications from [JDK-8347273](https://bugs.openjdk.org/browse/JDK-8347273)?
Please change the title of this issue to something more descriptive.
test/hotspot/jtreg/compiler/c2/TestPropagateTypeRefinementToIGVN.java line 35:
> 33: * -XX:CompileCommand=compileonly,compiler.c2.TestPropagateTypeRefinementToIGVN::test
> 34: * compiler.c2.TestPropagateTypeRefinementToIGVN
> 35: * @run main compiler.c2.TestPropagateTypeRefinementToIGVN
The test name is a bit too generic, maybe change to `TestModControlFoldedAfterCCP` or something.
-------------
Changes requested by thartmann (Reviewer).
PR Review: https://git.openjdk.org/jdk/pull/26017#pullrequestreview-2970961669
PR Review Comment: https://git.openjdk.org/jdk/pull/26017#discussion_r2174907075
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