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8308892: Bad graph detected in build_loop_late after JDK-8305635 #14196

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@chhagedorn chhagedorn commented May 28, 2023

The cleanup done in JDK-8305635 wrongly identifies unrelated Parse Predicates which are not cleaned up, yet. It just walks from the entry of the loop up and tries to find each of the three Parse Predicates once but in no particular order. This order insensitive walk is wrong as seen in the following graph (from the attached replay file of this bug):

image

We first find 116 Parse Predicate for Loop Predicates, then 84 Parse Predicate for Profiled Loop Predicates and then stop when finding 71 Parse Predicate for Loop Predicates because we've already found a Parse Predicate for Loop Predicates already. We then wrongly create Loop Predicates (above 116 Parse Predicate) which are below newly created Profiled Loop Predicates (above 84 Parse Predicate). This could lead to a bad graph because of data dependencies that rely on the fact that Loop Predicates are above Profiled Loop Predicates:

bool can_create_loop_predicates = true;
// We cannot add Loop Predicates if:
// - Already added Profiled Loop Predicates (Loop Predicates and Profiled Loop Predicates can be dependent
// through a data node, and thus we should only add new Profiled Loop Predicates which are below Loop Predicates
// in the graph).
// - There are currently no Profiled Loop Predicates, but we have a data node with a control dependency on the Loop
// Parse Predicate (could happen, for example, if we've removed an earlier created Profiled Loop Predicate with
// dominated_by()). We should not create a Loop Predicate for a check that is dependent on this data node because
// the Loop Predicate would end up above the data node with its dependency on the Loop Parse Predicate below. This
// would become unschedulable. However, we can still hoist the check as Profiled Loop Predicate which would end up
// below the Loop Parse Predicate.
if (Predicates::has_profiled_loop_predicates(parse_predicates)
|| (parse_predicates.loop_predicate_proj() != nullptr && parse_predicates.loop_predicate_proj()->outcnt() != 1)) {
can_create_loop_predicates = false;
}

The fix is straight forward to make the assignment of Parse Predicate projections in ParsePredicates aware of the relative ordering constraint. Note that this class will be refactored again in JDK-8305636. But I think properly fixing this first is better than waiting for JDK-8305636 to go in.

Testing: tier1-4, hs-precheckin-comp, hs-stress-comp

Thanks,
Christian


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  • JDK-8308892: Bad graph detected in build_loop_late after JDK-8305635

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👋 Welcome back chagedorn! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

@openjdk openjdk bot added the rfr Pull request is ready for review label May 28, 2023
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@chhagedorn The following label will be automatically applied to this pull request:

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Looks good!

Windows build failure in GHA testing is unrelated.

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@chhagedorn This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8308892: Bad graph detected in build_loop_late after JDK-8305635

Reviewed-by: rcastanedalo, roland, thartmann

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At the time when this comment was updated there had been 112 new commits pushed to the master branch:

  • dc8bc6c: 8308090: Add container tests for on-the-fly resource quota updates
  • 73e7af9: 8309287: Add fontconfig requirement to building.md for Debian
  • aeb53e6: 8308711: Develop additional Tests for KEM implementation
  • dcd9590: 8309224: Fix xlc17 clang 15 warnings in java.desktop
  • 8f1ce78: 8308752: Generational ZGC: Avoid final marking through stack chunks
  • 7b0a336: 8308387: CLD created and unloading list sharing _next node pointer leads to concurrent YC missing CLD roots
  • 60f3b87: 8309295: C2: MaxNode::signed_min() returns nullptr for int operands
  • 8007599: 8309093: Underscore with brackets
  • 5bd2af2: 8307478: Implementation of Prepare to Restrict The Dynamic Loading of Agents
  • 325940b: 8307105: JFileChooser InvalidPathException when selecting some system folders on Windows
  • ... and 102 more: https://git.openjdk.org/jdk/compare/90e57fd5a96199b01e7c058a4d8a82e467d7f41a...master

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Thanks Roberto for your review!

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The cleanup done in JDK-8305635 wrongly identifies unrelated Parse Predicates which are not cleaned up, yet.

So which of the 3 predicates will get cleaned up? Are some of the 3 predicates for another loop that doesn't exist anymore?

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chhagedorn commented May 30, 2023

The cleanup done in JDK-8305635 wrongly identifies unrelated Parse Predicates which are not cleaned up, yet.

So which of the 3 predicates will get cleaned up? Are some of the 3 predicates for another loop that doesn't exist anymore?

Sorry, the explanation of the bug was not precise enough - there is a missing piece: It should have cleaned up 84 Parse Predicate and 71 Parse Predicate in eliminate_useless_predicates() but it does not because we also use the broken ParsePredicates class to collect them:

ParsePredicates parse_predicates(entry);
ProjNode* predicate_proj = parse_predicates.loop_limit_check_predicate_proj();
if (predicate_proj != nullptr) { // right pattern that can be used by loop predication
assert(predicate_proj->in(0)->in(1)->in(1)->Opcode() == Op_Opaque1, "must be");
useful_predicates.push(predicate_proj->in(0)->in(1)->in(1)); // good one
}
if (UseProfiledLoopPredicate) {
predicate_proj = parse_predicates.profiled_loop_predicate_proj();
if (predicate_proj != nullptr) { // right pattern that can be used by loop predication
useful_predicates.push(predicate_proj->in(0)->in(1)->in(1)); // good one
get_assertion_predicates(predicate_proj, useful_predicates, true);
}
}
if (UseLoopPredicate) {
predicate_proj = parse_predicates.loop_predicate_proj();
if (predicate_proj != nullptr) { // right pattern that can be used by loop predication
useful_predicates.push(predicate_proj->in(0)->in(1)->in(1)); // good one
get_assertion_predicates(predicate_proj, useful_predicates, true);
}
}
}

So, we keep 116 Parse Predicate and 84 Parse Predicate as useful predicates while 71 Parse Predicate is removed before Loop Predication.

The useless predicates were added normally for the loop while the additional 116 Parse Predicate was added with maybe_add_predicate_after_if() (the profiled loop predicate was skipped due to too_may_trap(reason) being true).

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The useless predicates were added normally for the loop while the additional 116 Parse Predicate was added with maybe_add_predicate_after_if() (the profiled loop predicate was skipped due to too_may_trap(reason) being true).

Do I read that correctly that parsing inserts useless/duplicate predicates?

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After the first IGVN round, after parsing, they become useless because the If that triggered the insertion of the predicates with maybe_add_predicate_after_if() is folded away. I think it's unfortunate, though, that we eliminate 71 Parse Predicate and 84 Parse Predicate instead of 116 Parse Predicate (so, we cannot add profiled loop predicates anymore). But that might be an edge case.

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Looks good to me.

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Thanks for the explanation. Can a test case be added?

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Thanks Roland for your review!

I was able to come up with a test that fails with the same graph pattern. I've pushed an update.

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Looks good to me.

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Thanks Tobias for your review! I've simplified the test some more but the fix remains the same.

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Still looks good.

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Looks good to me.

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/integrate

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openjdk bot commented Jun 2, 2023

Going to push as commit 7dbdad5.
Since your change was applied there have been 112 commits pushed to the master branch:

  • dc8bc6c: 8308090: Add container tests for on-the-fly resource quota updates
  • 73e7af9: 8309287: Add fontconfig requirement to building.md for Debian
  • aeb53e6: 8308711: Develop additional Tests for KEM implementation
  • dcd9590: 8309224: Fix xlc17 clang 15 warnings in java.desktop
  • 8f1ce78: 8308752: Generational ZGC: Avoid final marking through stack chunks
  • 7b0a336: 8308387: CLD created and unloading list sharing _next node pointer leads to concurrent YC missing CLD roots
  • 60f3b87: 8309295: C2: MaxNode::signed_min() returns nullptr for int operands
  • 8007599: 8309093: Underscore with brackets
  • 5bd2af2: 8307478: Implementation of Prepare to Restrict The Dynamic Loading of Agents
  • 325940b: 8307105: JFileChooser InvalidPathException when selecting some system folders on Windows
  • ... and 102 more: https://git.openjdk.org/jdk/compare/90e57fd5a96199b01e7c058a4d8a82e467d7f41a...master

Your commit was automatically rebased without conflicts.

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@chhagedorn Pushed as commit 7dbdad5.

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