8296412: Special case infinite loops with unmerged backedges in IdealLoopTree::check_safepts #11706

eme64 · 2022-12-16T09:57:35Z

Context
During parsing, we insert SafePoints if we jump from higher to lower bci (maybe_add_safepoint is called for every if, goto etc).

jdk/src/hotspot/share/opto/parse.hpp

Lines 490 to 494 in 8c472e4

    
           void maybe_add_safepoint(int target_bci) { 
        
             if (target_bci <= bci()) { 
        
               add_safepoint(); 
        
             } 
        
           }

Generally, this alligns with backedges: the assumption is that the loop-head sits at the smallest bci of all blocks in the loop. So every jump back to the loop-head goes from higher to lower bci, hence we place a SafePoint just before the jump.

Also: the first build_loop_tree may not attach an infinite loop to the loop-tree. If during the same loop-opts-phase we go to beautify_loops and it requires us rebuilding the loop-tree (eg because some other loop did merge_many_backedges), we call build_loop_tree again, and this time around we do detect the infinite loop (it now has a NeverBranch exit, so it is attached because of that).
Afterwards, we call IdealLoopTree::check_safepts, which tries to find SafePoints on all backedges. Normally, we have SafePoints on all backedges, just before we go back to the head.

Problem case
My jasm fuzzer produced some infinite loops that have the following form:
The loop head is not at the smallest bci (bytecode index) of all blocks in the loop. So the SafePoints are placed somewhere in the body of the loop, just before an if branches into the two backedges. Because this is an infinite loop, it is only attached to the loop-tree in build_loop_tree after beautify_loops, so the two backedges were not merged.
When we call IdealLoopTree::check_safepts, we start with the inner loop, where we find the SafePoint above the if. Then we go to the outer loop. We don't find a SafePoint before we find the inner body. Now we decide to skip the inner body (which implies skipping the SafePoint in the body). The code assumes after skipping the inner loop, we are still in the outer loop. This is not true, because inner and outer loop have the same loop head (the backedges were not merged). We trigger an assert that checks that we are still in the outer loop (nested loop).

Why did we not find this earlier?
We have not extensively tested infinite loops before. Also, we have not tested loops with loop-heads that are not at the smallest bci of the loop. However, with my bytecode fuzzer I can find these issues. It is also more likely with irreducible loops: there at least one loop-entry cannot be at the smallest bci. Irreducible loops are not processed by maybe_add_safepoint, but once it only has a singe entry, it is not irreducible any more, and so it can happen that a loop-entry becomes loop head that does not have the smallest bci.

Solution
We could fix maybe_add_safepoint to not depend on bci, but rather the loop-tree from ciTypeFlow. That would be complex, and risky. That is not justified just for infinite loops, and even infinite loops where the loop head is not at the lowest bci.

I decided to simply special case infinite loops. I detect if we have an outer loop with the same head as an inner loop. This should not happen, as we must have merged those backedges. Except if it is an infinite loop: We can break the scan, as we have already reached the loop's head.

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Issue

JDK-8296412: Special case infinite loops with unmerged backedges in IdealLoopTree::check_safepts

Reviewers

Christian Hagedorn (@chhagedorn - Reviewer) ⚠️ Review applies to 2a02b338
Vladimir Kozlov (@vnkozlov - Reviewer) ⚠️ Review applies to 2a02b338
Tobias Hartmann (@TobiHartmann - Reviewer) ⚠️ Review applies to 2a02b338

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…LoopTree::check_safepts

bridgekeeper · 2022-12-16T09:59:07Z

👋 Welcome back epeter! 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 · 2022-12-16T10:03:53Z

@eme64 The following label will be automatically applied to this pull request:

hotspot-compiler

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mlbridge · 2022-12-16T11:17:13Z

Webrevs

chhagedorn

Nice analysis! That looks reasonable to go with this simpler fix instead of fixing maybe_add_safepoint() given that this case is rare.

src/hotspot/share/opto/loopnode.cpp

openjdk · 2022-12-16T11:30:42Z

@eme64 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:

8296412: Special case infinite loops with unmerged backedges in IdealLoopTree::check_safepts

Reviewed-by: chagedorn, kvn, thartmann

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 23 new commits pushed to the master branch:

3637660: 8298813: [C2] Converting double to float cause a loss of precision and resulting crypto.aes scores fluctuate
5e678f7: 8298824: C2 crash: assert(is_Bool()) failed: invalid node class: ConI
ba942c2: 8298244: AArch64: Optimize vector implementation of AddReduction for floating point
7938f8c: 8298639: Perform I/O operations in bulk for RandomAccessFile
bfa921a: 8160404: RelocationHolder constructors have bugs
bf9a8ce: 8249826: 5 javax/net/ssl/SSLEngine tests use @ignore w/o bug-id
0eeaeb8: 8298808: Check script code on detecting the base locales
81e23ab: 8298809: Clean up vm/compiler/InterfaceCalls JMH
3696711: Merge
f771c56: 8298797: Specification of some restricted methods is incorrect
... and 13 more: https://git.openjdk.org/jdk/compare/fa322e40b68abf0a253040d14414d41f4e01e028...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

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vnkozlov

Looks good.

test/hotspot/jtreg/compiler/loopopts/TestInfiniteLoopWithUnmergedBackedgesMain.java

TobiHartmann

Looks good to me.

…gedBackedgesMain.java remove redundant empty line Co-authored-by: Tobias Hartmann <tobias.hartmann@oracle.com>

eme64 · 2022-12-19T12:20:08Z

Thanks @TobiHartmann @chhagedorn @vnkozlov for the discussions and reviews!
/integrate

openjdk · 2022-12-19T12:22:13Z

Going to push as commit da38d43.
Since your change was applied there have been 27 commits pushed to the master branch:

8e49fcd: 8295661: CompileTask::compile_id() should be passed as int
86d588b: 8283740: C1: Convert flag TwoOperandLIRForm to a constant on all platforms
ec95991: 8298736: Revisit usages of log10 in compiler code
1622563: 8265688: Unused ciMethodType::ptype_at should be removed
3637660: 8298813: [C2] Converting double to float cause a loss of precision and resulting crypto.aes scores fluctuate
5e678f7: 8298824: C2 crash: assert(is_Bool()) failed: invalid node class: ConI
ba942c2: 8298244: AArch64: Optimize vector implementation of AddReduction for floating point
7938f8c: 8298639: Perform I/O operations in bulk for RandomAccessFile
bfa921a: 8160404: RelocationHolder constructors have bugs
bf9a8ce: 8249826: 5 javax/net/ssl/SSLEngine tests use @ignore w/o bug-id
... and 17 more: https://git.openjdk.org/jdk/compare/fa322e40b68abf0a253040d14414d41f4e01e028...master

Your commit was automatically rebased without conflicts.

openjdk · 2022-12-19T12:22:37Z

@eme64 Pushed as commit da38d43.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

8296412: Special case infinite loops with unmerged backedges in Ideal…

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…LoopTree::check_safepts

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eme64 marked this pull request as ready for review December 16, 2022 11:11

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Update test/hotspot/jtreg/compiler/loopopts/TestInfiniteLoopWithUnmer…

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…gedBackedgesMain.java remove redundant empty line Co-authored-by: Tobias Hartmann <tobias.hartmann@oracle.com>

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8296412: Special case infinite loops with unmerged backedges in IdealLoopTree::check_safepts #11706

8296412: Special case infinite loops with unmerged backedges in IdealLoopTree::check_safepts #11706

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openjdk bot commented Dec 19, 2022

	void maybe_add_safepoint(int target_bci) {
	if (target_bci <= bci()) {
	add_safepoint();
	}
	}

8296412: Special case infinite loops with unmerged backedges in IdealLoopTree::check_safepts #11706

8296412: Special case infinite loops with unmerged backedges in IdealLoopTree::check_safepts #11706

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