8252237: C2: Call to compute_separating_interferences has wrong argument order

[r-v-raghav]

# [.../src/hotspot/share/opto/coalesce.hpp] -
112 uint compute_separating_interferences(Node *dst_copy, Node *src_copy, Block *b, uint bindex, RegMask &rm, uint rm_size, uint reg_degree, uint lr1, uint lr2);

# [.../src/hotspot/share/opto/coalesce.cpp] -
..........
538 uint PhaseConservativeCoalesce::compute_separating_interferences(Node *dst_copy, Node *src_copy, Block *b, uint bindex, RegMask &rm, uint reg_degree, uint rm_size, uint lr1, uint lr2 ) {
.................
747 reg_degree = compute_separating_interferences(dst_copy, src_copy, b, bindex, rm, rm_size, reg_degree, lr1, lr2 );

# So fixing the argument order - [.../src/hotspot/share/opto/coalesce.cpp]
-uint PhaseConservativeCoalesce::compute_separating_interferences(Node *dst_copy, Node *src_copy, Block *b, uint bindex, RegMask &rm, uint reg_degree, uint rm_size, uint lr1, uint lr2 ) {
+uint PhaseConservativeCoalesce::compute_separating_interferences(Node *dst_copy, Node *src_copy, Block *b, uint bindex, RegMask &rm, uint rm_size, uint reg_degree, uint lr1, uint lr2 ) {

---

Progress

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Issue

• JDK-8252237: C2: Call to compute_separating_interferences has wrong argument order

Reviewers

• Tobias Hartmann (@TobiHartmann - Reviewer)
• Vladimir Kozlov (@vnkozlov - Reviewer)
• shade - Reviewer ⚠️ Added manually

Download

$ git fetch https://git.openjdk.java.net/jdk pull/1533/head:pull/1533
$ git checkout pull/1533

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[openjdk]

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[mlbridge]

Webrevs

• 00: Full (a56dedc)

[r-v-raghav]

/reviewer credit thartmann, shade

[openjdk]

@r-v-raghav
Reviewer thartmann successfully credited.

Reviewer shade successfully credited.

[shipilev]

This change looks good, but my question is more wide: does it actually break/fixes any compiler bug? I would have thought changing RA would have some impact. In other words, are there test failures on higher tiers?

[TobiHartmann]

Looks good to me but I'm also wondering why that was never an issue and if the fix could now trigger previously hidden problems. We should at least give it a good amount of testing before integration.

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@r-v-raghav This change now passes all automated pre-integration checks.

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

8252237: C2: Call to compute_separating_interferences has wrong argument order

Reviewed-by: thartmann, kvn, shade

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At the time when this comment was updated there had been 113 new commits pushed to the master branch:

• 6c9482e: 8257561: Some code is not vectorized after 8251925 and 8250607
• 70517c8: 8257642: CipherByteBufferOverwriteTest copyright issue
• e836396: 8257436: [aarch64] Regressions in ArrayCopyUnalignedDst.testByte/testChar for 65-78 bytes when UseSIMDForMemoryOps is on
• b170c83: 8257591: Remove suppression of record preview related warnings in java.lang
• 66a2e70: 8255845: Memory leak in imageFile.cpp
• fa58671: 8257020: [JVMCI] enable a JVMCICompiler to specify which GCs it supports
• 129c377: 8257594: C2 compiled checkcast of non-null object triggers endless deoptimization/recompilation cycle
• e4497c9: 8256718: Obsolete the long term deprecated and aliased Trace flags
• 4a267f1: 8244847: Linux/PPC: runtime/CompressedOops/CompressedClassPointers: smallHeapTest fails
• b44a329: 8256864: [windows] Improve tracing for mapping errors
• ... and 103 more: https://git.openjdk.java.net/jdk/compare/4e43b28858b0c7c9fb7ad91a506b61aa1d554f86...master

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[vnkozlov]

Looks good to me but I'm also wondering why that was never an issue and if the fix could now trigger previously hidden problems. We should at least give it a good amount of testing before integration.

compute_separating_interferences() is used only in one place and resulting reg_degree is only used in place to compare with max_juint:
https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/coalesce.cpp#L746
Due to this bug the returning value is most likely max_juint and we bailout coalesce of copies. As result we would have more LRG live ranges to work with but Graph still can be processed.
In short RA produce less optimal code.
I agree with @TobiHartmann we should do a lot of tiers testing for this fix because coalescing code after L755 may be not exercised as much as we want due to this bug.

[r-v-raghav]

Thanks for the details @shipilev, @TobiHartmann, @vnkozlov.
I too could not link any other unresolved or 'won't-fix/incomplete' old c2-regalloc bug to this case.
Yes I will initiate more tier testing with the change before integeration.

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