8255579: x86: Use cmpq(Register,Address) in safepoint_poll

[shipilev]

JDK-8253180 added a new block in safepoint_poll that uses broken cmpq (JDK-8255550): it effectively does the comparison with operands swapped. It makes sense to use the non-broken cmpq, as to avoid changing the condition code and thus making the code less understandable. See the discussion in #910.

Testing:

• tier1 with Z (some SA tests fail, and some other fail with OOME -- seem to be expected/problem-listed)

---

Progress

• Change must not contain extraneous whitespace
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Testing

	Linux x64	Linux x86	Windows x64	macOS x64
Build	✔️ (5/5 passed)	✔️ (2/2 passed)	✔️ (2/2 passed)	✔️ (2/2 passed)
Test (tier1)	✔️ (9/9 passed)	✔️ (9/9 passed)	✔️ (9/9 passed)	✔️ (9/9 passed)

Issue

• JDK-8255579: x86: Use cmpq(Register,Address) in safepoint_poll

Reviewers

• Erik Österlund (@fisk - Reviewer)
• Zhengyu Gu (@zhengyu123 - Reviewer)

Download

$ git fetch https://git.openjdk.java.net/jdk pull/924/head:pull/924
$ git checkout pull/924

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[mlbridge]

Webrevs

• 01: Full - Incremental (cc98eaa)
• 00: Full (1f156d7)

[fisk]

Looks good. I also got confused about this. When this instruction didn't work the way I thought it would, I thought it must be a weird style thing, that everything is right to left in the assembler, for consistency, so it would read from right to left. Then I convinced myself it made sense. But the instruction being incorrectly encoded also checks out. Sigh. Anyway, I ran some local testing that would just fall over immediately if this didn't work. And it seems to work.

[openjdk]

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

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

8255579: x86: Use cmpq(Register,Address) in safepoint_poll

Reviewed-by: eosterlund, zgu

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At the time when this comment was updated there had been 1 new commit pushed to the master branch:

• 72ff8e2: 8254782: Fix benchmark issues in java/lang/StringIndexOfChar.java benchmark

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[shipilev]

Thanks @fisk! Would you like someone else to look at this, or your review is enough?

[fisk]

Since the encodings of the two instructions are literally identical, I think this classifies as a trivial change. So I'm okay with one review.

[zhengyu123]

Tested with Shenandoah + concurrent stack processing on my personal branch, works fine.

[shipilev]

Tests look clean. Pushing, thanks @fisk, @zhengyu123.

/integrate

[openjdk]

@shipilev Since your change was applied there has been 1 commit pushed to the master branch:

• 72ff8e2: 8254782: Fix benchmark issues in java/lang/StringIndexOfChar.java benchmark

Your commit was automatically rebased without conflicts.

Pushed as commit 4b20e46.

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