8370459: C2: CompressBitsNode::Value produces wrong result on Windows (1UL vs 1ULL), found by ExpressionFuzzer

[eme64]

It seems we keep finding issues in CompressBitsNode::Value, using the TemplateFramework #26885.

This is a JDK26 regression of the bugfix #23947, which was itself reported by my prototype of the TemplateFramework.

The bug is simple: On windows 1UL is only a 32-bit value, and not a 64-bit value. We should use 1ULL instead. Impacted lines:

jdk/src/hotspot/share/opto/intrinsicnode.cpp

Line 276 in b02c125

hi = (1UL << bitcount) - 1;

jdk/src/hotspot/share/opto/intrinsicnode.cpp

Line 379 in b02c125

hi = MIN2((jlong)((1UL << result_bit_width) - 1L), hi);

This means that simple cases like these wrongly constant fold to zero:

• Long.compress(-2683206580L, Integer.toUnsignedLong(x))
• Long.compress(x, 0xffff_ffffL)

---

This sort of bug (1UL vs 1ULL) is of course very subtle, and easy to miss in a code review. So that is why testing is paramount.

Why was this not caught in the testing of #23947? After all there were quite a few tests there, right? There were simply not enough tests, or not the right ones ;)

I did at the time ask for a "range-based" test (#23947 (comment)). I then doubled down and even proposed a conctete test (#23947 (comment)) that would create "range-based" inputs:

public static test(int mask, int src) {
  mask = Math.max(CON1, Math.min(CON2, mask));
  src = Math.max(CON2, Math.min(CON4, src));
  result = Integer.compress(src, mask);
  int sum = 0;
  if (sum > LIMIT_1) { sum += 1; }
  if (sum > LIMIT_2) { sum += 2; }
  if (sum > LIMIT_3) { sum += 4; }
  if (sum > LIMIT_4) { sum += 8; }
  if (sum > LIMIT_5) { sum += 16; }
  if (sum > LIMIT_6) { sum += 32; }
  if (sum > LIMIT_7) { sum += 64; }
  if (sum > LIMIT_8) { sum += 128; }
  return new int[] {sum, result};
}

What is implortant here: both the src and mask must have random ranges. But the test that ended up being integrated only made the src "range-based" using the min/max. Without the mask being tested "range-based", the bug here could not have been caught by that test.

I was asked again for my review (#23947 (comment)), but I had to go on vacation, and was not able to catch the issue (#23947).

---

Progress

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Issue

• JDK-8370459: C2: CompressBitsNode::Value produces wrong result on Windows (1UL vs 1ULL), found by ExpressionFuzzer (Bug - P3)

Reviewers

• Jatin Bhateja (@jatin-bhateja - Reviewer) Review applies to 794ef7b3
• Tobias Hartmann (@TobiHartmann - Reviewer) Review applies to 41b8c365
• Dean Long (@dean-long - Reviewer)

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[bridgekeeper]

👋 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]

@eme64 This change now passes all automated pre-integration checks.

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

8370459: C2: CompressBitsNode::Value produces wrong result on Windows (1UL vs 1ULL), found by ExpressionFuzzer

Reviewed-by: dlong, jbhateja, thartmann

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At the time when this comment was updated there had been 46 new commits pushed to the master branch:

• 1781b18: 8343232: PKCS#12 KeyStore support for RFC 9879: Use of Password-Based Message Authentication Code 1 (PBMAC1)
• 8236800: 8370527: Memory leak after 8316694: Implement relocation of nmethod within CodeCache
• 16dafc0: 8370393: Cleanup handling of ancient Windows versions from GetJavaProperties java_props_md
• ... and 43 more: https://git.openjdk.org/jdk/compare/2c07214d7c075da5dd4a4e872aef29f58cef2bae...master

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Webrevs

• 03: Full - Incremental (8bf89e95)
• 02: Full - Incremental (d2b88bee)
• 01: Full - Incremental (41b8c365)
• 00: Full (794ef7b3)

[jatin-bhateja]

Hi @eme64,
Your fix looks good to me!
Thanks for addressing this.

Best Regards

[TobiHartmann]

Looks good to me. Thank you for your persistence on improving our testing! Good that we caught this in-time for JDK 26.

[dean-long]

It seems weird having - 1L mixed with ULL now. It might be better to use right_n_bits_typed() here and at line 276.

[eme64]

Great idea, I did not know about right_n_bits_typed :)

[openjdk]

⚠️ @eme64 This pull request contains merges that bring in commits not present in the target repository. Since this is not a "merge style" pull request, these changes will be squashed when this pull request in integrated. If this is your intention, then please ignore this message. If you want to preserve the commit structure, you must change the title of this pull request to Merge <project>:<branch> where <project> is the name of another project in the OpenJDK organization (for example Merge jdk:master).

[eme64]

@dean-long Does it look better now?

[dean-long]

@dean-long Does it look better now?

Yes, much better, thanks!

[eme64]

@dean-long @TobiHartmann @jatin-bhateja thanks for the quick reviews!

/integrate

[openjdk]

Going to push as commit 0ca0852.
Since your change was applied there have been 56 commits pushed to the master branch:

• 4cfabd6: 8370974: Command-line Hotspot debugger tests fail on Windows AArch64
• 05efc1f: 8370578: SystemMapTest.java#normal fails intermittendly on macOS aarch64
• e8a1a87: 8369186: HotSpot Style Guide should permit some uses of the C++ Standard Library
• ... and 53 more: https://git.openjdk.org/jdk/compare/2c07214d7c075da5dd4a4e872aef29f58cef2bae...master

Your commit was automatically rebased without conflicts.

[openjdk]

@eme64 Pushed as commit 0ca0852.

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