8288564: C2: LShiftLNode::Ideal produces wrong result after JDK-8278114

[chhagedorn]

JDK-8278114 added the following transformation for integer and long left shifts:

 "(x + x) << c0" into "x << (c0 + 1)"

However, in the long shift case, this transformation is not correct if c0 is 63:

(x + x) << 63 = 2x << 63

while

 (x + x) << 63 --transform--> x << 64 = x << 0 = x

which is not the same. For example, if x = 1:

2x << 63 = 2 << 63 = 0 != 1

This optimization does not account for the fact that x << 64 is the same as x << 0 = x. According to the Java spec, chapter 15.19, we only consider the six lowest-order bits of the right-hand operand (i.e. "right-hand operand" & 0b111111). Therefore, x << 64 is the same as x << 0 (64 = 0b10000000 & 0b0111111 = 0).

Integer shifts are not affected because we do not apply this transformation if c0 >= 16:

jdk19/src/hotspot/share/opto/mulnode.cpp

Lines 810 to 817 in 729164f

	// Transform is legal, but check for profit. Avoid breaking 'i2s'
	// and 'i2b' patterns which typically fold into 'StoreC/StoreB'.
	if( con < 16 ) {
	// Left input is an add of the same number?
	if (add1->in(1) == add1->in(2)) {
	// Convert "(x + x) << c0" into "x << (c0 + 1)"
	return new LShiftINode(add1->in(1), phase->intcon(con + 1));
	}

The fix I propose is to not apply this optimization for long left shifts if c0 == 63. I've added an additional sanity assertion for integer left shifts just in case this optimization is moved at some point and ending up outside the check for con < 16.

Thanks,
Christian

---

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Issue

• JDK-8288564: C2: LShiftLNode::Ideal produces wrong result after JDK-8278114

Reviewers

• Vladimir Kozlov (@vnkozlov - Reviewer)
• Igor Veresov (@veresov - Reviewer) ⚠️ Review applies to 37040f94
• Tobias Hartmann (@TobiHartmann - Reviewer)

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[bridgekeeper]

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[mlbridge]

Webrevs

• 01: Full - Incremental (45b1e994)
• 00: Full (37040f94)

[vnkozlov]

Good. Just one comment.

[vnkozlov]

The assert is useless because there is check at line 812 `(con < 16).

[chhagedorn]

Yes, that's true. I just wanted to make sure that we do not forget about the fact that we cannot apply this optimization for con == 31 if this optimization is moved at some point or we decide to remove the con < 16 restriction. But I guess I can also just add a comment instead.

[openjdk]

@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:

8288564: C2: LShiftLNode::Ideal produces wrong result after JDK-8278114

Reviewed-by: kvn, iveresov, thartmann

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At the time when this comment was updated there had been 7 new commits pushed to the master branch:

• ae030bc: 8288397: AArch64: Fix register issues in SVE backend match rules
• f12d044: 8288692: jdk/javadoc/doclet/testTagMisuse/TestTagMisuse.java fails after JDK-8288545
• 97544be: 8268398: 15% increase in JFR footprint in Noop-Base
• 983f75c: 8288545: Missing space in error message
• 53bf1bf: 8286176: Add JNI_VERSION_19 to jni.h and JNI spec
• c254c9d: 8287401: jpackage tests failing on Windows due to powershell issue
• ff3db52: 8288534: Out of bound errors for memory segment access mentions wrong values

Please see this link for an up-to-date comparison between the source branch of this pull request and the master branch.
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[vnkozlov]

Good

[chhagedorn]

Thanks Vladimir and Igor for your reviews!

[TobiHartmann]

Nice analysis. Looks good.

[chhagedorn]

Thanks Tobias for your review!

/integrate

[openjdk]

Going to push as commit ed714af.
Since your change was applied there have been 7 commits pushed to the master branch:

• ae030bc: 8288397: AArch64: Fix register issues in SVE backend match rules
• f12d044: 8288692: jdk/javadoc/doclet/testTagMisuse/TestTagMisuse.java fails after JDK-8288545
• 97544be: 8268398: 15% increase in JFR footprint in Noop-Base
• 983f75c: 8288545: Missing space in error message
• 53bf1bf: 8286176: Add JNI_VERSION_19 to jni.h and JNI spec
• c254c9d: 8287401: jpackage tests failing on Windows due to powershell issue
• ff3db52: 8288534: Out of bound errors for memory segment access mentions wrong values

Your commit was automatically rebased without conflicts.

[openjdk]

@chhagedorn Pushed as commit ed714af.

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