8267332: xor value should handle bounded values

[neliasso]

In the discussion of #3938 a limitation in C2 was found. C2 fails to eliminate obvious bound checks for indexes that are masked with xor.

The Xor for two values that have a lower bound of zero or more, the resulting lower bound is zero.
The Xor for two values that have a upperbound above zero, the resulting upper bound is the max of the next_power_of_2-1.

Test supplied.

Please review,
Best regards,
Nils Eliasson

---

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Issue

• JDK-8267332: xor value should handle bounded values

Reviewers

• Tobias Hartmann (@TobiHartmann - Reviewer) ⚠️ Review applies to 9be3b00
• Claes Redestad (@cl4es - Reviewer) ⚠️ Review applies to 9be3b00

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[cl4es]

I think you could use <= std::numeric_limits<T>::max() as the upper bound condition, and use (jint)(next_power_of_2((uint)t1i->_hi) - 1) to produce the mask in an overflow-conscious way.

[neliasso]

max_power_of_2() is less than std::numeric_limits::max().

But I noticed I got the bounds wrong - is should be strictly less:
t1i->_hi < max_power_of_2()

[cl4es]

Right, that's why I widened to an uint (since next_power_of_2((uint)jint::max()) is well-defined) then cast back to a jint after subtracting 1.

[cl4es]

Another way of expressing the same in an overflow-conscious way (without type conversion) is round_down_power_of_2(t1i->_hi) + (round_down_power_of_2(t1i->_hi) - 1)

[neliasso]

I agree - that is a much better solution! Thanks Claes!

[TobiHartmann]

Very nice. I've added some minor comments to the test.

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8267332: xor value should handle bounded values

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[TobiHartmann]

Looks good.

[cl4es]

Looks good!

[neliasso]

Thanks for the reviews Tobias and Claes!

[neliasso]

/integrate

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@neliasso Pushed as commit 4ba7613.

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