8368349: Add library notes discussing abs and IEEE 754 operations

[jddarcy]

Add apiNote linking {Math, StrictMath}.abs to the IEEE 754 abs operation.

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• JDK-8368349: Add library notes discussing abs and IEEE 754 operations (Enhancement - P4)

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• Iris Clark (@irisclark - Reviewer)

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8368349: Add library notes discussing abs and IEEE 754 operations

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[rgiulietti]

@jddarcy Why can't we simply require a zero sign bit, like IEEE 754 does?
After all, IIUC this is exactly what the implementation does.

[jddarcy]

@jddarcy Why can't we simply require a zero sign bit, like IEEE 754 does? After all, IIUC this is exactly what the implementation does.

As an intrinsic candidate, the method could be replaced with a hardware instruction and earlier editions of IEEE 754 didn't require the NaN behavior so hardware might not implement that policy. I could check on what contemporary chips are specified to do.

[rgiulietti]

I checked 754-2008, which requires a 0 sign bit, and 754-1985 (reaffirmed in 1990), which in the annex on recommended functions reads

Copysign(x,y) returns x with the sign of y. Hence, abs(x) = copysign(x,1.0), even it x is NaN.

So it seems that hardware that does not behave this way is not compliant.
I'm curious what other platforms specify.

[jddarcy]

I checked 754-2008, which requires a 0 sign bit, and 754-1985 (reaffirmed in 1990), which in the annex on recommended functions reads

Copysign(x,y) returns x with the sign of y. Hence, abs(x) = copysign(x,1.0), even it x is NaN.

So it seems that hardware that does not behave this way is not compliant. I'm curious what other platforms specify.

Let me do some more research and get back.