JDK 9 RFR of JDK-4851642: Add fused mac to Java math library
dmitry.nadezhin at gmail.com
Fri Apr 15 02:43:49 UTC 2016
It looks good except a guard expression in the line Math:1550
(tmp == 0.0 && a == 0.0 || b == 0.0)
Variables a and b occur asymmetrically in this expression.
A symmetrical expression would be either
(a == 0.0 || b == 0.0)
(product.signum() == 0)
On Fri, Apr 15, 2016 at 3:14 AM, joe darcy <joe.darcy at oracle.com> wrote:
> In response to the review comments from Dmitry and Brian, I've prepared
> another iteration of the webrev:
> Summary of the changes:
> * Renamed the method to "fma" to follow the precedent of the C library.
> * Added API note citing IEEE 754 operation.
> * More test cases
> * More comments
> * Restructured handling of finite double value to be more straightforward.
> On 4/12/2016 5:21 PM, joe darcy wrote:
>> Please review the changes for
>> JDK-4851642: Add fused mac to Java math library
>> Fused mac (multiply-accumulate) is a ternary floating-point operation
>> which accepts three inputs, a, b, c, and computes
>> a * b + c
>> with a single rounding error rather than the usual two rounding errors (a
>> first for the multiply, a second one for the add). The fused mac operation
>> was added in the 2008 update to the IEEE 754 floating-point standard and
>> hardware support for the operation is becoming more and more common in
>> different processor families.
>> When present as a hardware instruction, a fused mac can speed up loops
>> such as those for polynomial evaluation. A fused mac can also be used to
>> support a correctly rounding floating-point divide and support various
>> higher-precision operations such as "doubled-double" arithmetic.
>> With the increasing availability of fused mac as a hardware primitive,
>> the time has come to add fused mac to the Java math library. Fused mac is
>> an ideal candidate to be intrinsified where hardware support is available.
>> However, this initial implementation does not attempt to add any such
>> intrinsics support in HotSpot; a follow-up RFE has been filed for that work
>> (JDK-8154122). The current library implementation favors code simplicity
>> over speed; a more performant implementation could be written by directly
>> decomposing the floating-point inputs rather than turning to BigDecimal and
>> may be written in the future. More extensive tests could be added in the
>> future as well.
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