On Wed, 29 Apr 2026 08:34:37 GMT, Anton Artemov <[email protected]> wrote:
> Hi, please consider the following changes: > > This is a fix for the `StrictMath.pow()` method, which corrects its numerical > behavior in certain cases. > > It was recently reported by [Gladman et > al](https://members.loria.fr/PZimmermann/papers/accuracy.pdf) that the > `pow()` method in the original FDLIBM returns with error of 636 ulps in > certain cases. `StrictMath.pow()` is a direct port of FDLIBM `pow()`. > > It turned out that `ivln2` constant split into high and low parts (`ivln2_h` > and `ivln2_l` respectively) in the original method is not sufficient to > compute `u = ivln2_h * t` exactly in certain cases, namely when the span of > non-zero bits in `t` is wide. > > By the default split, the high part `ivln2_h` contains 24 high bits of > `ivln2`, and by changing the split so that `ivln2_h` contains only 21 bits of > `ivln2` we ensure that `u = ivln2_h * t` is computed exactly in all cases > when pow(x,y) does no overflow. > > New testcases added. > > --------- > - [x] I confirm that I make this contribution in accordance with the [OpenJDK > Interim AI Policy](https://openjdk.org/legal/ai). src/java.base/share/classes/java/lang/FdLibm.java line 2210: > 2208: if (y_abs > 0x1.00000_ffff_ffffp31) { // if |y| > ~2**31 > 2209: final double INV_LN2 = 0x1.7154_7652_b82fep0; // > 1.44269504088896338700e+00 = 1/ln2 > 2210: final double INV_LN2_H = 0x1.715476p0; // > 1.44269502162933349609e+00 = 24 bits of 1/ln2 Please update the comment at the class-level of this file 31 * Port of the "Freely Distributable Math Library", version 5.3, from 32 * C to Java. to mention a fix to pow. Otherwise, I think the change looks good to go back. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/30984#discussion_r3262278845
