On Wed, May 31, 2023 at 12:28 PM Chris Sidebottom <chris.sidebot...@arm.com> wrote:
> Matthew Brett wrote: > > Hi, > > On Wed, May 31, 2023 at 8:40 AM Matti Picus matti.pi...@gmail.com wrote: > > > On 31/5/23 09:33, Jerome Kieffer wrote: > > > Hi Sebastian, > > > I had a quick look at the PR and it looks like you re-implemented the > sin-cos > > > function using SIMD. > > > I wonder how it compares with SLEEF (header only library, > > > CPU-architecture agnostic SIMD implementation of transcendental > > > functions with precision validation). SLEEF is close to the Intel SVML > > > library in spirit but extended to multi-architecture (tested on > PowerPC > > > and ARM for example). > > > This is just curiosity ... > > > Like Juan, I am afraid of this change since my code, which depends on > > > numpy for sin/cos used for rotation is likely to see large change of > > > behavior. > > > Cheers, > > > Jerome > > > I think we should revert the changes. They have proved to be > disruptive, > > > and I am not sure the improvement is worth the cost. > > > The reversion should add a test that cements the current user > expectations. > > > The path forward is a different discussion, but for the 1.25 release I > > > think we should revert. > > > Is there a way to make the changes opt-in for now, while we go back to > > see if we can improve the precision? > > This would be similar to the approach libmvec is taking ( > https://sourceware.org/glibc/wiki/libmvec), adding the > `--disable-mathvec` option, although they favour the 4ULP variants rather > than the higher accuracy ones by default. If someone can advise as to the > most appropriate place for such a toggle I can look into adding it, I would > prefer for the default to be 4ULP to match libc though. > We have a build-time toggle for SVML (`disable-svml` in `meson_options.txt` and an `NPY_DISABLE_SVML` environment variable for the distutils build). This one should look similar I think - and definitely not separate Python API with `np.fastmath` or similar. The flag can then default to the old (higher-precision, slower) behavior for <2.0, and the fast version for >=2.0 somewhere halfway through the 2.0 development cycle - assuming the tweak in precision that Sebastian suggests is possible will remove the worst accuracy impacts that have now been identified. The `libmvec` link above is not conclusive it seems to me Chris, given that the examples specify that one only gets the faster version with `-ffast-math`, hence it's off by default. Cheers, Ralf
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