[geos-devel] Remove ttmath?

[Regina Obe]

+1 to ripping out in 3.9 and eventually backporting to 3.8 after some testing.

> -----Original Message-----
> From: geos-devel [mailto:geos-devel-bounces at lists.osgeo.org] On Behalf
> Of Paul Ramsey
> Sent: Wednesday, April 15, 2020 5:17 PM
> To: GEOS Development List <geos-devel at lists.osgeo.org>
> Subject: [geos-devel] Remove ttmath?
> 
> So, we ran into some issues with GEOS 3.8 on a Power architecture, and
> based on what tests failed, I suspect the issue is with our use of ttmath.
> 
> In particular, this comment says to me (and just eyeballing what's going on)
> that there is some in-baked assumptions about endianness.
> 
> https://github.com/libgeos/geos/blob/master/include/geos/algorithm/ttma
> th/ttmathbig.h#L2607-L2609
> 
> The only reason we're using ttmath is because Mateusz happened to choose
> it for his initial experiments on supporting the higher precision code in JTS.
> Turns out there's no reason we cannot use exactly the same routines as JTS
> does, though, and they have NO ENDIAN assumptions in them, so they'll be
> portable.
> 
> The port of the JTS code is here:
> 
> https://github.com/libgeos/geos/pull/303
> 
> Amazingly, all the regression tests still pass.
> 
> Also, just based on running the full test suite, the DD code seems 5-10%
> faster than ttmath. This isn't entirely surprising, since ttmath is an arbitrary
> precision system, while doubledouble uses nothing but standard double math
> and the assumption that all double representations and operations are to
> IEEE spec.
> 
> I'm going to port the JTS unit tests so we have a little more foundation under
> this work, but then I'd like to rip out ttmath in 3.9.
> 
> I'm a little worried that we cannot support big-endian machines in 3.8, so I
> think we might consider back-porting this work to 3.8 and switch over to it
> for big-endian architectures.
> 
> Paul
> 
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