Hi all,

We have to do something about long double support. This is something I wanted to propose a long time ago already, and moving build systems has resurfaced the pain yet again.

This is not a full proposal yet, but the start of a discussion and gradual plan of attack.

The problem
The main problem is that long double support is *extremely* painful to maintain, probably far more than justified. I could write a very long story about that, but instead I'll just illustrate with some of the key points:

(1) `long double` is the main reason why we're having such a hard time with building wheels on Windows, for SciPy in particular. This is because MSVC makes long double 64-bit, and Mingw-w64 defaults to 80-bit. So we have to deal with Mingw-w64 toolchains, proposed compiler patches, etc. This alone has been a massive time sink. A couple of threads:
The first issue linked above is one of the key ones, with a lot of detail about the fundamental problems with `long double`. The Scientific Python thread focused more on Fortran, however that Fortran + Windows problem is at least partly the fault of `long double`. And Fortran may be rejuvenated with LFortran and fortran-lang.org; `long double` is a dead end.

(2) `long double` is not a well-defined format. We support 9 specific binary representations, and have a ton of code floating around to check for those, and manually fiddle with individual bits in long double numbers. Part of that is the immediate pain point for me right now: in the configure stage of the build we consume object files produced by the compiler and parse them, matching some known bit patterns. This check is so weird that it's the only one that I cannot implement in Meson (short of porting the hundreds of lines of Python code for it to C), see https://github.com/mesonbuild/meson/issues/11068 for details. To get an idea of the complexity here:
Typically `long double` has multiple branches, and requires more code than float/double.

(3) We spend a lot of time dealing with issues and PR to keep `long double` working, as well as dealing with hard-to-diagnose build issues. Which sometimes even stops people from building/contributing, especially on Windows. Some recent examples:

(4) `long double` isn't all that useful. On both Windows and macOS `long double` is 64-bit, which means it is just a poor alias to `double`. So it does literally nothing for the majority of our users, except confuse them and take up extra memory. On Linux, `long double` is 80-bit precision, which means it doesn't do all that much there either, just a modest bump in precision.

Let me also note that it's not just the user-visible dtypes that we have to consider; long double types are also baked into the libnpymath static library that we ship with NumPy. That's a thing we have to do something about anyway (shipping static libraries is not the best idea, see https://github.com/numpy/numpy/issues/20880). We just have to make sure to not forget about it when thinking about solutions here.

Potential solutions

(A) The ideal solution would be to have a proper, portable quad-precision (128 bits of precision) dtype. It's now possible to write that externally, after all the work that Sebastian and others have put into the dtype infrastructure. The dtype itself already exists (https://github.com/peytondmurray/quaddtype, maybe there are more implementations floating around). It just need the people who have an actual need for it to drive that. It's still a significant amount of work, so I'll not go into this one more right now.

(B) Full deprecation and removal of all `long double` support from NumPy (and SciPy), irrespective of whether the quad dtype comes to life.

Given the current state, I'm personally convinced that that is easily justified. However, I know some folks are going to be hesitant, given that we don't know how many remaining users we have or what use cases they have. So let's see if we can find more granular solutions (note: these are ideas, not all fully researched solutions that we can pick from and are guaranteed to work out well).

(C) Only support `long double` where it has proper compiler support (C99/C++11), so using it "just works". And remove all the support for old formats and accessing bit representations directly. This also implies making some optional functions mandatory. For example, the issue I ran into today showed up at runtime in a fallback path for systems that don't have `strtold_l`. We don't test such fallback paths in CI, so they're going to be fragile anyway.

(D) Add a build mode with a command-line flag, where we typedef `long double` to `double`. I'll note that we already touched on that once (see https://github.com/numpy/numpy/issues/20348); I'm not sure though if it's fundamentally not a good idea or that we just didn't do it intentionally enough.

Next steps

First, it would be great to hear from folks who have use cases for long double support in NumPy. So far we have very little knowledge of that, we only see the problems and work it causes us.

Second, I'm going to have to add support for (C) or (D) temporarily to the Meson build anyway, as we run into things. It can be worked around if we really have to by implementing support for long double format detection in Meson, or by rewriting all the detection logic so it's all-in-one in C. But that takes a significant amount of time.

Third, let's figure out which way we'd like to go. Do you see alternative solutions? Or like any of the ones I listed more than others? Are you willing to jump in and work on a quad dtype?