Feasability of patching libm with OS X 10.5.8 libc for IBM double-double in PPC

[ardi <ardillasdelmonte@gmail.com>]

Hi!!

Disclaimer: I've read previous threads about long double support in
musl, as well as web pages and I understand that musl won't support
the IBM double-double, by design choice. I respect that decision (I've
just started to use musl and I can only say I admire the quality and
the design), and I'm not writing this as my opinion about that
decision.

Said this, I need IBM double-double in PowerPC targets (using clang as
compiler), and I'd like to use musl because of its well written code.

I'd also have the option of using the libc from OS X 10.5.8, which,
AFAIK, is the latest libc from Apple with full support for PPC and
PPC64 before they dropped them. This is libc 498.1.7 from the Apple
open source site.

However, building Darwin components was never easy (build instructions
are most of the times missing, ask the PureDarwin people), so I
*believe* I can get this libc working, but who knows...

As a (perhaps) better solution, I thought that maybe I don't need to
patch too many musl source files in order to get double-double
working. If I'm understanding the situation correctly, the
double-double implementation is actually generated by clang at compile
code. The libc library should only need to provide the interface
definitions (the definitions for mantissa bits, exponent bits, max
values, etc...), and perhaps the implementation of some functions
whose code wouldn't be generated by the compiler (not sure of this, as
clang generates code for a set of functions through builtins in
compiler_rt).

Maybe there would be some issues with converting floating point to
char string, but it would depend on what code you'd use for that
conversion.

So, as a personal patch (not pushing for official support, as I said
above), do you consider it would be feasible to patch the necessary
musl source files with contents from the OS X 10.5.8 libc?

Do you believe it would be a matter of a reasonably small set of
files, or rather an overwhelming task?

And... do you know of any confidence test I could run for checking
that it's working as expected?

Thanks a lot!