Re: Complex division, compiler-rt and logarithm

[Matthias Drochner <M.Drochner%fz-juelich.de@localhost>]

Joerg Sonnenberger <joerg%britannica.bec.de@localhost> wrote:
>(1) Use the better algorithmic approach and provide a version of the
>functions in question in libc.
>(2) Use the simpler approach of libgcc.

I'd suggest to implement (2) for now. While it is always preferrable
to have a more accurate implementation, the matter is somewhat complex:
-scalbn() is used internally by libm, thus needs the rename/weak_alias
 protection. Would have to span the 2 libraries then, like the
 pthread stub stuff.
-The implementation needs special handling at least for non-IEEE FP,
 long double and perhaps for some FPUs -- it needs research whether
 eg the i387 version or one using bit operations performs better
 on amd64. (Such research is easier done if it only affects libm.)
-The Sun-based implementation in libm has its own support macros
 and error handling support. (I could live without the non-POSIX
 error handling modes, but I wouldn't remove an existing feature
 without good reasons.)

Christos Zoulas <christos%astron.com@localhost> wrote:
> We already have a stealth scalbn in libc for compat_ldexp...

Yes, and in my tests it has been as correct as the Sun-libm
implementation. Performance could make a difference.

The bottom line is that dealing with these problems would
distract you massively from getting compiler_rt in shape.
And since the easy solution wouldn't be a regression, I'd
choose that.

best regards
Matthias



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