Allow lib/msun/logarithm_test to pass on ld128 platforms

[arichardson]

Summary

The log1pl() implementation is less accurate than logl(), but does at least
guarantee precision >= the ld80 implementation. Increase the tolerance
to the ld80 equivalent for now.

Depends on D28798

PR: 253984

Test Plan

test passes now

[arichardson]

ping?

[ngie]

Not my domain of expertise. Maybe someone like @dim or Steve Kargl would have better input 🤷...

[dim]

It's not very likely Steve Kargl can be convinced to create a Phabricator account. :)

That said, while I'm fine with the approach of "fix the test so it works", does anybody have an idea as to why lib/msun/ld128/s_logl.c has worse accuracy than the 80 bit version? AFAIK the whole point of log1p() is better accuracy...

[arichardson]

In D29039#668715, @dim wrote:

It's not very likely Steve Kargl can be convinced to create a Phabricator account. :)

That said, while I'm fine with the approach of "fix the test so it works", does anybody have an idea as to why lib/msun/ld128/s_logl.c has worse accuracy than the 80 bit version? AFAIK the whole point of log1p() is better accuracy...

I'm afraid I also don't understand the implementation enough to figure out why this is the case.

This should really be an XFAIL, which I'll do for log1p_accuracy_tests. However, for accuracy_tests that would mean losing coverage for the other function so I'll keep this approach.