[PPL-devel] Fwd: Re: kern/133583: [libm] fma(3) does not respect rounding mode using extended precision

[Abramo Bagnara]

Too much worrisome to be true?!!?


-------- Messaggio originale --------
Oggetto: Re: kern/133583: [libm] fma(3) does not respect rounding mode
using extended precision
Data: Fri, 3 Dec 2010 07:02:04 GMT
Mittente: das at FreeBSD.org
A: abramo.bagnara at gmail.com, das at FreeBSD.org, freebsd-bugs at FreeBSD.org

Synopsis: [libm] fma(3) does not respect rounding mode using extended
precision

State-Changed-From-To: open->closed
State-Changed-By: das
State-Changed-When: Fri Dec 3 07:01:28 UTC 2010
State-Changed-Why:
Thanks for the report! This limitation is described in the source for
fma(), and unfortunately, it is unlikely to ever change. There are
several reasons:

- We are a long way from having the necessary compiler support to make
  dynamic precision changes work as expected.
- Dynamic FPU precision changes aren't officially supported, and
  fpsetprec() has been documented as deprecated for many years.
- The only supported architecture that can have this problem due to
  dynamic precision changes is i386, and even then only for non-SSE2
  builds.
- The cost and complexity associated with making every function in
  libm detect and adapt to dynamic precision changes is prohibitive.

I have updated the manpage for fpsetprec() to explain that changing
the FPU precision isn't supported by the compiler or libraries.

http://www.freebsd.org/cgi/query-pr.cgi?pr=133583