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From: Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: [patch][0/5] powerpc: Add support to fully comply with
	IEEE-754standard
Date: Fri, 12 Jan 2007 13:03:53 +0100
To: "Zhu Ebony-r57400" <ebony.zhu@freescale.com>
Cc: linuxppc-dev@ozlabs.org, Paul Mackerras <paulus@samba.org>
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> When floating-point underflow exceptions are disabled
> (SPEFSCR[FUNFE] is cleared) and the result of a floating-point 
> operation
> underflows, a signed zero result is produced.

You probably want to make at least this one tweakable
per-process; on some important algorithms (some FFTs
etc.) the results are perfectly acceptable with underflow-
to-zero and the performance difference is huge.  AltiVec
can do this per-process too, for example -- it has a
user register for setting this, you need some other
interface though.


Segher