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math-emu issue with fp divide
- From: Kumar Gala <galak at kernel dot crashing dot org>
- To: David Miller <davem at davemloft dot net>
- Cc: lkml List <linux-kernel at vger dot kernel dot org>, libc-alpha at sourceware dot org
- Date: Wed, 4 Jun 2008 15:38:22 -0500
- Subject: math-emu issue with fp divide
David,
You seem to be the last person to touch the math-emu code in the
kernel so I figured you might have some insights. I'm trying to move
the powerpc math-emu code to use the include/math-emu bits.
In doing so I've been using TestFloat to see how good or bad we are
doing. For the most part the current math-emu code that PPC uses has
a number of issues that the code in include/math-emu seems to solve
(plus bugs we've had for ever that no one every realized).
Anyways, I've come across a case that we are flagging underflow and
inexact because we think we have a denormalized result from a double
precision divide:
000.FFFFFFFFFFFFF / 3FE.FFFFFFFFFFFFE
soft: 001.0000000000000 ..... syst: 001.0000000000000 ...ux
What it looks like is the results out of FP_DIV_D are:
D:
sign: 0
mantissa: 01000000 00000000
exp: -1023 (0)
The problem seems like we aren't normalizing the result and bumping
the exp. So I was wondering if you had any idea if FP_DIV_D should be
doing that or if when we call FP_PACK_DP/_FP_PACK_CANONICAL if that
could should be doing it.
thanks
- k