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Kumar Gala 930cc144a0 math-emu: Fix signalling of underflow and inexact while packing result. 
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.

Now that I'm digging into this a bit I'm thinking my issue has to do with
the fix DaveM put in place from back in Aug 2007 (commit
405849610f):

[MATH-EMU]: Fix underflow exception reporting.

    2) we ended up rounding back up to normal (this is the case where
       we set the exponent to 1 and set the fraction to zero), this
       should set inexact too
...

    Another example, "0x0.0000000000001p-1022 / 16.0", should signal both
    inexact and underflow.  The cpu implementations and ieee1754
    literature is very clear about this.  This is case #2 above.

Here is the distilled glibc test case from Jakub Jelinek which prompted that
commit:

--------------------
#include <float.h>
#include <fenv.h>
#include <stdio.h>

volatile double d = DBL_MIN;
volatile double e = 0x0.0000000000001p-1022;
volatile double f = 16.0;
int
main (void)
{
  printf ("%x\n", fetestexcept (FE_UNDERFLOW));
  d /= f;
  printf ("%x\n", fetestexcept (FE_UNDERFLOW));
  e /= f;
  printf ("%x\n", fetestexcept (FE_UNDERFLOW));
  return 0;
}
--------------------

It looks like the case I have we are exact before rounding, but think it
looks like the rounding case since it appears as if "overflow is set".

000.FFFFFFFFFFFFF / 3FE.FFFFFFFFFFFFE = 001.0000000000000

I think the following adds the check for my case and still works for the
issue your commit was trying to resolve.

Signed-off-by: David S. Miller <davem@davemloft.net>
2008-10-22 21:53:53 -07:00
..
acpi include: replace __FUNCTION__ with __func__ 2008-10-16 11:21:30 -07:00
asm-arm
asm-cris container freezer: add TIF_FREEZE flag to all architectures 2008-10-20 08:52:33 -07:00
asm-frv ide: fix support for IDE PCI controllers using MMIO on frv 2008-10-17 18:09:14 +02:00
asm-generic fix WARN() for PPC 2008-10-20 15:29:57 -07:00
asm-h8300 h8300: update timer handler - new files 2008-10-16 11:21:29 -07:00
asm-m32r [PATCH] remove unused ibcs2/PER_SVR4 in SET_PERSONALITY 2008-10-16 15:40:05 +02:00
asm-m68k Merge git://git.kernel.org/pub/scm/linux/kernel/git/bart/ide-2.6 2008-10-20 13:12:39 -07:00
asm-mn10300 [PATCH] remove unused ibcs2/PER_SVR4 in SET_PERSONALITY 2008-10-16 15:40:05 +02:00
asm-um container freezer: add TIF_FREEZE flag to all architectures 2008-10-20 08:52:33 -07:00
asm-x86 Merge branch 'tracing-v28-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip 2008-10-20 13:35:07 -07:00
asm-xtensa container freezer: add TIF_FREEZE flag to all architectures 2008-10-20 08:52:33 -07:00
crypto
drm radeon: fix PCI bus mastering support enables. 2008-10-18 07:10:54 +10:00
keys
linux Merge branch 'linux-next' of git://git.kernel.org/pub/scm/linux/kernel/git/jbarnes/pci-2.6 2008-10-20 13:40:47 -07:00
math-emu math-emu: Fix signalling of underflow and inexact while packing result. 2008-10-22 21:53:53 -07:00
media V4L/DVB (9276): videobuf-dvb: two functions are now static 2008-10-17 17:39:14 -03:00
mtd
net Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/ericvh/v9fs 2008-10-20 09:39:47 -07:00
pcmcia
rdma
rxrpc
scsi [SCSI] iscsi_tcp: return a descriptive error value during connection errors 2008-10-13 09:29:00 -04:00
sound Merge branch 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab/linux-2.6 2008-10-13 14:03:59 -07:00
trace sched: clean up tracepoints 2008-10-14 10:33:14 +02:00
video video: add sh_mobile_lcdc platform flags 2008-10-20 11:38:50 +09:00
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