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Subject[patch 19/57] math-emu: Fix signalling of underflow and inexact while packing result.
2.6.27-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Kumar Gala <galak@kernel.crashing.org>
[ Upstream commit 930cc144a043ff95e56b6888fa51c618b33f89e7 ]

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
405849610fd96b4f34cd1875c4c033228fea6c0f):

[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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
include/math-emu/op-common.h | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
--- a/include/math-emu/op-common.h
+++ b/include/math-emu/op-common.h
@@ -139,18 +139,27 @@ do { \
if (X##_e <= _FP_WFRACBITS_##fs) \
{ \
_FP_FRAC_SRS_##wc(X, X##_e, _FP_WFRACBITS_##fs); \
- _FP_ROUND(wc, X); \
if (_FP_FRAC_HIGH_##fs(X) \
& (_FP_OVERFLOW_##fs >> 1)) \
{ \
X##_e = 1; \
_FP_FRAC_SET_##wc(X, _FP_ZEROFRAC_##wc); \
- FP_SET_EXCEPTION(FP_EX_INEXACT); \
} \
else \
{ \
- X##_e = 0; \
- _FP_FRAC_SRL_##wc(X, _FP_WORKBITS); \
+ _FP_ROUND(wc, X); \
+ if (_FP_FRAC_HIGH_##fs(X) \
+ & (_FP_OVERFLOW_##fs >> 1)) \
+ { \
+ X##_e = 1; \
+ _FP_FRAC_SET_##wc(X, _FP_ZEROFRAC_##wc); \
+ FP_SET_EXCEPTION(FP_EX_INEXACT); \
+ } \
+ else \
+ { \
+ X##_e = 0; \
+ _FP_FRAC_SRL_##wc(X, _FP_WORKBITS); \
+ } \
} \
if ((FP_CUR_EXCEPTIONS & FP_EX_INEXACT) || \
(FP_TRAPPING_EXCEPTIONS & FP_EX_UNDERFLOW)) \
--


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