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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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