• David S. Miller's avatar
    [MATH-EMU]: Fix underflow exception reporting. · 40584961
    David S. Miller authored
    The underflow exception cases were wrong.
    
    This is one weird area of ieee1754 handling in that the underflow
    behavior changes based upon whether underflow is enabled in the trap
    enable mask of the FPU control register.  As a specific case the Sparc
    V9 manual gives us the following description:
    
    --------------------
    If UFM = 0:     Underflow occurs if a nonzero result is tiny and a
                    loss of accuracy occurs.  Tininess may be detected
                    before or after rounding.  Loss of accuracy may be
                    either a denormalization loss or an inexact result.
    
    If UFM = 1:     Underflow occurs if a nonzero result is tiny.
                    Tininess may be detected before or after rounding.
    --------------------
    
    What this amounts to in the packing case is if we go subnormal,
    we set underflow if any of the following are true:
    
    1) rounding sets inexact
    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
    3) underflow is set in FPU control register trap-enable mask
    
    The initially discovered example was "DBL_MIN / 16.0" which
    incorrectly generated an underflow.  It should not, unless underflow
    is set in the trap-enable mask of the FPU csr.
    
    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.
    
    However, if underflow is set in the trap enable mask, only underflow
    should be set and reported as a trap.  That is handled properly by the
    prioritization logic in
    
    arch/sparc{,64}/math-emu/math.c:record_exception().
    
    Based upon a report and test case from Jakub Jelinek.
    Signed-off-by: 's avatarDavid S. Miller <davem@davemloft.net>
    40584961
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