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
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 <[e-mail]>

include/math-emu/op-common.h

···
 	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))	\