Hi!
On Fri, 5 Apr 2013 23:55:37 +0100, "Maciej W. Rozycki" <ma...@codesourcery.com>
wrote:
> On Fri, 5 Apr 2013, Thomas Schwinge wrote:
> > > Index: gcc/config/fp-bit.c
> > > ===================================================================
> > > RCS file: /cvs/uberbaum/gcc/config/fp-bit.c,v
> > > retrieving revision 1.39
> > > diff -u -p -r1.39 fp-bit.c
> > > --- gcc/config/fp-bit.c 26 Jan 2003 10:06:57 -0000 1.39
> > > +++ gcc/config/fp-bit.c 1 Apr 2003 21:35:00 -0000
> > > @@ -210,7 +210,11 @@ pack_d ( fp_number_type * src)
> > > exp = EXPMAX;
> > > if (src->class == CLASS_QNAN || 1)
> > > {
> > > +#ifdef QUIET_NAN_NEGATED
> > > + fraction |= QUIET_NAN - 1;
> > > +#else
> > > fraction |= QUIET_NAN;
> > > +#endif
> I think the intent of this code is to preserve a NaN's payload (it
> certainly does for non-QUIET_NAN_NEGATED targets)
I agree. For preserving the payload, both the unpack/pack code also has
to shift by NGARDS.
> Complementing the change above I think it will also make
> sense to clear the qNaN bit when extracting a payload from fraction in
> unpack_d as the class of a NaN being handled is stored separately.
I agree.
> Also I find the "|| 1" clause in the condition immediately above the
> pack_d piece concerned suspicious -- why is a qNaN returned for sNaN
> input? Likewise why are __thenan_sf, etc. encoded as sNaNs rather than
> qNaNs? Does anybody know?
I also stumbled over that, but for all these, I suppose the idea is that
when a sNaN is "arithmetically processed" (which includes datatype
conversion), an INVALID exception is to be raised (though, »[fp-bit]
implements IEEE 754 format arithmetic, but does not provide a mechanism
[...] for generating or handling exceptions«), and then converted into a
qNaN.
Also, I found that the bit to look at for distinguishing qNaN/sNaN is
defined wrongly for float. Giving me some "interesting" test results...
;-)
Manual testing looks good. Automated testing is still running; in case
nothing turns up, is this OK to check in?
libgcc/
* fp-bit.c (unpack_d, pack_d): Properly preserve and restore a
NaN's payload.
* fp-bit.h [FLOAT] (QUIET_NAN): Correct value.
Index: libgcc/fp-bit.c
===================================================================
--- libgcc/fp-bit.c (revision 402061)
+++ libgcc/fp-bit.c (working copy)
@@ -214,11 +214,18 @@ pack_d (const fp_number_type *src)
else if (isnan (src))
{
exp = EXPMAX;
+ /* Restore the NaN's payload. */
+ fraction >>= NGARDS;
+ fraction &= QUIET_NAN - 1;
if (src->class == CLASS_QNAN || 1)
{
#ifdef QUIET_NAN_NEGATED
- fraction |= QUIET_NAN - 1;
+ /* The quiet/signaling bit remains unset. */
+ /* Make sure the fraction has a non-zero value. */
+ if (fraction == 0)
+ fraction |= QUIET_NAN - 1;
#else
+ /* Set the quiet/signaling bit. */
fraction |= QUIET_NAN;
#endif
}
@@ -574,8 +581,10 @@ unpack_d (FLO_union_type * src, fp_number_type * d
{
dst->class = CLASS_SNAN;
}
- /* Keep the fraction part as the nan number */
- dst->fraction.ll = fraction;
+ /* Now that we know which kind of NaN we got, discard the
+ quiet/signaling bit, but do preserve the NaN payload. */
+ fraction &= ~QUIET_NAN;
+ dst->fraction.ll = fraction << NGARDS;
}
}
else
Index: libgcc/fp-bit.h
===================================================================
--- libgcc/fp-bit.h (revision 402061)
+++ libgcc/fp-bit.h (working copy)
@@ -190,7 +190,7 @@ typedef unsigned int UTItype __attribute__ ((mode
# define EXPBIAS 127
# define FRACBITS 23
# define EXPMAX (0xff)
-# define QUIET_NAN 0x100000L
+# define QUIET_NAN 0x400000L
# define FRAC_NBITS 32
# define FRACHIGH 0x80000000L
# define FRACHIGH2 0xc0000000L
@@ -298,7 +298,7 @@ typedef unsigned int UTItype __attribute__ ((mode
/* numeric parameters */
/* F_D_BITOFF is the number of bits offset between the MSB of the mantissa
of a float and of a double. Assumes there are only two float types.
- (double::FRAC_BITS+double::NGARDS-(float::FRAC_BITS-float::NGARDS))
+ (double::FRAC_BITS+double::NGARDS-(float::FRAC_BITS+float::NGARDS))
*/
#define F_D_BITOFF (52+8-(23+7))
Grüße,
Thomas
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