Re: About GSOC.

[Tejas Joshi <tejasjoshi9673@gmail.com>]

I should have taken all the test cases into consideration. Fool of me. I
will try to make changes taking all the test cases into consideration along
with the testsuite.
Thanks.

On Wed, 8 May 2019 at 02:31, Joseph Myers <joseph@codesourcery.com> wrote:

> On Wed, 8 May 2019, Tejas Joshi wrote:
>
> > Hello.
> > As per my understanding, 3.5 would be represented in GCC as follows :
> > r->uexp  = 2
> > and
> > r->sig[2] = 1110000....00 in binary 64 bit. (first 2 bits being 3 and
> > following 1000....0 being 0.5, which later only happens for halfway
> cases)
> > So, if we clear out the significand part and the immediate bit to the
> right
> > which represent 0.5, the entire significand would become 0 due to
> bit-wise
> > ANDing.
> >
> > > +  tempsig[w] &= (((unsigned long)1 << ((n % HOST_BITS_PER_LONG) - 1))
> -
> > > 1);
> > >
> >
> > That is what the following line intend to do. The clearing part would
> > change the significand, that's why significand was copied to a temporary
>
> That much is fine.  My issues are two other things:
>
> * The function would wrongly return true for 3, not just for 3.5, because
> it never checks the bit representing 0.5.  (If you don't care what it
> returns for 3, see my previous point about every function needing a
> comment defining its semantics.  Without such a comment, I have to guess,
> and my guess here is that the function should return true for 3.5 but
> false for 3 and for 3.5000...0001.)
>
> * The function would wrongly return true for 3.5000...0001, if there are
> enough 0s that all those low bits in sig[2] are 0, but some low bits in
> sig[1] or sig[0] are not 0.
>
> And also:
>
> * You should never need to modify parts of (a copy of) the significand in
> place.  Compare low parts of the significand (masked as needed) with 0.
> If not 0, just return false.  Likewise for comparing the 0.5 bit with 1.
> It's not that copying and modifying in place results in incorrect logic,
> it's simply excessively convoluted compared to things like:
>
>   if ((something & mask) != 0)
>     return false
>
> (the function is probably twice as long as necessary because of that
> copying).
>
> > array for checking. This logic is inspired by the clear_significand_below
> > function. Or isn't this the way it was meant to be implemented? Also, why
> > unsigned long sig[SIGSZ] has to be an array?
>
> What would it be other than an array?  It can't be a single scalar because
> floating-point significands may be longer than any supported integer type
> on the host (remember the IEEE binary128 case).  And if you made it a
> sequence of individually named fields, a load of loops would need to be
> manually unrolled, which would be much more error prone and hard to read.
>
> --
> Joseph S. Myers
> joseph@codesourcery.com
>