About GSOC.

[Tejas Joshi]

Hello.
I can't figure out from the documentation how to add test cases in the
testsuite and inspect the results. How can I do that? Although, Taking
the mentioned conditions under consideration, I have made another
patch, attached.

Thanks,
-Tejas


On Wed, 8 May 2019 at 09:01, Tejas Joshi <tejasjoshi9673@gmail.com> wrote:
>
> 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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: roundeven.patch
Type: text/x-patch
Size: 5121 bytes
Desc: not available
URL: <https://gcc.gnu.org/pipermail/gcc/attachments/20190508/315a47c6/attachment.bin>