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 <jos...@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 > jos...@codesourcery.com >
