https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93806
--- Comment #41 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- On Wed, 4 Mar 2020, rguenther at suse dot de wrote: > We're actually careful about the sign of zero here when recording > requivalences for propagation. I don't see anything preventing > equivalence based propagation for decimal floats though. > Also not sure if actual FP formats are required to be always > normalized ... All finite DFP values that can be represented with fewer decimal digits than the maximum for the format, and where the least significant nonzero decimal digit has an exponent greater than the smallest possible for the format, have representations with more than one possible quantum exponent, which are not equivalent even though they compare equal, and thus comparing equal should not result in an equivalence being recorded. E.g. _Decimal32 has 7-digit precision. 1.0DF and 1.00DF are different members of the same cohort, with quantum exponents -1 and -2, and compare equal but are not equivalent. 1.000001DF uses all 7 digits so is the unique member of its cohort, so it is valid to use 1.000001DF in place of anything that compared equal to 1.000001DF. 1E-101DF is the least positive subnormal value; as -101 is the lowest quantum exponent, anything with a nonzero digit in the 10^-101 place is the unique member of its cohort and so 1E-101DF can be used in place of anything that compared equal to 1E-101DF. This is a separate matter from noncanonical encodings of DFP values. A noncanonical encoding is a valid representation that *is* fully equivalent to some canonical encoding - but most operations are supposed to produce only canonically encoded results. If a cohort has only one member but there are noncanonical encodings of that member, a canonical encoding can replace a noncanonical one (in general it's not required to propagate noncanonical encodings, even when permitted), but not vice versa.
