Unless an IEEE 754 is NaN, +-0.0 or +-Infinity, is dividing by itself guaranteed to result in exactly 1.0?
Similarly, is subtracting itself guaranteed to always result in +-0.0?
IEEE 754-2008 4.3 says:
… Except where stated otherwise, every operation shall be performed as if it first produced an intermediate result correct to infinite precision and with unbounded range, and then rounded that result according to one of the attributes in this clause.
When an intermediate result is representable, all of the rounding attributes round it to itself; rounding changes a value only when it is not representable. Rules for subtraction and division are given in 5.4, and they do not state exceptions for the above.
Zero and one are representable per 3.3, which specify the sets of numbers representable in any format conforming to the standard. In particular, zero may be represented by a significand with all zero digits, and one may be represented with a significand starting with 1 and followed by “.000…000” and an exponent of zero. The minimum and maximums exponents of a format are defined so that they always include zero (emin is 1−emax, so zero is between them, inclusive, unless emax is less than 1, in which case no numbers are representable, i.e. the format is empty and not an actual format).
Since rounding does not change representable values and zero and one are representable, dividing a finite non-zero value by itself always produces one, and subtracting a finite value from itself always produces zero.