Rainer Deyke wrote:
Daniel Keep wrote:
Rainer Deyke wrote:
This is, in my opinion, a significant design error in the language. The
difference between '!(a < b)' and 'a !< b' is not obvious. There is
nothing about the operator '<' that suggests that it should set a global
exception state, and there is nothing about '!<' that suggests that it
should /not/ set a global exception state. (Is global state for error
reporting ever a good idea in a high-level language?) It also adds
awkward expressions to the language, not just in the form '!(a < b)',
but in the form '!(a !< b)'.
I believe this is, or is the result of, an aspect of IEEE floating point.
I don't have a copy of the IEEE floating point standard, but I strongly
suspect it would have allowed syntax like:
a < b // sets global state
a !< b // sets global state
less_than_no_state(a, b) // does not set global state
!less_than_no_state(a, b) // does not set global state
Or:
a < b // does not set global state
a !< b // does not set global state
less_than_set_state(a, b) // sets global state
!less_than_set_state(a, b) // sets global state
Or:
a < b // sets global state
a !< b // sets global state
a [<] b // does not set global state
a [!<] b // does not set global state
Or any number of other syntax choices, all less confusing than the
syntax actually used.
This is assuming we need two sets of comparison operators, one of which
uses global state to report NaN operands and one which does not. I'm
not convinced that this is the case.
I don't have the final standard, but the last publically avilable draft
(IEE754R Draft 1.2.5, Oct 2006) states:
"The unordered-signaling predicates in Table 9, intended for use by
programs not written to take into account
the possibility of NaN operands, signal an invalid exception on quiet
NaN operands:" [table 9]
"The unordered-quiet predicates in Table 10, intended for use by
programs written to take into account the
possibility of NaN operands, do not signal an exception on quiet NaN
operands:" [table 10]
"There are two ways to write the logical negation of a predicate, one
using NOT explicitly and the other
reversing the relational operator. Thus in programs written without
considering the possibility of a NaN
operand, the logical negation of the unordered-signaling predicate (X <
Y) is just the unordered-signaling
predicate NOT(X < Y); the unordered-quiet reversed predicate (X ?>= Y)
is different in that it does not signal
an invalid operation exception when X and Y are unordered. In contrast,
the logical negation of (X = Y) may
be written either NOT(X = Y) or (X ?<> Y); in this case both expressions
are functionally equivalent to (X != Y).
The "global state" you're referring to is just a flag in the
floating-point status register.
Although I believe you could implement these comparisons as functions
rather than operators, they'd have to be intrinsics. On x87, the normal
comparisons are done with the FCOM instruction, and the others are done
with FCOMI. That's the only difference.
