NetNews Usenet Archive 1992 #26/NN_1992_26.iso/spool/comp/lang/fortran/4272

Text File | 1992-11-11 | 2.7 KB | 77 lines

Newsgroups: comp.lang.fortran
Path: sparky!uunet!noc.near.net!meiko.com!jim
From: [e-mail] (James Cownie)
Subject: IEEE arithmetic [was Re: unformatted output]
Message-ID: <[e-mail]>
Followup-To: [e-mail]
Sender: jim@canopus (James Cownie)
Organization: meiko
References: <[e-mail]> <[e-mail]> <[e-mail]> <[e-mail]> <[e-mail]> <[e-mail]>
Date: Wed, 11 Nov 1992 10:26:11 GMT
Lines: 64

In article comp.lang.fortran <[e-mail]>,
[e-mail] (J. Giles) writes:

|> Unfortunately, the Fortran standard does not conform to
|> IEEE (in fact, it conflicts with it in some places - like with regard
|> to negative zero).  And the Fortran standard does not require the same
|> conversion accuracy as the IEEE floating point standard.

This is a less significant difference than some of the others, in particular
Fortran definition of NINT and ANINT directly conflicts with the IEEE definitions
of the same functions.

The Fortran definition could be written as
    INTEGER FUNCTION NINT(X)

    IF (X > 0.0) THEN
       NINT = INT(X+0.5)
        ELSE
           NINT = INT(X-0.5)
        ENDIF
        END

IEEE (in round nearest mode, which is the default) specifies round even,
so
    IEEENINT(1.5) == 2.0  (as Fortran NINT)
BUT     IEEENINT(2.5) == 2.0  (Fortran gives 3.0)

Another area where IEEE seems never to be implemented correctly by compilers
is in the handling of Not a Numbers (NaNs).

In the IEEE standard NaNs are values which can be encoded into a floating point
variable, which represent the concept that it is not a reasonable number.
(They are generated for operations such as sqrt(-ve), 0.0/0.0 etc.)
IEEE specifies that a NaN is unordered with respect to anything else,
EVEN ITSELF. Therefore ALL comparisons involving floating variables MUST be
generated by the compiler WITHOUT introducing logical negation, since in the face
of NaNs (and their unordered relationship with other numbers)

    (.NOT. (X .LT. 2.0)) does NOT imply (X .GE 2.0)

(If X is a NaN (X .LT. 2.0) is FALSE (it's not less it's unordered),
 but (X .GE. 2.0) is also FALSE (it's not greater equal it's unordered)).

Similarly (and I've never seen this handled right in an optimising compilation),

    IF (X .ne. X) THEN
            print *,'X is a NaN'
        ELSE
            print *,'X is a number'
        ENDIF

should generate code which has a run time test.

-- Jim
James Cownie             Normal disclaimers apply.
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