J3/10-238
To: J3
From: Malcolm Cohen
Subject: IEEE divide by zero
Date: 2010 October 12

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NUMBER:  F03/0030
TITLE:  IEEE divide by zero
KEYWORDS: IEEE-754, divide-by-zero
DEFECT TYPE: Erratum
STATUS: J3 consideration in progress

QUESTION:

Is infinity / 0.0 a divide by zero exception?
Is NaN / 0.0 a divide by zero exception?

Fortran 2003 defines (in 14.2) infinity / zero and NaN / zero
cases as IEEE_DIVIDE_BY_ZERO.  IEEE-754 defines (in 6.1 and 6.2)
those two as unexceptional.

ANSWER:

On an IEEE-conformant processor, these cases do not raise exceptions
(see clauses 6.1 and 6.2 of IEEE-754).

The definitions in 14.2 were intended to describe IEEE exceptions with
sufficient latitude to allow use on near-IEEE and non-IEEE machines.
However, the definition of IEEE_DIVIDE_BY_ZERO is not consistent with
the IEEE International Standard.

Furthermore, the definition of the IEEE_OVERFLOW flag is also not
consistent with the IEEE standard, because this exception is not
raised for operations on infinite operands.

Additionally, if the data type is not an IEEE data type, but the
exception is supported, the circumstances under which the exception is
raised are processor dependent.

Edits are provided.

EDITS:

Page and line numbers refer to 10-007.

[403:7-9] Clause 14.3, first paragraph, first bullet (IEEE_OVERFLOW),
  Replace with
  "IEEE_OVERFLOW occurs in an intrinsic real operation or conversion
   as specified by IEC 60559:1989 if IEEE_SUPPORT_DATATYPE is true for
   the result of the operation or conversion, and as determined by the
   processor if IEEE_SUPPORT_DATATYPE for the result is not true.  It
   occurs in an intrinsic complex operation or conversion if it caused
   by the calculation of the real or imaginary part of the result."

[403:10-11] Replace with
  "IEEE_DIVIDE_BY_ZERO occurs in a real division as specified by IEC
   60559:1989 if IEEE_SUPPORT_DATATYPE is true for the result of the
   division, and as determined by the processor if
   IEEE_SUPPORT_DATATYPE for the result is not true.  It occurs in a
   complex division if it is caused by the calculation of the real or
   imaginary part of the result."

[462:24+] Insert new bullet points
  "- the conditions under which IEEE_OVERFLOW is raised in a
     calculation involving non-IEC 60559:1989 floating-point data;
   - the conditions under which IEEE_DIVIDE_BY_ZERO is raised in a
     calculation involving non-IEC 60559:1989 floating-point data;"

SUBMITTED BY: Fred Tydeman

HISTORY: 05-109    m171  F03/0030 submitted
         05-109r1  m171  Revised to include IEEE_OVERFLOW,
                         Passed by J3 meeting
         05-170    m172  Passed J3 letter ballot #11
         N1622     m172  Failed WG5 ballot N1629
         10-nnn    m193  Revised answer.

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