To: J3 J3/21-122r2
From: Robert Corbett
Subject: Integration with ISO/IEC 60559:2020
Date: 2021-March-01
References: 21-007
1. Introduction
A new edition of the ISO/IEC floating-point standard,
ISO/IEC 60559:2020, was published in May of 2020. The
Fortran standard includes language support for the
ISO/IEC floating-point standard. Therefore, the Fortran
standard should track changes to the ISO/IEC floating-point
standard.
The new edition of the ISO/IEC floating-point standard is,
for the most part, backward compatible with the previous
edition. The previous edition of the floating-point
standard placed restrictions on exception handling in the
case of comparison operations that could not implemented
efficiently on existing hardware. Those restrictions are
relaxed in the new edition. The change should not be
noticeable by users or implementors, because no one paid
attention to the restrictions. Members of the IEEE 754 WG
themselves expressed surprise when the restrictions were
pointed out to them.
The maximum and minimum operations have changed in the new
edition. In the previous standard, the maximum and minimum
operations applied to operands whose values are zeroes with
different signs could return either positive or negative zero.
In the new edition, the maximum and minimum operations
treat negative zero as if it is less than positive zero.
The change is unlikely to affect user codes, but it might
affect implementations. Some CPUs include instructions for
the maximum and minimum operations of the previous edition.
Those instructions could still be used for the MAX and MIN
functions of the Fortran standard, but they cannot be used to
implement the maximum and minimum operations of the new edition
of the floating-point standard.
The names of the functions IEEE_MAX_NUM_MAG and IEEE_MIN_NUM_MAG
were based on the names of the corresponding operations in the
previous edition of the floating-point standard. The names are
not intuitive. In the new edition of the floating-point
standard, the names of the operations were changed to be more
intuitive. I would like the names of the functions in the
Fortran standard to match the new names of the operations, but I
do not want to introduce a incompatibility.
The floating-point standard includes two versions of the
maximum and minimum operations. In the plain versions, NaNs are
treated as error values. In the number versions, NaNs are
treated as null values that are to be ignored. In the previous
edition of the floating-point standard, the plain versions of
the maximum and minimum operations did not propagate NaNs.
That made the plain versions unsuitable for some of their
intended uses. The new edition of the floating-point standard
requires the plain versions of the maximum and minimum
operations to propagate NaNs. The new edition also requires
the number versions to favor quiet NaNs over signaling NaNs
when both operands are NaNs.
The Fortran 2018 standard does not include functions
corresponding to the plain versions of the maximum and minimum
operations max, maxMag, min, and minMag specified in
ISO/IEC/IEEE 60559:2011. It does include functions
corresponding the number versions maxNum, maxNumMag, minNum,
and minNumMag.
The descriptions of maximum and minimum functions in the
Fortran 2018 standard do not go into the details of producing
NaN results described in section 6.2 of ISO/IEC/IEEE 60559:2011.
The edits proposed in this paper hono(u)r this tradition by
not going into the details described in section 6.2 of
ISO/IEC 60559:2020.
The new edition of the floating-point standard includes a new
rounding mode roundTiesTowardZero. The new rounding mode is
optional and is associated with the new augmented arithmetic
operations. The new rounding mode and the new operations
cannot be efficiently emulated in software. No hardware yet
available supports the new rounding mode or the new operations.
Therefore, they are not addressed in this paper.
The new edition of the floating-point standard includes other
new features. While some of the new features might be useful
for Fortran applications, they are not addressed in this paper.
2. Edits
[Global] Replace all occurrences of "ISO/IEC/IEEE 60559:2011"
with "ISO/IEC 60559:2020".
[Introduction, p2+, [xiii]] At the end of paragraph 2, insert
"Changes to the intrinsic module IEEE_ARITHMETIC for
conformance with ISO/IEC 60559:2020:
The new functions IEEE_MAX, IEEE_MAX_MAG, IEEE_MIN, and
IEEE_MIN_MAG corresponding to the operations maximum,
maximumMagnitude, minimum, and miminumMagnitude in
ISO/IEC 60559:2020 have been added. The semantics of the
functions IEEE_MAX_NUM, IEEE_MAX_NUM_MAG, IEEE_MIN_NUM,
and IEEE_MIN_NUM_MAG have revised to conform to the
operations maximumNumber, maximumMagnitudeNumber,
minimumNumber and minimumMagnitudeNumber in
ISO/IEC 60559:2020; the changes affect the treatment of
zeroes and NaNs."
[4.3.3 Fortran 2018 compatibility, p2, [27:24]] Replace
"feature might have a different interpretation" with
"features might have different interpretations".
[4.3.3 Fortran 2018 compatibility, p2, [27:28+]] Add the item
" * The value of a reference of IEEE_MAX_NUM, IEEE_MAX_NUM_MAG,
IEEE_MIN_NUM, or IEEE_MIN_NUM_MAG where one argument is a
number and the other is a signaling NaN is specified to be
a number in this document. Fortran 2018 specified that the
value is a NaN."
to the end of the list.
[17.10 Summary of the procedures, Table 17.2, [458]] Before
IEEE_MAX_NUM insert
"IEEE_MAX(X, Y) E Maximum value.
IEEE_MAX_MAG(X, Y) E Maximum magnitude value."
[17.10 Summary of the procedures, Table 17.2, [458]] Before
IEEE_MIN_NUM insert
"IEEE_MIN(X, Y) E Minimum value.
IEEE_MIN_MAG(X, Y) E Minimum magnitude value."
[17.11.16+, IEEE_LOGB(X), [465:27+]] After the description
of IEEE_LOGB, insert
"17.11.16a IEEE_MAX(X, Y)
Description. Maximum value.
Class. Elemental function.
Arguments.
X shall be of type real.
Y shall be of the same type and kind type parameter as X.
Restriction. IEEE_MAX shall not be invoked if
IEEE_SUPPORT_DATATYPE(X) has the value false.
Result Characteristics. Same as X.
Result value. The result has the value specified for the
maximum operation in ISO/IEC 60559:2020; that is
* if X < Y the result has the value of Y;
* if Y < X the result has the value of X;
* if either operand is a NaN, the result is a quiet Nan;
* if X = Y and the signs are the same, the result is the
value of either X or Y.
For this function, negative zero compares less than positive
zero. If one or both of X and Y are signaling NaNs,
IEEE_INVALID signals; otherwise, no exception is signaled.
Example. The value of
IEEE_MAX(1.5, IEEE_VALUE(1.0, QUIET_NAN)) is a quiet NaN.
17.11.16b IEEE_MAX_MAG(X, Y)
Description. Maximum magnitude value.
Class. Elemental function.
Arguments.
X shall be of type real.
Y shall be of the same type and kind type parameter as X.
Restriction. IEEE_MAX_MAG shall not be invoked if
IEEE_SUPPORT_DATATYPE(X) has the value false.
Result Characteristics. Same as X.
Result value. The result has the value specified for the
maximumMagnitude operation in ISO/IEC 60559:2020; that is
* if |X| < |Y| the result has the value of Y;
* if |Y| < |X| the result has the value of X;
* otherwise, the result has the value of IEEE_MAX(X, Y).
If one or both of X and Y are signaling NaNs, IEEE_INVALID
signals; otherwise, no exception is signaled.
Example. The value of IEEE_MAX_MAG(1.5, -2.5) is -2.5."
[17.11.17 IEEE_MAX_NUM(X, Y), p6, [466:1]] Replace
"the maxNum operation in ISO/IEC/IEEE 60559:2011" with
"the maximumNumber operation in ISO/IEC 60559:2020"
[17.11.17 IEEE_MAX_NUM(X, Y), p6 [466:5]] Replace
"is a quiet NaN" with "is a NaN"
[17.11.17 IEEE_MAX_NUM(X, Y), p6 [466:6]] Replace
"if one or both ... is a NaN;" with
"if both X and Y are NaNs the result is a quiet NaN;"
[17.11.17 IEEE_MAX_NUM(X, Y), p6 [466:7]] replace
"otherwise, ... dependent)." with
"if X = Y and the signs are the same, the result is either
X or Y."
[17.11.17 IEEE_MAX_NUM(X, Y), p6 [466:7+]] After the list,
insert
"For this function, negative zero compares less than
positive zero."
[17.11.17 IEEE_MAX_NUM(X, Y), p7 [466:8]] Replace
"Except when ... is signaled." with
"If one or both of X and Y are signaling NaNs, IEEE_INVALID
signals, but unless X and Y are both signaling NaNs, the
signaling NaN is otherwise ignored and not converted to a
quiet NaN. No other exceptions are signaled."
Make paragraph 7 a part of paragraph 6.
[17.11.18 IEEE_MAX_NUM_MAG(X, Y), p6, [466:19]] Replace
"the maxNumMag operation in ISO/IEC/IEEE 60559:2011" with
"the maximumMagnitudeNumber operation in ISO/IEC 60559:2020"
[17.11.18 IEEE_MAX_NUM_MAG(X, Y), p6, [466:21]] Replace
"ABS(X) < ABS(Y)" with "|X| < |Y|".
[17.11.18 IEEE_MAX_NUM_MAG(X, Y), p6, [466:22]] Replace
"ABS(Y) < ABS(X)" with "|Y| < |X|".
[17.11.18 IEEE_MAX_NUM_MAG(X, Y), p7, [466:24]] Replace
the sentence "Except when ... signaled." with
"If one or both of X and Y are signaling NaNs, IEEE_INVALID
signals, but unless X and Y are both signaling NaNs, the
signaling NaN is otherwise ignored and not converted to a
quiet NaN. No other exceptions are signaled."
Make paragraph 7 a part of paragraph 6.
[17.11.18+, IEEE_MAX_NUM_MAG(X, Y), p8+, [466:25+]] After the
description of IEEE_MAX_NUM_MAG, insert
"17.11.18a IEEE_MIN(X, Y)
Description. Minimum value.
Class. Elemental function.
Arguments.
X shall be of type real.
Y shall be of the same type and kind type parameter as X.
Restriction. IEEE_MIN shall not be invoked if
IEEE_SUPPORT_DATATYPE(X) has the value false.
Result Characteristics. Same as X.
Result value. The result has the value specified for the
minimum operation in ISO/IEC 60559:2020; that is
* if X < Y the result has the value of X;
* if Y < X the result has the value of Y;
* if either operand is a NaN, the result is a quiet NaN;
* if X = Y and the signs are the same, the result is the
value of either X or Y.
For this function, negative zero compares less than positive
zero. If one or both of X and Y are signaling NaNs,
IEEE_INVALID signals; otherwise, no exception is signaled.
Example. The value of
IEEE_MIN(1.5, IEEE_VALUE(1.0, QUIET_NAN)) is a quiet NaN.
17.11.18b IEEE_MIN_MAG(X, Y)
Description. Minimum magnitude value.
Class. Elemental function.
Arguments.
X shall be of type real.
Y shall be of the same type and kind type parameter as X.
Restriction. IEEE_MIN_MAG shall not be invoked if
IEEE_SUPPORT_DATATYPE(X) has the value false.
Result Characteristics. Same as X.
Result value. The result has the value specified for the
minimumMagnitude operation in ISO/IEC 60559:2020; that is
* if |X| < |Y| the result has the value of X;
* if |Y| < |X| the result has the value of Y;
* otherwise, the result has the value of IEEE_MIN(X, Y).
If one or both of X and Y are signaling NaNs, IEEE_INVALID
signals; otherwise, no exception is signaled.
Example. The value of IEEE_MIN_MAG(1.5, -2.5) is 1.5."
[17.11.19 IEEE_MIN_NUM(X, Y), p6, [466:32]] Replace
"the minNum operation in ISO/IEC/IEEE 60559:2011" with
"the minimumNumber operation in ISO/IEC 60559:2020"
[17.11.19 IEEE_MIN_NUM(X, Y), p6 [467:1]] Replace
"is a quiet NaN" with "is a NaN"
[17.11.19 IEEE_MIN_NUM(X, Y), p6 [467:2]] Replace
"if one or both ... is a NaN;" with
"if both X and Y are NaNs the result is a quiet NaN;"
[17.11.19 IEEE_MIN_NUM(X, Y), p6 [467:3]] replace
"otherwise, ... dependent)." with
"if X = Y and the signs are the same, the result is either
X or Y."
[17.11.19 IEEE_MIN_NUM(X, Y), p6 [467:3+]] After the list,
insert
"For this function, negative zero compares less than
positive zero."
[17.11.19 IEEE_MIN_NUM(X, Y), p7 [467:7]] Replace
"Except when ... is signaled." with
"If one or both of X and Y are signaling NaNs, IEEE_INVALID
signals, but unless X and Y are both signaling NaNs, the
signaling NaN is otherwise ignored and not converted to a
quiet NaN. No other exceptions are signaled."
Make paragraph 7 a part of paragraph 6.
[17.11.20 IEEE_MIN_NUM_MAG(X, Y), p6, [467:15]] Replace
"the minNumMag operation in ISO/IEC/IEEE 60559:2011" with
"the minimumMagnitudeNumber operation in ISO/IEC 60559:2020"
[17.11.20 IEEE_MIN_NUM_MAG(X, Y), p6, [467:17]] Replace
"ABS(X) < ABS(Y)" with "|X| < |Y|".
[17.11.20 IEEE_MIN_NUM_MAG(X, Y), p6, [467:18]] Replace
"ABS(Y) < ABS(X)" with "|Y| < |X|".
[17.11.20 IEEE_MIN_NUM_MAG(X, Y), p7, [467:20]] Replace
the sentence "Except when ... signaled." with
"If one or both of X and Y are signaling NaNs, IEEE_INVALID
signals, but unless X and Y are both signaling NaNs, the
signaling NaN is otherwise ignored and not converted to a
quiet NaN. No other exceptions are signaled."
Make paragraph 7 a part of paragraph 6.