To:		SC22

From:		Miles Ellis, Convenor: SC22/WG5

Date:		1995-07-24

Subject:Request for subdivision of project JTC1.22.02.01 to produce a

Technical Report (Type 2) on Floating-Point Exception Handling





WG5 requests SC22 to subdivide project JTC1.22.02.01 in order to permit work on

the following sub-project:





1.	Title



Portable Floating-Point Exception Handling in Fortran





2.	Scope



To define language facilities for detecting and treating, in a programmer-selected

way, conditions and exceptions that can occur in Fortran programs.





3.	Purpose and justification



Exception handling is required for the development of robust and efficient

numerical software.  In particular, it is necessary in order to be able to write

portable scientific libraries.  In numerical Fortran programming, current practice

is to employ whatever exception handling mechanisms are provided by the

system/vendor.  This clearly inhibits the production of fully portable numerical

libraries and programs.  



The Fortran language as currently defined is deficient and irregular in this area. 

There are some minimal exception handling facilities for I/O and allocation

exceptions, but they do not encourage well-structured programming.  In

particular, the lack of any feature for floating-point exceptions means that

existing floating-point arithmetic hardware cannot be exploited to its full

potential.  More generally, in the absence of portable exception handling, there is

no way to render numerical code robust without incurring a substantial runtime

penalty.



At its meeting in Tokyo, Japan, 1995-04-17/21, WG5 resolved to adopt a new

approach for accelerating standardization of certain key features which were

deemed to be too urgent to wait for the next revision of the Fortran Standard. 

This approach, which is based on the production of Type 2 Technical Reports that

would be incorporated unaltered in the next revision unless experience in their

implementation and use demanded changes, is described in SC22 N1862.



It is proposed, therefore, to produce a type 2 technical report to define the

necessary language extensions and to place them in the public domain.  It is

intended that these extensions will be included without change in a subsequent

revision of the Fortran standard, unless experience in implementing and using

them indicates that there are serious inadequacies in the definition.  It is hoped

that this will allow for a rapid technical development and for early testing well

before a subsequent language revision.  





4.	Programme of work



A Type 2 Technical Report will be developed for this project.





5.	Relevant documents to be considered



ISO/IEC 1539-1 : Draft CD 1995Programming Language Fortran Part 1 (SC22

N1842)



The following are the major relevant WG5 papers (minor papers responding to

these are not cited here):



N974 (1994-04)	Enable proposal (J K Reid)

N1031 (1994-08-01)	Exception Handling Proposal (DIN Fortran Working Group)

N1042 (1994-08-10)	Enable proposal (J K Reid)

N1095 (1995-03)	Floating Point Subset of ENABLE (J L Wagener)





6.	Cooperation and liaison



IFIP WG 2.5

ANSI/X3J3 _ US Fortran Technical Committee





7.	Preparatory work offered with target date(s)



This topic has been considered in considerable detail by WG5 already and a

largely complete proposal already exists.  This will form the basis of the proposed

Technical Report.



It is expected that a document will be ready for balloting as a PDTR by April 1996.



John Reid (UK) is the proposed editor for this technical report.



There will be no requirement for a maintenance agency or registration authority

for this project.



There are no known requirements for coding.



The proposed technical report does not concern any known patented items.