J3/04-127

Date:    13 Dec. 2003
To:      J3
From:    Dan Nagle
Subject: VOLATILE Functions


For some time now, there has been an ongoing debate
over the exact circumstances where a programmer is guaranteed
that a function in an expression must be executed.  There
are, however, some advantages to the current ambiguity,
to wit: the freedom given optimizers to generate faster code.
Some papers have been written suggesting new language
to remove said ambiguities.  This paper proposes another approach,
specifically, the VOLATILE attribute for functions.  This was
considered for Fortran 2003 at the last minute, but was viewed
as too extensive to be incorporated at that time.  Interoperability
with C, where function side-effects are common, and the language
definition gives the explicit requirement that all function references
must result in function execution, only exacerbates the problem.

Number:

Title: VOLATILE Functions

Submitted By: J3

Status: For Consideration

References: J3/03-163

Basic Functionality: A VOLATILE function must be executed,
                     whether the expression containing the reference
                     requires the function value for computation
                     of the value of the expression, or not.

Rationale:  The current language is ambiguous, yet the ambiguity
            serves a purpose well.  Following the example set
            by SAVE and (the future) AUTOMATIC attributes
            for variables, and in parallel with the meaning
            of VOLATILE for variables, the proposal is to create
            the VOLATILE attribute for functions.  That is, defer
            the issue to the programmer, who may indicate that
            a function is PURE or VOLATILE.  Functions with neither
            attribute may be treated by compilers
            in a processor dependent way.

Estimated Impact: Compilers will have to recognize the keyword
                  in a new situation, and support a new attribute
                  for functions.  The handling of expressions is affected.
                  For this price, programmers are given full control
                  of whether a function reference may be elided.

Detailed Specification: J3/03-163 contains the specification.
                        I will not attempt to improve upon it,
                        as this paper requests reconsideration of 03-163
                        in the next standard.

History: