To: J3                                                     J3/18-151
From: Robert Corbett
Subject: A VIRTUOUS or SUPER_PURE attribute for functions
Date: 2018-February-15
References: 18-122r1

Introduction
------------

A procedure with the PURE attribute is allowed to read the
values of data entities that are not accessed through dummy
arguments and are not local to the procedure.  The feature
requested in 18-122r1 is for a stronger attribute that
includes all of the restrictions on pure procedures and also
prohibits access to data entities that might have varying
values and that are external to the procedure.

The rationale for adding the new attribute in 18-122r1 is to
allow a user to create functions that the processor can move
without regard to context.  This capability might allow
processors to perform optimizations that would not be allowed
for pure procedures.

Note that floating-point operations might depend on global
rounding modes and trap mode settings.  Performing a
floating-point operation might set an exeception flags.
Therefore, when a processor invokes a super-pure function
that performs floating-point oprations, it must reset the
floating-point rounding mode and the trap modes on entry and
must restore the exception flags on exit.

The name of the new attribute is not yet set in concrete.