Newsgroups: comp.std.c
Path: cs.tu-berlin.de!fu-berlin.de!zib-berlin.de!news.mathworks.com!uunet!in1.uu.net!allegra!alice!dmr
From: dmr@research.att.com (Dennis Ritchie <7549-15328> 0112710)
Subject: Re: Restricted Pointers? (Re: New proposal: Optimizer hints)
Message-ID: <D7DADv.5MG@research.att.com>
Organization: AT&T Bell Labs, Murray Hill, NJ
Date: Fri, 21 Apr 1995 04:13:06 GMT
Lines: 86

For the sake of history, I record the portion dealing with the
semantics of noalias in X3J11's draft C standard as of Jan 1988.
It's been manually de-troffed; apologies for errors.

I had problems convincing myself that the notion of handle (see below)
was well defined, and in understanding the consequences of the
described semantics.  As my diatribe tells, though, the ultimate
problem was that the model these semantics propose is wrong:
"noaliasness" is not a useful property of objects, but
is better understood in terms of promises about what pointers
can point to.

	Dennis Ritchie

Here's the snippet from the draft:
--------
Semantics

The properties associated with qualified types are
meaningful only for expressions that are lvalues.

Each lvalue that is a unary expression with the unary * indirection
operator (either explicitly or as a result of the array subscript []
operator) contains one or more "handles".  These handles have pointer
or array type and are the smallest expressions contained within the
lvalue (ignoring the left operand of the comma operator, the operand
of the sizeof operator, all argument expressions, and all expressions
with integral type), and are associated with the entire object to
which the lvalue refers, in part or as a whole.  The handles for
lvalues with noalias-qualified type are "noalias handles" .  A handle
is distinguished by the spelling of its operands (ignoring the
spelling of any contained argument expressions) and by the storage
duration of its "handle objects", those objects whose values are
accessed to evaluate the handle (excluding any objects accessed to
evaluate any contained argument expressions).

The "actual object" of a noalias handle is the entire object of which
a part is designated by the same lvalue except that its type is the
non-noalias-qualified version.  For each distinct noalias handle, it
is unspecified whether the handle is associated with the actual object
or is associated with a "virtual object" , a distinct object with the
same size and address as the actual object.  The behavior of a program
that depends upon a specific implementation choice is undefined.

A virtual object, if and when created, acquires the last-stored value
of the actual object.  A virtual object may be created at any sequence
point within a function for which there exists a sequence point at
which the storage for all handle objects is guaranteed to be reserved.
A virtual object has a "pending value" if it has been modified through
use of its noalias handle and the actual object does not have the same
value.  All virtual objects with pending values for a noalias handle
have their values assigned to their respective actual objects at the
return of a function after whose execution storage is no longer
guaranteed to be reserved for a handle object of the noalias handle.

If an argument expression is a noalias handle, or is a noalias handle
plus or minus an expression with integral type (either explicitly or
as a result of combinations of the unary & address operator and the
array subscript [] operator), just after the return from the function,
the value stored in the actual object is assigned to the currently
associated virtual object, if any exists.\*F\
[footnote]
If the called function has a type that includes a prototype and the
type of the parameter is a pointer to a const- and noalias-qualified
type, this assignment can be suppressed, as the called function cannot
modify the designated object.
[end footnote]
Furthermore, if, at the sequence point just before the call to the
function, the noalias handle is currently associated with a virtual
object with a pending value, its value is assigned to the actual
object at that sequence point.

[Dicussion of volatile qualifier deleted--dmr]

The following are examples of some lvalues and their handles:

	int a[2], b[3][4], i;
	int *p, **q, *f();

	a[1];        /* same as *(a + 1), handle: a */
	b[i][2];     /* same as *(b[i] + 2), handle: b */
	*p;          /* handle: p */
	*(p + i);    /* handle: p */
	*(i = 4, p); /* handle: p */
	*f();        /* handle: f() */
	**(q + i);   /* handle: *(q + i) */