Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!aurora!labrea!agate!ucbvax!hoptoad!gnu
From: gnu@hoptoad.uucp (John Gilmore)
Newsgroups: comp.lang.c
Subject: Order of evaluation of nested function arguments
Message-ID: <3995@hoptoad.uucp>
Date: 2 Feb 88 13:02:47 GMT
Organization: Nebula Consultants in San Francisco
Lines: 51

Richard Stallman and I have a disagreement over whether the ANSI
standard specifies an order of evaluation for this statement
(taken from an implementation of Ackermann's function):

	return(A(--x,A(x,--y)));

In the oct 86 draft standard, it says that (3.3) "the evaluation of the
operands of an operator that involves a sequence point shall not be
interleaved with other evaluations".  The function call operator
clearly involves a sequence point (3.3.2.2) "The order of evaluation of
the function designator, the arguments, and subexpressions within the
arguments is unspecified, but there is a sequence point before the
actual call".

This means to me that A(x,--y) must be evaluated before the --x,
because the evaluation of A, x, and --y for the inner function call
cannot be interleaved with the evaluation of A, --x, and A(x,--y) for
the outer one.

This is probably an unintended consequence of the "no interleaving"
rule, which as I recall was written for cases involving ?: in getc().
For all I know, the wording has been modified in future draft standards
which are as yet unavailable to the public.

Richard claims the order of evaluation here is undefined.  Anybody
know?  Anybody care?  Here's the test program (gnu12.c), which gcc-1.17
"fails" if I am correct.

	John

/* Ackerman's function */
main()
{
	int i;

	i = A(3,6);
	if (i == 509)
		printf("Test passed\n");
	else
		printf("FAILED ackerman's(3, 6): %d\n", i);
}

A(x,y)
int x,y;
{
    if(x==0) return(++y);
    if(y==0) return(A(--x,1));
    return(A(--x,A(x,--y)));
    }
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