Xref: utzoo comp.lang.c:5848 comp.lang.fortran:322
Path: utzoo!mnetor!uunet!husc6!hao!ames!ptsfa!ihnp4!ttrdc!levy
From: levy@ttrdc.UUCP (Daniel R. Levy)
Newsgroups: comp.lang.c,comp.lang.fortran
Subject: Fortran copy-in/copy-out (was Re: some objections to 'noalias')
Message-ID: <2068@ttrdc.UUCP>
Date: 25 Dec 87 02:40:08 GMT
References: <485@cresswell.quintus.UUCP> <1453@cuuxb.ATT.COM> <9934@mimsy.UUCP>
Organization: AT&T, Skokie, IL
Lines: 60

In article <9934@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) writes:
>In article <1453@cuuxb.ATT.COM> mmengel@cuuxb.ATT.COM (Marc W. Mengel) writes:
>>Fortran uses copy-in copy-out address passing....

>Some FORTRAN compilers may indeed use copy-in/copy-out (I believe this
>is usually called `value-result'), but I think it is not mandated.

UNIX f77 uses pass-by-address for FORTRAN parameter passing, but still
implements a copy-in copy-out scheme where this would make a difference
(i.e., where an operation is not atomic, as for arithmetic on complex
numbers).  Therefore, a routine like this:

	subroutine multd(d1,d2,d3)
	double precision d1, d2, d3
	d3 = d1 * d2
	return
	end

produces object code like this:

multd_:
	fmuld3	*4(%ap),*0(%ap),*8(%ap)
	ret	&0

whereas

	subroutine multc(c1,c2,c3)
	complex c1, c2, c3
	c3 = c1 * c2
	return
	end

produces:

multc_:
	fmuls3	*4(%ap),*0(%ap),%r0	/* r0 = c1.real * c2.real */
	movw	0(%ap),%r1
	movw	4(%ap),%r2
	fmuls3	4(%r2),4(%r1),%r1	/* r1 = c1.imag * c2.imag */
	fsubs2	%r1,%r0			/* real part of result = r0 - r1 */
	movw	%r0,0(%fp)		/* save it in a temporary */
	movw	4(%ap),%r0
	fmuls3	*0(%ap),4(%r0),%r0	/* r0 = c1.real * c2.imag */
	movw	0(%ap),%r1
	fmuls3	*4(%ap),4(%r1),%r1	/* r1 = c1.imag * c2.real */
	fadds2	%r1,%r0			/* imag. part of result = r0 + r1 */
	movw	%r0,4(%fp)		/* save it in a temporary */
	movw	0(%fp),*8(%ap)		/* store real part of result */
	movw	8(%ap),%r0
	movw	4(%fp),4(%r0)		/* then imaginary part of result */
	ret	&0

So, you see, the FORTRAN compiler has covered its tail with respect to
aliasing of arguments.  (I believe this is the way the compiler is
"supposed" to behave.)
-- 
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