Squaring a function call calls the function twice in f77

[donn at sdchema.UUCP]

Subject: f(x) ** 2 produces f(x) * f(x) in f77
Index:	usr.bin/f77/src/f77pass1/optim.c 4.2BSD

Description:
	When an f77 program is compiled with the optimizer enabled,
	side effects of an expression are evaluated twice if that
	expression is squared.  This was originally pointed out to
	me by Jerry Berkman at UC Berkeley.

Repeat-By:
	Put the following f77 subroutine in a file fsq.f and compile
	it to assembly language with the optimizer turned on:

	----------------------------------------------------------------
		subroutine fsq( x )

		real f, x

		x = f( x ) ** 2

		return
		end
	----------------------------------------------------------------

	If the bug is present in your compiler you should see the
	following (although probably not so pretty):

	----------------------------------------------------------------
		.globl	_fsq_
		.set	LF1,4
	_fsq_:
		.word	LWM1
		subl2	$LF1,sp
		jmp	L12
	L15:
		pushl	4(ap)
		calls	$1,_f_
		movl	r0,-4(fp)
		pushl	4(ap)
		calls	$1,_f_
		mulf2	-4(fp),r0
		movl	r0,*4(ap)
		ret
		.align	1
	L12:
		jmp	L15
	----------------------------------------------------------------

Fix:
	In the code for buffpower() in optim.c, the comment mentions
	that the routine was ripped off from putpower() in putpcc.c;
	unfortunately the thieves did not also steal the bug fix when
	it was installed in putpower()...  Sigh.  Here is the fix:

	----------------------------------------------------------------
	*** /tmp/,RCSt1009277	Mon May 28 00:02:50 1984
	--- optim.c	Fri May  4 21:03:06 1984
	***************
	*** 632,638
		fatal("buffpower: bad call");
	  base = p->exprblock.leftp;
	  
	! if (k == 2)
	  {
		expptr prod;
		prod = mkexpr (OPSTAR,cpexpr(base),cpexpr(base));

	--- 660,666 -----
		fatal("buffpower: bad call");
	  base = p->exprblock.leftp;
	  
	! if (k == 2 && base->tag == TADDR && ISCONST(base->addrblock.memoffset))
	  {
		expptr prod;
		prod = mkexpr (OPSTAR,cpexpr(base),cpexpr(base));
	----------------------------------------------------------------

Another one bites the dust,

Donn Seeley    UCSD Chemistry Dept.       ucbvax!sdcsvax!sdchema!donn
32 52' 30"N 117 14' 25"W  (619) 452-4016  sdcsvax!sdchema!donn at nosc.ARPA