Constant exponents sometimes fail in f77

[donn at sdchema.UUCP]

Subject: Bug in copy propagation causes constant exponents to break in f77
Index:	usr.bin/f77/src/f77pass1/bb.c 4.2BSD

Description:
	An f77 program that uses certain constant integer exponents
	greater than 4 will not get the proper results if the optimizer
	is turned on.  In general the values produced are much larger
	than they should be.

Repeat-By:
	Compile the following program with the optimizer turned on
	and run it:

	----------------------------------------------------------------
		program powbug

		integer i

		i = 10
		i = i ** 5

		print *, i

		stop
		end
	----------------------------------------------------------------

	If your f77 is broken it will print '1000000' instead of
	'100000'.

Fix:
	F77 handles constant integer exponentiation with inline
	multiplies.  The way that inline multiplies are implemented
	requires two temporaries, one to hold the original value and
	one to hold a copy which gets repeatedly squared; if the
	exponent is not a power of two, then the first temporary is
	multiplied into the second temporary the necessary remaining
	number of times.  The copy propagation code notices that the
	first temporary is initialized from the second; unfortunately
	it misses the fact that the second copy is modified and reduces
	the two temporaries to one, eliminating the assignment from the
	one temporary to the other.  The reason it misses the
	modification of the second temporary is that the squarings are
	done with an OPSTAREQ operator instead of an ordinary
	OPASSIGN.  The solution is to make the copy propagation code
	know about the other assignment operators.  The changes are in
	routine ckexpr() in bb.c:

	----------------------------------------------------------------
	rcsdiff -c -r1.1 bb.c
	*** /tmp/,RCSt1015453	Mon Apr  2 12:47:44 1984
	--- bb.c	Fri Mar 30 05:02:44 1984
	***************
	*** 343,348
	  
	  {
	  Tempp	lp,rp;
	  
	  if (expr->exprblock.opcode == OPASSIGN)
		{

	--- 343,349 -----
	  
	  {
	  Tempp	lp,rp;
	+ int	oc = expr->exprblock.opcode;
	  
	  if (oc == OPASSIGN || oc == OPPLUSEQ || oc == OPSTAREQ)
		{
	***************
	*** 344,350
	  {
	  Tempp	lp,rp;
	  
	! if (expr->exprblock.opcode == OPASSIGN)
		{
		lp = (Tempp) expr->exprblock.leftp;
		rp = (Tempp) expr->exprblock.rightp;

	--- 345,351 -----
	  Tempp	lp,rp;
	  int	oc = expr->exprblock.opcode;
	  
	! if (oc == OPASSIGN || oc == OPPLUSEQ || oc == OPSTAREQ)
		{
		lp = (Tempp) expr->exprblock.leftp;
		rp = (Tempp) expr->exprblock.rightp;
	***************
	*** 349,355
		lp = (Tempp) expr->exprblock.leftp;
		rp = (Tempp) expr->exprblock.rightp;
		if (lp->tag == TTEMP)
	! 		if (rp->tag == TTEMP)
				enter (lp->memalloc, rp->memalloc);
			else
				enter (lp->memalloc, ENULL);

	--- 350,356 -----
		lp = (Tempp) expr->exprblock.leftp;
		rp = (Tempp) expr->exprblock.rightp;
		if (lp->tag == TTEMP)
	! 		if (rp->tag == TTEMP && oc == OPASSIGN)
				enter (lp->memalloc, rp->memalloc);
			else
				enter (lp->memalloc, ENULL);
	----------------------------------------------------------------

	Thanks should go to Peter Gross at the High Altitude
	Observatory for bringing this to my attention...

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