f77 has problems when a DO loop variable is also a loop parameter

[donn at sdchema.UUCP]

Subject: f77 has problems when a DO loop variable is also a loop parameter
Index:	usr.bin/f77/src/f77pass1/exec.c 4.2BSD

Description:
	This bug only strikes when the optimizer is OFF.  If a DO
	loop variable is used for other purposes in a routine and in
	particular is used in the loop initialization, limit or
	increment at the same time it is used as the loop variable, the
	parameter containing the loop variable will appear to be
	garbage.  This bug was originally sent to me by Robert Elz
	of the University of Melbourne.

Repeat-By:
	Compile the following program with 'f77 -S' (examples are
	courtesy of Robert Elz):

	----------------------------------------------------------------
		k = 1
		i = 1
		do 10 k = k, 10
		i = i + k
	10	continue
		print i
		end
	----------------------------------------------------------------

	In the assembly language output we see that the register is
	initialized from itself, instead of the memory version of the
	loop variable:

	----------------------------------------------------------------
		movl	$1,{k}(r11)
		movl	$1,{i}(r11)
		movl	r10,r10				<<<<<<<<<<<
		cmpl	r10,$10
		jgtr	L16
	L17:
		movl	r10,{k}(r11)
		addl3	r10,{i}(r11),r0
		movl	r0,{i}(r11)
	L15:
		incl	r10
		cmpl	r10,$10
		jleq	L17
	L16:
		movl	r10,{k}(r11)
		clrl	-104(fp)
		movl	$6,-100(fp)
		movl	{i}(r11),-92(fp)
		addl3	$-104,fp,r0
		pushl	r0
		calls	$1,_s_wsfe
		calls	$0,_e_wsfe
	L14:
		ret
	----------------------------------------------------------------

Fix:
	The problem is that the compiler puts the DO loop variable in
	register before fixing the DO loop parameters.  The bug goes
	away if you just swap the two chunks of code in exdo() in
	f77pass1/exec.c:

	----------------------------------------------------------------
	*** /tmp/,RCSt1019970	Thu Jun  7 22:27:42 1984
	--- exec.c	Thu Jun  7 22:12:00 1984
	***************
	*** 379,385
	      }
	  
	    dovarp = mkplace(np);
	!   if( ! ONEOF(dovarp->vtype, MSKINT|MSKREAL) )
	      {
		err("bad type on DO variable");
		return;

	--- 379,387 -----
	      }
	  
	    dovarp = mkplace(np);
	!   dotype = dovarp->vtype;
	! 
	!   if( ! ONEOF(dotype, MSKINT|MSKREAL) )
	      {
		err("bad type on DO variable");
		return;
	***************
	*** 387,404
		return;
	      }
	  
	-   ctlstack->donamep = np;
	- 
	-   np->vdovar = YES;
	-   if( !optimflag && enregister(np) )
	-     {
	-       /* stgp points to a storage version, varp to a register version */
	-       dostgp = dovarp;
	-       dovarp = mkplace(np);
	-     }
	-   else
	-     dostgp = NULL;
	-   dotype = dovarp->vtype;
	- 
	- 
	    for(i=0 , cp = spec->nextp ; cp!=NULL && i<3 ; cp = cp->nextp)
	      {
		p = fixtype((expptr) cpexpr((tagptr) q = cp->datap));

	--- 388,393 -----
		return;
	      }
	  
	    for(i=0 , cp = spec->nextp ; cp!=NULL && i<3 ; cp = cp->nextp)
	      {
		p = fixtype((expptr) cpexpr((tagptr) q = cp->datap));
	***************
	*** 440,445
		return;
	      }
	  
	  
	    for (i = 0; i < 4; i++)
	      ctlstack->ctlabels[i] = newlabel();

	--- 429,445 -----
		return;
	      }
	  
	+   ctlstack->donamep = np;
	+ 
	+   np->vdovar = YES;
	+   if( !optimflag && enregister(np) )
	+     {
	+       /* stgp points to a storage version, varp to a register version */
	+       dostgp = dovarp;
	+       dovarp = mkplace(np);
	+     }
	+   else
	+     dostgp = NULL;
	  
	    for (i = 0; i < 4; i++)
	      ctlstack->ctlabels[i] = newlabel();
	----------------------------------------------------------------

And I bet you thought it was safe as long as the optimizer was off,

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
		    (Moving soon to utah-cs)