4.2 BSD f77 compiler bug (2)

[donn at sdchema.UUCP]

Subject: Computed GOTOs can cause f77 to dump core
Index:	/usr/src/usr.bin/f77/src/f77pass1/regalloc.c 4.2BSD

Description:
	This problem occurs in the f77 compiler supplied on a tape
	made on 8/23/83.

	While compiling a program with 'f77 -O ...' that contains
	a computed GOTO, f77 stops with a 'Termination code 11' or
	'Termination code 132' and a core dump of f77pass1 is left
	behind.  This only occurs with a certain very large program
	which I have that is generated with a ratfor-like pre-
	processor and contains zillions of gotos and DO loops.

Repeat-By:
	The program that causes this is much too large to send by
	mail or news, but it is available on request.  I have tried
	replicating the problem with smaller programs, to no avail.
	The program contains the line:

		GOTO(16,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15),COMBO

Fix:
	Fortunately the error is fairly obvious.  Running dbx on
	f77pass1 shows that it dies with the PC in the routine alreg()
	of regalloc.c.  At that point it is in code that handles
	computed GOTOs; it is running through a list of all the labels
	used by the computed GOTO statement.  If it finds that a label
	is outside of the current DO loop, it flags it and breaks the
	loop.  If it runs through the list and finds no such branches,
	it should just fall out.  Unfortunately the test for the last
	label at the top of the for loop has an off-by-one error and
	skips off the end into never-never land.  This test IS handled
	correctly in very similar code earlier in the same routine, so
	it is clear what the correct code should be (in
	src/f77pass1/regalloc.c):

	***************
	*** 835,841
	  
		    case SKCMGOTO:
		      lp = (struct Labelblock **) sp->ctlinfo;
	! 	      for (i = 0; i <= sp->label; i++, lp++)
			if (!locallabel((*lp)->labelno))
			  {
			    gensetall(sp);

	--- 861,867 -----
	  
		    case SKCMGOTO:
		      lp = (struct Labelblock **) sp->ctlinfo;
	! 	      for (i = 0; i < sp->label; i++, lp++)
			if (!locallabel((*lp)->labelno))
			  {
			    gensetall(sp);

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