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Subject: Fix for 4.3 BSD C compiler schain botch bug
Index: lib/mip/pftn.c 4.3BSD

Description:
	Very infrequently, the distributed 4.3 BSD C compiler will
	die unexpectedly, printing an error message like this:

		"foo.c", line 65: compiler error: schain botch

Repeat-By:
	It's very difficult to repeat this error.  All the examples
	that are known to induce the bug involve huge functions with
	many gotos; the very first example seen was a direct
	translation from a Fortran utility.  The nature of the error
	makes it likely that the problem was due to a goto from an
	inner block to a subsequent label in an outer block, but the
	bug is hash dependent and won't appear consistently even under
	these conditions.  If the spelling of local variable names in
	the function is changed, or new local variables added, the
	problem often disappears.

Fix:
	The following fix was worked out between John P. Linderman
	and me, and we think it fixes the bug properly.  Make the
	following changes to the clearst() routine in lib/mip/pftn.c
	(your line numbers may vary):

	----------------------------------------------------------------
	*** /tmp/,RCSt1002168	Mon Nov 23 17:55:18 1987
	--- pftn.c	Mon Sep 21 02:23:55 1987
	***************
	*** 1921,1927 ****
		/* step 2: fix any mishashed entries */
		p = clist;
		while( p ){
	! 		register struct symtab *r, *next;
	  
			q = p;
			next = p->snext;
	--- 1925,1931 ----
		/* step 2: fix any mishashed entries */
		p = clist;
		while( p ){
	! 		register struct symtab *next, **t, *r;
	  
			q = p;
			next = p->snext;
	***************
	*** 1929,1934 ****
	--- 1933,1946 ----
				if( ++q >= &stab[SYMTSZ] )q = stab;
				if( q == p || q->stype == TNULL )break;
				if( (r = relook(q)) != q ) {
	+ 				/* move q in schain list */
	+ 				t = &schain[q->slevel];
	+ 				while( *t && *t != q )
	+ 					t = &(*t)->snext;
	+ 				if( *t )
	+ 					*t = r;
	+ 				else
	+ 					cerror("schain botch 2");
					*r = *q;
					q->stype = TNULL;
					}
	----------------------------------------------------------------

	Donn Seeley    University of Utah CS Dept    donn@cs.utah.edu
	40 46' 6"N 111 50' 34"W    (801) 581-5668    utah-cs!donn