c2

[utzoo!duke!vax135!ucbvax!mark]

>From uucp Wed Sep 26 13:16:38 1979
>From tom Tue Sep 25 22:01:30 1979 remote from ucsfcgl
To: Bob Kridle, Ed Gould, Bill Joy
Re: Bug in version 7 optimizer of dmr's pdp-11 compiler

A bug involving unsigned integers has surfaced in the version 7 C compiler.
After installing the following fix you should recompile:
	mkfs.c
	dcheck.c
	passwd.c
	getty.c
	mail.c
	pstat.c

>From uucp Mon Jul 23 22:53:59 1979
>From dmr Mon Jul 23 23:59:07 1979 remote from research
Here is the change needed to fix the optimizer.
The file is /usr/src/cmd/c/c21.c.
This is in routine rmove, after case CBR, the first two
lines in the 'if r==0' clause:

190,191c
< 				p->back->back->forw = p->forw;
< 				p->forw->back = p->back->back;
---
> 				if (p->forw->op==CBR
> 				  || p->forw->op==SXT
> 				  || p->forw->op==CFCC) {
> 					p->back->forw = p->forw;
> 					p->forw->back = p->back;
> 				} else {
> 					p->back->back->forw = p->forw;
> 					p->forw->back = p->back->back;
> 				}

The old code deleted a test or compare with constant operands
and a following conditional branch that would always fail.
The new code only deletes the branch (leaves the test)
if the combination is followed by another instruction that
needs the condition codes.  The test and second branch are liable
to be deleted later.