chris@trantor.umd.edu (Chris Torek) (04/18/88)
Index: lib/c2/c21.c 4.3BSD Fix
Description:
When rearranging a branch to branch past a test whose result
is already known, the optimiser forgets to update the label
number in the branch node. If the test itself is later
deleted, the label disappears entirely. Hence given input like
movl r9,r8
L2:
tstl r8
movl r7,r8
jbr L2
it removes the label L2, replacing it with the internal
label L2000001, but never generates L2000001s, and winds
up producing
movl r9,r8
movl r7,r8
jbr L2
instead of the correct infinite loop
movl r9,r8
L2000000:movl r7,r8
jbr L2000000
Repeat-by:
See above.
Fix:
Someone (jfr?) simply forgot one statement: `p->labno = p1->labno'.
Every other place that alters p->ref also carefully fixes p->labno.
While I was verifying this, I also added comments, and reformatted
a few lines.
RCS file: RCS/c21.c,v
retrieving revision 1.2
diff -c2 -r1.2 c21.c
*** /tmp/,RCSt1007542 Mon Apr 18 01:39:55 1988
--- c21.c Mon Apr 18 01:39:08 1988
***************
*** 1296,1299 ****
--- 1296,1300 ----
*/
+ /* find branches to comparisons whose result is already known */
redunbr(p)
register struct node *p;
***************
*** 1306,1310 ****
return;
p1 = nonlab(p1);
! if (p1->op==TST) {
splitrand(p1);
savereg(RT2, "$0", p1->subop);
--- 1307,1311 ----
return;
p1 = nonlab(p1);
! if (p1->op==TST) { /* test == compare $0 */
splitrand(p1);
savereg(RT2, "$0", p1->subop);
***************
*** 1314,1317 ****
--- 1315,1319 ----
return;
if (p1->forw->op==CBR) {
+ /* branching to compare+branch ... can we short circuit? */
ap1 = findcon(RT1, p1->subop);
ap2 = findcon(RT2, p1->subop);
***************
*** 1318,1321 ****
--- 1320,1324 ----
p1 = p1->forw;
if (compare(p1->subop, ap1, ap2)) {
+ /* p1 always branches, so go to p1's dst, not p's */
nredunj++;
nchange++;
***************
*** 1331,1337 ****
}
} else if (p1->op==TST && equstr(regs[RT1],ccloc+1) &&
! equtype(ccloc[0],p1->subop)) {
! p1=insertl(p1->forw); decref(p->ref); p->ref=p1;
! nrtst++; nchange++;
}
}
--- 1334,1345 ----
}
} else if (p1->op==TST && equstr(regs[RT1],ccloc+1) &&
! equtype(ccloc[0],p1->subop)) {
! /* branch past the test, since ccodes already set properly */
! p1 = insertl(p1->forw);
! decref(p->ref);
! p->ref = p1;
! p->labno = p1->labno;
! nrtst++;
! nchange++;
}
}
***************
*** 1483,1485 ****
if (*--p == '+' && *--p == ')') return(1);
return(0);
! }
--- 1491,1493 ----
if (*--p == '+' && *--p == ')') return(1);
return(0);
! }
--
In-Real-Life: Chris Torek, Univ of MD Computer Science, +1 301 454 7163
Domain: chris@mimsy.umd.edu Path: ...!uunet!mimsy!chris