4bsd-f77@utah-cs.UUCP (08/25/84)
From: Donn Seeley <donn@utah-cs.arpa>
Subject: F77 sometimes won't optimize subscripts
Index: usr.bin/f77/src/f77pass1 4.2BSD
Description:
Expressions in subscripts are not subject to expansion prior to
optimization in f77, and as a result some optimizations are
missed.
Repeat-By:
Clip out the following program and compile it down to assembly
language with the optimizer on:
----------------------------------------------------------------
program memo
integer i, j, m(8)
i = 2
j = i ** 3
m(i ** 3) = j + 1
print *, m(8)
stop
end
----------------------------------------------------------------
You should see something like the following (this has been
prettied up for readability):
----------------------------------------------------------------
movl $2,{i}
mull3 {i},{i},r0
mull3 {i},r0,{j}
movl {i},-4(fp)
movl -4(fp),-8(fp)
addl3 $1,{j},r0
mull3 -4(fp),-4(fp),r1
mull2 -8(fp),r1
movl r0,&{m}-4[r1]
----------------------------------------------------------------
Results will vary depending on whether you installed the fix
for exponentiations and CSE which I posted earlier.
Fix:
The problem is that the routine expand() in optim.c is used to
convert complicated operations like min/max or exponentiations
into simpler forms that can be optimized, but it fails to
convert subscripts. The following change fixes this problem:
----------------------------------------------------------------
*** /tmp/,RCSt1002935 Wed Aug 1 16:07:11 1984
--- optim.c Wed Aug 1 16:06:45 1984
***************
*** 535,540
case TTEMP:
if (p->tempblock.istemp)
frtemp(p);
break;
default:
--- 538,547 -----
case TTEMP:
if (p->tempblock.istemp)
frtemp(p);
+ break;
+
+ case TADDR:
+ p->addrblock.memoffset = expand( p->addrblock.memoffset );
break;
default:
----------------------------------------------------------------
This change by itself is not enough. Since expansions can
create 'comma' operators, and the common subexpression
elimination code requires that expressions not contain comma
operators, rmcommaop() in optcse.c must be changed so that
comma operators in subscripts get removed too:
----------------------------------------------------------------
*** /tmp/,RCSt1002953 Wed Aug 1 16:11:29 1984
--- optcse.c Wed Aug 1 16:05:30 1984
***************
*** 859,864
case TLIST:
for (cp = p->listblock.listp; cp; cp = cp->nextp)
cp->datap = (tagptr) rmcommaop (cp->datap,sl);
return (p);
default:
--- 862,871 -----
case TLIST:
for (cp = p->listblock.listp; cp; cp = cp->nextp)
cp->datap = (tagptr) rmcommaop (cp->datap,sl);
+ return (p);
+
+ case TADDR:
+ p->addrblock.memoffset = rmcommaop (p->addrblock.memoffset,sl);
return (p);
default:
----------------------------------------------------------------
Donn Seeley University of Utah CS Dept donn@utah-cs.arpa
40 46' 6"N 111 50' 34"W (801) 581-5668 decvax!utah-cs!donn