donn@sdchema.UUCP (03/27/84)
Subject: Ghastly f77 bug in common subexpression elimination -- IMPORTANT
Index: usr.bin/f77/src/f77pass1/optcse.c 4.2BSD
Description:
F77 considers two variables from the same COMMON block to be
the same variable for the purposes of common subexpression
elimination. This is almost always wrong.
Repeat-By:
Copy the following program into a file com.f. Compile it with
the optimizer turned on.
----------------------------------------------------------------
program com
common /x/ a, b, c, d
integer result, a, b, c, d
a = 2
b = 3
c = 4
d = 5
result = a * b + c * d
print *, result
stop
end
----------------------------------------------------------------
The expected result of running the program is '26'. What the
program actually prints is '12'. To see why, here is the code
produced (comments and other prettification added):
----------------------------------------------------------------
.globl _MAIN_
.set LF1,4
_MAIN_:
.word LWM1
subl2 $LF1,sp
jmp L12
L13:
movl $2,_x_
movl $3,_x_+4
movl $4,_x_+8
movl $5,_x_+12
mull3 _x_+4,_x_,-4(fp)
addl3 -4(fp),-4(fp),{result} # Mistake!
pushal v.3
calls $1,_s_wsle
pushl $4
pushab {result}
pushal {1}
pushal {3}
calls $4,_do_lio
calls $0,_e_wsle
pushl $0
pushal {00,00}
calls $2,_s_stop
ret
.align 1
_com_:
.word LWM1
L12:
moval v.1,r11
jmp L13
----------------------------------------------------------------
The two multiplies were treated as common subexpressions, and
the same temporary [-4(fp)] was used to store both results.
Fix:
I agonized over this one. Since this is a very urgent problem
I have decided to can my earlier inconclusive changes and make
a single, one-line change that has the effect of making COMMON
variables pretty much ineligible for CSE (actually it gives
them the status of arrays, which is convenient since COMMON
blocks look just like arrays to the compiler). This is fast
and has the advantage of introducing few new bugs. The change
is to optcse.c in routine scantree():
----------------------------------------------------------------
***************
*** 564,570
ap = (Addrp) p->exprblock.leftp;
idp = findid(ap);
killdepnodes(idp);
! if( ! ap->isarray ) {
if(rnode->is_dead)idp->assgnval=idp->initval;
else idp->assgnval = rnode;
}
--- 584,590 -----
ap = (Addrp) p->exprblock.leftp;
idp = findid(ap);
killdepnodes(idp);
! if( ! (ap->isarray || ap->vstg == STGCOMMON) ) {
if(rnode->is_dead)idp->assgnval=idp->initval;
else idp->assgnval = rnode;
}
----------------------------------------------------------------
This bug is responsible for many seemingly unrelated errors,
such as the previously reported 'exponentiation' error where
'x1 ** alpha + x2 ** alpha + x3 ** alpha' always turned out
to be '3 * x1 ** alpha'. You should fix this as soon as you
can...
Donn Seeley UCSD Chemistry Dept. ucbvax!sdcsvax!sdchema!donn
32 52' 30"N 117 14' 25"W (619) 452-4016 sdcsvax!sdchema!donn@nosc.ARPA