donn@sdchema.UUCP (06/08/84)
Subject: f77 has problems when a DO loop variable is also a loop parameter
Index: usr.bin/f77/src/f77pass1/exec.c 4.2BSD
Description:
This bug only strikes when the optimizer is OFF. If a DO
loop variable is used for other purposes in a routine and in
particular is used in the loop initialization, limit or
increment at the same time it is used as the loop variable, the
parameter containing the loop variable will appear to be
garbage. This bug was originally sent to me by Robert Elz
of the University of Melbourne.
Repeat-By:
Compile the following program with 'f77 -S' (examples are
courtesy of Robert Elz):
----------------------------------------------------------------
k = 1
i = 1
do 10 k = k, 10
i = i + k
10 continue
print i
end
----------------------------------------------------------------
In the assembly language output we see that the register is
initialized from itself, instead of the memory version of the
loop variable:
----------------------------------------------------------------
movl $1,{k}(r11)
movl $1,{i}(r11)
movl r10,r10 <<<<<<<<<<<
cmpl r10,$10
jgtr L16
L17:
movl r10,{k}(r11)
addl3 r10,{i}(r11),r0
movl r0,{i}(r11)
L15:
incl r10
cmpl r10,$10
jleq L17
L16:
movl r10,{k}(r11)
clrl -104(fp)
movl $6,-100(fp)
movl {i}(r11),-92(fp)
addl3 $-104,fp,r0
pushl r0
calls $1,_s_wsfe
calls $0,_e_wsfe
L14:
ret
----------------------------------------------------------------
Fix:
The problem is that the compiler puts the DO loop variable in
register before fixing the DO loop parameters. The bug goes
away if you just swap the two chunks of code in exdo() in
f77pass1/exec.c:
----------------------------------------------------------------
*** /tmp/,RCSt1019970 Thu Jun 7 22:27:42 1984
--- exec.c Thu Jun 7 22:12:00 1984
***************
*** 379,385
}
dovarp = mkplace(np);
! if( ! ONEOF(dovarp->vtype, MSKINT|MSKREAL) )
{
err("bad type on DO variable");
return;
--- 379,387 -----
}
dovarp = mkplace(np);
! dotype = dovarp->vtype;
!
! if( ! ONEOF(dotype, MSKINT|MSKREAL) )
{
err("bad type on DO variable");
return;
***************
*** 387,404
return;
}
- ctlstack->donamep = np;
-
- np->vdovar = YES;
- if( !optimflag && enregister(np) )
- {
- /* stgp points to a storage version, varp to a register version */
- dostgp = dovarp;
- dovarp = mkplace(np);
- }
- else
- dostgp = NULL;
- dotype = dovarp->vtype;
-
-
for(i=0 , cp = spec->nextp ; cp!=NULL && i<3 ; cp = cp->nextp)
{
p = fixtype((expptr) cpexpr((tagptr) q = cp->datap));
--- 388,393 -----
return;
}
for(i=0 , cp = spec->nextp ; cp!=NULL && i<3 ; cp = cp->nextp)
{
p = fixtype((expptr) cpexpr((tagptr) q = cp->datap));
***************
*** 440,445
return;
}
for (i = 0; i < 4; i++)
ctlstack->ctlabels[i] = newlabel();
--- 429,445 -----
return;
}
+ ctlstack->donamep = np;
+
+ np->vdovar = YES;
+ if( !optimflag && enregister(np) )
+ {
+ /* stgp points to a storage version, varp to a register version */
+ dostgp = dovarp;
+ dovarp = mkplace(np);
+ }
+ else
+ dostgp = NULL;
for (i = 0; i < 4; i++)
ctlstack->ctlabels[i] = newlabel();
----------------------------------------------------------------
And I bet you thought it was safe as long as the optimizer was off,
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
(Moving soon to utah-cs)