donn@sdchema.UUCP (04/07/84)
Subject: Multiplying something by the constant 0 can cause f77 to crash
Index: usr.bin/f77/src/f77pass1/expr.c 4.2BSD
Description:
This bug is the same as the one reported by trq@astrovax.UUCP
with the subject 'problem with mixed-mode in f77'. Basically
it randomly applies to f77 programs that have an expression in
which a value of a type other than INTEGER is multiplied by the
constant 0. Admittedly few real programs explicitly multiply
by zero -- the main sources of examples are programs which have
PARAMETER variables that are set to zero.
Repeat-By:
Put the following program in a file named constbug.f:
----------------------------------------------------------------
program constbug
integer iwrblk
iwrblk = 1 + 0 * 1.0
print *, iwrblk
stop
end
----------------------------------------------------------------
If you compile this program with the optimizer on, the compiler
says:
----------------------------------------------------------------
constbug.f:
MAIN constbug:
Compiler error line 5 of constbug.f: Impossible type 0 in routine mkconv
compiler error.
----------------------------------------------------------------
If you compile the program with the optimizer off, it prints '0'.
(What you would expect it to print is '1'.)
Fix:
What is happening is that f77 is stomping on its data
structures. F77 wants to convert '0 * 1.0' to '0.0' when it
evaluates constant expressions. The data structure for the '0'
is freed after the conversion to '0.0', but the compiler
doesn't pay attention to the new value and continues to use the
old data structure for the '0' instead. Since the compiler is
free to reuse this space, it later seems to behave strangely
(in this case the '0' shares with the optimizer data structure
for the same statement it appears in!). This is trivial to fix
-- the change is in function mkexpr() in expr.c:
----------------------------------------------------------------
RCS file: RCS/expr.c,v
retrieving revision 1.1
diff -c -r1.1 expr.c
*** /tmp/,RCSt1015888 Fri Apr 6 20:08:55 1984
--- expr.c Fri Apr 6 19:10:53 1984
***************
*** 1601,1607
{
if(rp->constblock.const.ci == 0)
{
! mkconv(etype, rp);
goto retright;
}
if ((lp->tag == TEXPR) &&
--- 1601,1607 -----
{
if(rp->constblock.const.ci == 0)
{
! rp = mkconv(etype, rp);
goto retright;
}
if ((lp->tag == TEXPR) &&
----------------------------------------------------------------
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