bug in some 68000 C-compilers

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2 total messages Started by Mark Mendel Tue, 11 Sep 1984 15:57

#299

Author: Mark Mendel
Date: Tue, 11 Sep 1984 15:57

35 lines
1744 bytes

I have discovered a bug in our C compiler (Pixel Computer Inc., Version 1.2,
3/9/84).  Since this bug caused a core dump in the net.sources game PHANTASIA
and many people have been CLUTTERING that group with complaints about a core
dump, I am guessing that this bug is more wide spread.  I believe our compiler
is of Unisoft derivation.

Attached is a short program to determine if you suffer from the bug.

The bug is:  fcmp(), the compiler's floating point comparison routine, destroys
regiser d0.  This is only a problem if a floating comparison is used as PART of
an expression: previously computed sub-expressions are trashed. 
[in PHANTASIA, the routine printloc() uses such an expression as an array 
subscipt, which caused a core dump].

The fix (if you can't fix the compiler):  given an expressions like:

    ...+(x>1.0)+...

where x is a floating point variable, replace it with:

    tmpint = (x>1.0); ...+ tmpint + ...

===================== cut here =====================
#include <stdio.h>

main()
{
    double x = 1.0, y = 1.0;
    int i;

    i = (x > 0) + 2*(y>0);
    printf ("%d\n", i);
    if (i != 3)
	printf("your compiler has problems.\n");
}

Re: bug in some 68000 C-compilers

#302

Author: hamilton
Date: Thu, 13 Sep 1984 07:00

12 lines
1263 bytes

i reported this problem a while back (in net.micro, i think); it's effects
are even more widespread.  my UniSoft C compiler reuses one half of the
double result "register" as a scratchpad.  you'll see similar problems
with code like:
	double x[], atof();
	x[i] = atof(...);
the low-order fraction bits from the "atof" will get stomped by the
index calculation.  i "solved" my problem by avoiding "double"s
altogether.  i reported the problem to my micro vendor, who promised
to pass the word on to unisoft.  this was like last march... haven't
heard another word about it since.
	wayne ({decvax,ucbvax}!pur-ee!uiucdcs!uiucuxc!)hamilton

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