tp@td2cad.UUCP (t patterson) (05/04/88)
/*
following program demonstrates a bug in C optimizer found in Ultrix 2.0.
it does not occur under UTS on IBM mainframe, on 4.3 BSD on a VAX, under
SunOS 3.2 (or maybe 3.4), nor under Bell Technologies System V on Intel 301 box.
does anybody know if Ultrix 2.2 fixes this?
(we have filed an SPR on it; no reply from DEC; we filed thru DSIN ... hahaha
...at least we got a receipt that the report was mailed properly; I presume
the whole affair is over one of those old 4.2 BSD optimizer bugs that DEC has
never fixed)
Don't trust your C programs! (or use gcc!)
*/
/*
* $Compile: cc -O -o opttest opttest.c
*
* From myu@tdcad Wed Apr 6 08:41:56 1988
* Subject: simple c program for testing -O in cc
*
* if the following program is compiled with -O option, it will give
* results different from when it is compiled without that option.
*
* if you dis-assemble the object file, it's pretty obvious that the
* optimizer is producing incorrect code -- it goofs up a branch statement
* so it always passes over one of the printf statements.
*
* the program can be made to run correctly by compiling with "-DHUMOR"
* or by tinkering withthe array c in various ways.
*
* this problem was discovered when someone finally noticed that a
* piece of production code was producing wildly incorrect results.
*/
#include <stdio.h>
int c[9] = { 2, 2, 1, 5, 20, 18, 20, 17, 8 };
main()
{
int id, b, a;
id = 4;
b = 0;
if ( (id != 8) && (id != 9) ) {
b = c[ id ];
} else {
printf("id is either 8 or 9\n");
}
#ifdef HUMOR
a = 10; /* if you compile with "-DHUMOR", it's OK */
#endif
if( b == 20 ) {
printf("This statement skipped with optimizer; b=%d\n", b);
}
printf("the last statement: id=%d b=%d\n",id,b);
}
--
..tp.. t patterson path: {ihnp4,uunet}!wucs1!td2cad!tp
OR {pyramid,hoptoad}!td2cad!tp
domain: tp%td2cad.intel.com@relay.cs.net OR decwrl!intelca!mipos3!td2cad!tp