[NTLUG:Discuss] Thinking ALPHA

[Richard Cobbe]

cbbrowne at godel.brownes.org wrote on 9-26-1999:

> On Sat, 25 Sep 1999 09:25:35 CDT, the world broke into rejoicing as
> Steve Baker <sjbaker1 at airmail.net>  said:
> > The biggest problem area seemed to be related to doing floating point
> > math on undefined variables - even in situations where the results
> > don't matter:
> > 
> > Here is a much simplified example:
> > 
> >     float x ;  /* Undefined */
> > 
> >     if ( condition )
> >       x = 6.0f ;
> > 
> >     x += 2.0f ;  /* Maybe an operation on an undefined variable */
> > 
> >     if ( condition )
> >       printf ( "%f\n", x ) ;  /* But who cares? */
> > 
> > 
> > This kind of thing would sometimes core dump on the Alpha if
> > condition==FALSE, where x86, PPC, 68000, MIPS, etc CPU's would
> > happily sail on past.  Clearly it's sloppy programming on my part
> > - but it's a pain to debug if you don't have an Alpha.
> 
> I always found it something of a convenience that Pascal would always
> "puke" on that sort of thing; 'tis quite useful to *know* that you've not
> initialized a variable.

At the risk of re-opening an old flame war, gcc will detect this sort of
thing.  You have to compile with optimizations on, though, because it
relies on data-flow analysis that isn't done without optimizations.

Take the following foo.cpp:

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    int i;

    // the following is a lousy condition, but it suffices for this
    // example. 

    if (rand() & 0x01)
    {
	printf("initing i\n");
	i = 3;
    }

    i += 2;

    printf("%d\n", i);

    return 0;
}


And a compilation:

[minbar:~]$ g++ -Wall -O2 -o foo foo.cpp
foo.cpp: In function `int main()':
foo.cpp:6: warning: `int i' might be used uninitialized in this function


Actually, the -Wall isn't necessary; a -Wuninitialized will do.  Also,
according to the info page, -O, -O1, or -O3 will work fine as well,
although I haven't tested this.

Richard