Re: Unreachable code warning conflicts with optomisation

["David Brown" <david at westcontrol dot com>]

> On Fri, Sep 26, 2003 at 01:49:23PM +0200, David Brown wrote:
> > I've filled out a complete example below.  The "-Wunreachable-code" is
not
> > included in the -Wall flags, so it has to be specified seperately.
>
> Indeed. I don't actually have it at all on my gcc.
>
> > >m68k-elf-gcc -W -O2 -c test.c -o test.o -Wunreachable-code
> > test.c: In function `test1':
> > test.c:13: warning: will never be executed
>
> This one should actually always turn up (unsigned can't be < 0 )
>

Yes, that function is for testing the opposite case.  I suppose it is also
an issue that it does not turn up until using -O2 optomisation, but that's a
matter of the compiler only running the more advanced code-flow analysis
passes when doing more optomisation.  I can't expect the compiler to be able
to catch *all* unreachable code!  Incidently, on my first try at that test
case (using "(u >= 0)" ), the compiler always spotting the unnecessary
comparison.


> > // *******************************
> > extern void sendString(char *p);
> > extern void sendNewline(void);
>
> Can we also get those 2 functions? Ideally you want to
> reduce the test case to one small file, the gcc people will
> appreciate that if it turns out to be a bug.
>

I don't think it would help at all, and to give you the full code would mean
you'd also need all my uart handling code, circular buffer code, interrupt
routines, hardware configuration, ....  The code is just artificial test
code, and these functions are merely a break from the usual foo() and bar(),
and could happily be stubbed out.  An alternative would be to set some
volatile variables (I like volatiles for this sort of thing - it stops the
optomiser being too helpful :-)

// *******************************
volatile int vola;
volatile int volb;

void test1(unsigned int u)
{
 if (u > 0) {
  vola = 1;
 } else if (u == 0) {
  vola = 2;
 } else {
  vola = 3;
 };
}

void test2(int i)
{
 switch (i) {
 case 0 :
  vola = 0;
  volb = 1;
  break;
 case 1 :
  vola = 1;
  volb = 1;
  break;
 case 2 :
  vola = 2;
  volb = 1;
  break;
  }
}
// *******************************




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