[Bug middle-end/33315] If condition not getting eliminated

["j at uriah dot heep dot sax dot de" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #4 from j at uriah dot heep dot sax dot de  2007-11-23 10:57 -------
Is the missed optimization in the following code the same?

volatile unsigned char *reg_a = (unsigned char *)42;
volatile unsigned char *reg_b = (unsigned char *)34;

extern void a(void);
extern void b(void);
extern void c(void);

void
decide(void)
{
        signed char diff;

        diff = *reg_a - *reg_b;

        if (diff < 0)
                a();
        else if (diff == 0)
                b();
        else if (diff > 0)
                c();
}

The third "if" statement is partially executed: it apparently remembered
that diff could not be less than 0, but it still tests against 0 even
though that test has just been done before.  Verified on both, the AVR
and i386 target.

Interestingly, by just reordering the code, the third condition will
be eliminated by GCC 4.x (but not by GCC 3.x):

volatile unsigned char *reg_a = (unsigned char *)42;
volatile unsigned char *reg_b = (unsigned char *)34;

extern void a(void);
extern void b(void);
extern void c(void);

void
decide(void)
{
        signed char diff;

        diff = *reg_a - *reg_b;

        if (diff < 0)
                a();
        else if (diff > 0)
                c();
        else if (diff == 0)
                b();
}

If someone thinks that's an entirely different thing than the subject of this
bug, please tell me so, and I'll submit a separate one.


-- 

j at uriah dot heep dot sax dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |j at uriah dot heep dot sax
                   |                            |dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33315