[Bug rtl-optimization/34791] [avr] optimisation of 8-bit logic sometimes fails

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34791

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.07.09 11:55:16
                 CC|                            |gjl at gcc dot gnu.org
   Target Milestone|---                         |4.7.0
     Ever Confirmed|0                           |1
      Known to fail|                            |4.6.1

--- Comment #2 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2011-07-09 
11:55:16 UTC ---
I still see this in 4.6.1.

(In reply to comment #1)
> There are many other cases where 8-bit optimisation is missing - C's integer
> promotion gets in the way.  This is particularly common when dealing with a
> compile-time constant - there is no way in C to say that "0x3f" is 8-bit 
> rather
> than a 16-bit int.
> 
> Another example of code with this problem is:
> 
> void foo(void) {
>     static unsigned char count;
> 
>     if (++count & 0x3f) {
>         PORTC &= ~0x01;
>     } else {
>         PORTC |= 0x01;
>     }
> }
> 
> Both the "&" and the comparison with zero are done as 16-bit.

Please, don't use stuff like PORTC that is nor available to the general GCC
developer.  They won't be able to reproduce this artifact!  This is not
specific to the avr part and related to PR43088.

Write yout test case like that:

typedef unsigned char uint8_t;
#define PORTC (*((uint8_t volatile*) 0x28))

void foo(void) {
    static unsigned char count;

    if (++count & 0x3f) {
        PORTC &= ~0x01;
    } else {
        PORTC |= 0x01;
    }
}

void foo3e(void) {
    static unsigned char count;

    if (++count & 0x3e) {
        PORTC &= ~0x01;
    } else {
        PORTC |= 0x01;
    }
}