[Bug target/51002] New: SP_H register is used even on targets that do not have it (eg attiny26)

[wiml at hhhh dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51002

             Bug #: 51002
           Summary: SP_H register is used even on targets that do not have
                    it (eg attiny26)
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: wiml@hhhh.org
            Target: avr


Not all of the AVR parts implement a full 16-bit-wide stack pointer; on (for
example) the ATtiny26/ATtiny261, the stack pointer is 8 bits wide and the SP_H
register (at 0x3E) is not implemented. GCC 4.6.2 is smart enough to manipulate
only the lower 8 bits of the stack pointer in the function prologue/epilogue,
but it still attempts to load the upper 8 bits from the nonexistent SP_H
register, resulting in the upper 8 bits of any stack-relative pointer
containing a garbage value:


extern void bar(char *c);
void foo()
{
    char buf[16];
    bar(buf);
}

$ avr-gcc -Os -fverbose-asm -S -mmcu=attiny26 stkbuf.c  

foo:
        push r28         ; 
        push r29         ; 
        in r28,__SP_L__  ; 
        in r29,__SP_H__  ; 
        subi r28,lo8(-(-16))     ; ,
        out __SP_L__,r28         ; 
/* prologue: function */
/* frame size = 16 */
/* stack size = 18 */
.L__stack_usage = 18
        mov r24,r28      ; ,
        mov r25,r29      ; ,
        adiw r24,1       ; ,
        rcall bar        ; 
[etc]