[Bug target/50289] New: [avr]: call-prologues saveing/restoring global register variables

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50289

             Bug #: 50289
           Summary: [avr]: call-prologues saveing/restoring global
                    register variables
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: g...@gcc.gnu.org
                CC: eric.wedding...@atmel.com
            Target: avr


void bar (long long, long long, void*);

register char x asm ("7");

void foo (char a)
{
    asm volatile ("":::"6");
    bar (0, 0, &a);
}


This small program, compiles with
   avr-gcc-4.6.1 -mcall-prologues -S -Os -mmcu=atmega8 
to

foo:
    ldi r26,lo8(1)
    ldi r27,hi8(1)
    ldi r30,lo8(gs(1f))
    ldi r31,hi8(gs(1f))
    jmp __prologue_saves__+((18 - 13) * 2)
1:

This means that __prologue_saves__ from libgcc saves R7 (i.e. R6..R17 and Y)
even though R7 is a global register variable and must not saved/restored by
function prologue/epilogue.

The function to fix is avr.c:sequent_regs_live()