[Bug target/19686] New: avr-gcc 4.0: loop performance decrease (extra conversion)

["dmixm at marine dot febras dot ru" <gcc-bugzilla at gcc dot gnu dot org>]

Program: 
 
int foo (int x) 
{ 
    int i= x; 
    do { 
        x *= 2; 
    } while (--i); 
    return x; 
} 
 
avr-gcc 4.0 (20050116) with `-Os' option: 
 
foo: 
/* prologue: frame size=0 */ 
/* prologue end (size=0) */ 
        ldi r18,lo8(0) 
        ldi r19,hi8(0) 
        clr r20 
        clr r21 
        sub r20,r24 
        sbc r21,r25 
.L2: 
        add r24,r24 
        adc r25,r25 
        subi r18,lo8(-(-1)) 
        sbci r19,hi8(-(-1)) 
        cp r18,r20 
        cpc r19,r21 
        brne .L2 
/* epilogue: frame size=0 */ 
        ret 
 
avr-gcc 3.3.4: 
 
foo: 
/* prologue: frame size=0 */ 
/* prologue end (size=0) */ 
        mov r18,r24 
        mov r19,r25 
.L2: 
        add r24,r24 
        adc r25,r25 
        subi r18,lo8(-(-1)) 
        sbci r19,hi8(-(-1)) 
        brne .L2 
/* epilogue: frame size=0 */ 
        ret

-- 
           Summary: avr-gcc 4.0: loop performance decrease (extra
                    conversion)
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dmixm at marine dot febras dot ru
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19686