Re: [avr-libc-dev] Strange asm output

[Dmitry K.]

8 Nov 2004 02:52 Nicolas Schodet wrote:
> Hello,
>
> I have a little piece of code which produce a very strange code:
>
> int32_t a;
>
>     if (a & (1L << 23))
>  1f6:   da 01           movw    r26, r20
>  1f8:   c9 01           movw    r24, r18
>  1fa:   77 e1           ldi     r23, 0x17       ; 23
>  1fc:   b6 95           lsr     r27
>  1fe:   a7 95           ror     r26
>  200:   97 95           ror     r25
>  202:   87 95           ror     r24
>  204:   7a 95           dec     r23
>  206:   d1 f7           brne    .-12            ; 0x1fc
>  208:   81 70           andi    r24, 0x01       ; 1
>  20a:   90 70           andi    r25, 0x00       ; 0
>  20c:   89 2b           or      r24, r25
>  20e:   51 f0           breq    .+20            ; 0x224
>
> I can not understand why gcc do not just test the 23th bit.

I have try:
   int foo (long x)
   {
       if (x & (1L<<23))
           return 1001;
       else
           return 1002;
   }

with:
   avr-gcc 3.3.4 / mcu=atmega8

I have receive for -O1/-O2/-Os:
   foo:
        movw r26,r24
        movw r24,r22
        sbrs r26,7
        rjmp .L2
        ldi r24,lo8(1001)
        ldi r25,hi8(1001)
        ret
  .L2:
        ldi r24,lo8(1002)
        ldi r25,hi8(1002)
        ret

And for -O3:
   foo:
        sbrs r24,7
        rjmp .L2
        ldi r24,lo8(1001)
        ldi r25,hi8(1001)
        ret
   .L2:
        ldi r24,lo8(1002)
        ldi r25,hi8(1002)
        ret

Give, please, more piece of this program.