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

[Paul Schlie]

GCC Bug 18424 has been added to the database:

3.4.3 generates code which may be ~6x+ slower and larger than 3.3.1 @ -0s
as it apparently no longer evaluates constant expression trees in anything
other than simple expressions for some reason, which may result in serious
performance and code size regressions, which should really be fixed if
possible.

// 000000c6 <foo>:
int foo ( int a ){
        if (a & (1L << 23))
//  c6: aa 27           eor     r26, r26
//  c8: 97 fd           sbrc    r25, 7
//  ca: a0 95           com     r26
//  cc: ba 2f           mov     r27, r26
//  ce: 27 e1           ldi     r18, 0x17       ; 23
//  d0: b6 95           lsr     r27
//  d2: a7 95           ror     r26
//  d4: 97 95           ror     r25
//  d6: 87 95           ror     r24
//  d8: 2a 95           dec     r18
//  da: d1 f7           brne    .-12            ; 0xd0
//  dc: 81 70           andi    r24, 0x01       ; 1
//  de: 90 70           andi    r25, 0x00       ; 0
//  e0: 89 2b           or      r24, r25
//  e2: 19 f0           breq    .+6             ; 0xea
                return 1;
//  e4: 81 e0           ldi     r24, 0x01       ; 1
//  e6: 90 e0           ldi     r25, 0x00       ; 0
//  e8: 08 95           ret
          else
                return 2 ;
//  ea: 82 e0           ldi     r24, 0x02       ; 2
//  ec: 90 e0           ldi     r25, 0x00       ; 0
}
//  ee: 08 95           ret
//  f0: 08 95           ret
// where the second return is odd as well?

vs GCC 3.3.1 @ -0s

// 000000c6 <foo>:
int foo ( int a ){
        if (a & (1L << 23))
                return 1;
          else
                return 2 ;
}
//  c6: 82 e0           ldi     r24, 0x02       ; 2
//  c8: 90 e0           ldi     r25, 0x00       ; 0
//  ca: 08 95           ret

(where for referance int targeted to avr is 16-bits wide,
 but the above problem is not likely target sensitive.)

Which I hope will get someone's attention.

> From: Nicolas Schodet <address@hidden>
> Date: Thu, 11 Nov 2004 00:43:35 +0100
> To: <address@hidden>
> Subject: Re: [avr-libc-dev] Strange asm output
> 
> * Dmitry K. <address@hidden> [041110 23:50]:
>>> I have a little piece of code which produce a very strange code:
>>> I can not understand why gcc do not just test the 23th bit.
>> I have try:
>> [...]
> 
> I change my original code, so I writed a new one:
> 
> /* toto.c */
> #include <inttypes.h>
> 
> int32_t
> toto (int32_t a)
> {
>     if (a & (1L << 23))
>         return 1;
>     else
>         return 0;
> }
> 
> Command line:
> 
> avr-gcc -g -Wall -O2 -mmcu=atmega128 -MMD -S -o toto.s toto.c
> 
> Gcc version:
> 
> avr-gcc (GCC) 3.4.1
> 
> Result:
> 
> .global toto
>         .type   toto, @function
> toto:   
>         .stabn 68,0,6,.LM1-toto
> .LM1:
> /* prologue: frame size=0 */
> /* prologue end (size=0) */
>         movw r26,r24
>         movw r24,r22
>         .stabn 68,0,7,.LM2-toto
> .LM2:   
>         ldi r18,23
> 1:      lsr r27
>         ror r26
>         ror r25
>         ror r24
>         dec r18
>         brne 1b
>         andi r24,lo8(1)
>         andi r25,hi8(1)
>         or r24,r25
>         breq .L2
>         .stabn 68,0,8,.LM3-toto
> .LM3:   
>         ldi r22,lo8(1)
>         ldi r23,hi8(1)
>         ldi r24,hlo8(1)
>         ldi r25,hhi8(1)
>         ret
> .L2:
>         .stabn 68,0,10,.LM4-toto
> .LM4:   
>         ldi r22,lo8(0)
>         ldi r23,hi8(0)
>         ldi r24,hlo8(0)
>         ldi r25,hhi8(0)
>         .stabn 68,0,11,.LM5-toto
> .LM5:   
>         ret
> /* epilogue: frame size=0 */
>         ret
> /* epilogue end (size=1) */
> /* function toto size 24 (23) */
>         .size   toto, .-toto
> 
> Tell me if you need more informations.
> 
> /Nicolas
> 
> 
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