[avr-gcc-list] odd optimization differences of shift between C and C++

[William \"Chops\" Westfield]

I'm told that I should post questions here rather than on avr-freaks.
Here's one...
(some additional discussion at http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=98888 
 )  4.3.4 behaves the same, and
there are NO right shifts of SIGNED variables involved...


Is there something I'm not understanding about C++ ? I have a trivial  
program:

Code:
> extern void foo(unsigned char c);
> extern void bar(int c);
>
> extern unsigned char c;
>
> int main()
> {
>     foo(c>>4);
>     bar(c>>4);
> }


If I compile with avr-gcc I get the expected optimization using SWAP:
Code:
> gcc --version
> i686-apple-darwin9-gcc-4.0.1 (GCC) 4.0.1 (Apple Inc. build 5493)
>
>  avr-gcc -Os -g -mmcu=atmega8 -c -o shift.o shift.c
> BillW-MacOSX-2<1155> avr-objdump -S shift.o
>
> 00000000 <main>:
>
> extern unsigned char c;
>
> int main()
> {
>     foo(c>>4);
>    0:   80 91 00 00    lds   r24, 0x0000
>    4:   82 95          swap   r24
>    6:   8f 70          andi   r24, 0x0F   ; 15
>    8:   00 d0          rcall   .+0         ; 0xa <main+0xa>
>     bar(c>>4);
>    a:   80 91 00 00    lds   r24, 0x0000
>    e:   82 95          swap   r24
>   10:   8f 70          andi   r24, 0x0F   ; 15
>   12:   90 e0          ldi   r25, 0x00   ; 0
>   14:   00 d0          rcall   .+0         ; 0x16 <main+0x16>
> }
>   16:   08 95          ret


If I compile the SAME FILE using g++, it apparently does the promotion  
to int much earlier, and generates much bulkier and slower code:

> g++ --version
> i686-apple-darwin9-g++-4.0.1 (GCC) 4.0.1 (Apple Inc. build 5493)
>
>  avr-g++ -Os -g -mmcu=atmega8 -c -o shift.o shift.c
>
> avr-objdump -S shift.o
>
> 00000000 <main>:
>
> extern unsigned char c;
>
> int main()
> {
>     foo(c>>4);
>    0:   80 91 00 00    lds   r24, 0x0000
>    4:   82 95          swap   r24
>    6:   8f 70          andi   r24, 0x0F   ; 15
>    8:   00 d0          rcall   .+0         ; 0xa <main+0xa>
>     bar(c>>4);
>    a:   80 91 00 00    lds   r24, 0x0000
>    e:   90 e0          ldi   r25, 0x00   ; 0
>   10:   24 e0          ldi   r18, 0x04   ; 4
>   12:   96 95          lsr   r25
>   14:   87 95          ror   r24
>   16:   2a 95          dec   r18
>   18:   01 f4          brne   .+0         ; 0x1a <main+0x1a>
>   1a:   00 d0          rcall   .+0         ; 0x1c <main+0x1c>
> }
>   1c:   80 e0          ldi   r24, 0x00   ; 0
>   1e:   90 e0          ldi   r25, 0x00   ; 0
>   20:   08 95          ret


Is this expected? Is it bug-worthy ?