Re: [avr-gcc-list] -fpacked-struct compiler option

[E. Weddington]

Anton Erasmus wrote:

> Hi,
>
> I ran the following commands on the test code below:  
> (Lates WinAVR version)
>
> avr-gcc -c test1.c -Os -funsigned-char -funsigned-bitfields -fpack-struct 
> -fshort-enums -
> Wall -Wstrict-prototypes -Wa,-ahlms=test_s.lst
> avr-gcc -c test1.c -O1 -funsigned-char -funsigned-bitfields -fpack-struct 
> -fshort-enums -
> Wall -Wstrict-prototypes -Wa,-ahlms=test_1.lst
> avr-gcc -c test1.c -O2 -funsigned-char -funsigned-bitfields -fpack-struct 
> -fshort-enums -
> Wall -Wstrict-prototypes -Wa,-ahlms=test_2.lst
> avr-gcc -c test1.c -O3 -funsigned-char -funsigned-bitfields -fpack-struct 
> -fshort-enums -
> Wall -Wstrict-prototypes -Wa,-ahlms=test_3.lst
>
> avr-gcc -c test1.c -Os -funsigned-char -funsigned-bitfields  -fshort-enums 
> -Wall -
> Wstrict-prototypes -Wa,-ahlms=test1_s.lst
> avr-gcc -c test1.c -O1 -funsigned-char -funsigned-bitfields  -fshort-enums 
> -Wall -
> Wstrict-prototypes -Wa,-ahlms=test1_1.lst
> avr-gcc -c test1.c -O2 -funsigned-char -funsigned-bitfields  -fshort-enums 
> -Wall -
> Wstrict-prototypes -Wa,-ahlms=test1_2.lst
> avr-gcc -c test1.c -O3 -funsigned-char -funsigned-bitfields  -fshort-enums 
> -Wall -
> Wstrict-prototypes -Wa,-ahlms=test1_3.lst
>
> typedef struct
> {
>   unsigned char c;
>   unsigned long L;
>   unsigned char b[4];
> }test_t;
>
>
> test_t foo;
>
> void bar(void)
> {
>  volatile unsigned char xx;
>  volatile unsigned long xxx;
>  volatile int i;
>
>  xx=foo.c;
>  xxx=foo.L;
>  i=sizeof(foo);
>
> }
>
> It generates exactely the same code in all cases with no 4 or 2 byte alignment visible 
> even without the -fpack-struct option. So it appeard as if the -fpack-struct is redundent
> for the avr. ( On arm-elf-gcc it generates extremely non-optimal code when 
> using this
> option)
>
>  

Thanks for doing this test!
But at least it doesn't hurt to have the switch explicity on the command 
line. Unless otherwise, it'll probably stay in the WinAVR Makefile 
template. For paranoia's sake. :-)