Thomas, George wrote:
> Section 20.13.7 of the Xmega manual speaks about the requirement of 16 bit
> alignment.
>
> http://www.atmel.com/images/doc8331.pdf
>
> As the MAXIMUM_ALIGNMENT in avr.h is set to 1, it throws an error when
> trying to compile.
>
> I have attached a patch which sets it to 2 by default in the compiler.
> I have also attached a case which fails.
>
> struct testStruct {
>
> char a;
> int b;
> float c;
> };
>
>
> /*volatile struct testStruct testObj; */
> char a __attribute__ ((aligned (2)));
>
> int main () {
> a = 2;
> /*
> testObj.a = 'c';
> testObj.b = 6;
> return testObj.a;
> */
>
> }
I cannot reproduce this a fail. With the oldest release series that supports
xmega, which is 4.7, your code compiles correctly and without warnings, both
with -fcommon and -fno-common:
-fcommon (default):
.comm a,1,2
-fno-common :
.global a
.section .bss
.p2align 1
.type a, @object
.size a, 1
a:
.zero 1
In 4.7.0 we have PR53448 [1] which is fixed in 4.7.1, but notice that 4.7.0
won't work with XMEGA, anyway, so that you don't want to use 4.7.0, anyway.
> diff -Naurp gcc/config/avr/avr.h gcc/config/avr/avr.h
> --- gcc/config/avr/avr.h 2012-11-16 13:43:25.000000000 +0530
> +++ gcc/config/avr/avr.h 2012-11-16 14:12:32.000000000 +0530
> @@ -245,8 +245,8 @@ enum
>
> #define EMPTY_FIELD_BOUNDARY 8
>
> -/* No data type wants to be aligned rounder than this. */
> -#define BIGGEST_ALIGNMENT 8
> +/* For Xmega USB buffers */
> +#define BIGGEST_ALIGNMENT 16
>
> #define TARGET_VTABLE_ENTRY_ALIGN 8
>
> Would this be an acceptable change ?
I don't think this is appropriate because it leads to memory waste in noswitch
sections.
Johann
[1] http://gcc.gnu.org/PR53448
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