[PATCH] rs6000: Fix default alignment ABI break caused by MMA base support

[Paul A. Clarke]

On Fri, Nov 06, 2020 at 04:18:00PM -0600, Peter Bergner via Gcc-patches wrote:
> As part of the MMA base support, we incremented BIGGEST_ALIGNMENT in
> order to align the __vector_pair and __vector_quad types to 256 and 512
> bits respectively.  This had the unintended effect of changing the
> default alignment used by __attribute__ ((__aligned__)) which causes
> an ABI break because of some dodgy code in GLIBC's struct pthread
> (GLIBC is going to fix that too).  The fix in GCC is to revert the
> BIGGEST_ALIGNMENT change and to force the alignment on the type itself
> rather than the mode used by the type.
[snip]
> diff --git a/gcc/config/rs6000/rs6000.h b/gcc/config/rs6000/rs6000.h
> index bbd8060e143..5a47aa14722 100644
> --- a/gcc/config/rs6000/rs6000.h
> +++ b/gcc/config/rs6000/rs6000.h
> @@ -776,8 +776,10 @@ extern unsigned rs6000_pointer_size;
>  /* Allocation boundary (in *bits*) for the code of a function.  */
>  #define FUNCTION_BOUNDARY 32
>  
> -/* No data type wants to be aligned rounder than this.  */
> -#define BIGGEST_ALIGNMENT (TARGET_MMA ? 512 : 128)
> +/* No data type is required to be aligned rounder than this.  Warning, if
> +   BIGGEST_ALIGNMENT is changed, then this may be an ABI break.  An example
> +   of where this can break an ABI is in GLIBC's struct _Unwind_Exception.  */

Instead of calling out something that is expected to be fixed, should you
instead call out that it will change the alignment of anything using
"__attribute__ ((__aligned__))"?

> +#define BIGGEST_ALIGNMENT 128

[snip]

PC