Re: [patch] m68k: Fix binary compatibility problem with -mno-strict-align. (Take 2)

[Roman Zippel <zippel at linux-m68k dot org>]

Hi,

On Sat, 25 Aug 2007, Kazu Hirata wrote:

> Hi,
> 
> Attached is a revised patch to fix binary compatibility problem with
> -mno-strict-align on m68k-elf.
> 
> The previous iteration of this patch is at:
> 
>   http://gcc.gnu.org/ml/gcc-patches/2007-07/msg01437.html
> 
> In this iteration, Nathan has made a change to effectively comment out
> TARGET_RETURN_IN_MEMORY and m68k_return_in_memory if
> M68K_HONOR_TARGET_STRICT_ALIGNMENT is defined.  This way, this patch
> has no effect on m68k-linux, preserving the ABI.

Looking at this a little closer, I'm not sure why this is related to 
alignment in first place. Whether a structure is returned in a register 
should be controlled by -fpcc-struct-return/-freg-struct-return and the 
DEFAULT_PCC_STRUCT_RETURN value in the config. In 
function.c:aggregate_value_p there is shortly after the return_in_memory 
hook the following test:

  if (flag_pcc_struct_return && AGGREGATE_TYPE_P (type))
    return 1;

IMO the alignment should have no influence here, so where is this coming 
from? 

Looking at the default value of DEFAULT_PCC_STRUCT_RETURN:

$ grep DEFAULT_PCC_STRUCT_RETURN gcc/*.c gcc/config/m68k/*.h
gcc/toplev.c:#ifndef DEFAULT_PCC_STRUCT_RETURN
gcc/toplev.c:#define DEFAULT_PCC_STRUCT_RETURN 1
gcc/toplev.c:int flag_pcc_struct_return = DEFAULT_PCC_STRUCT_RETURN;
gcc/config/m68k/linux.h:#define DEFAULT_PCC_STRUCT_RETURN 0
gcc/config/m68k/m68kemb.h:#define DEFAULT_PCC_STRUCT_RETURN 0
gcc/config/m68k/netbsd-elf.h:#undef DEFAULT_PCC_STRUCT_RETURN
gcc/config/m68k/netbsd-elf.h:#define DEFAULT_PCC_STRUCT_RETURN 1
gcc/config/m68k/openbsd.h:#define DEFAULT_PCC_STRUCT_RETURN 0

m68k-elf includes m68kemb.h, which sets this to 0, so why isn't this 
honored?

bye, Roman