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

[Kazu Hirata <kazu@codesourcery.com>]

Hi Roman,

> 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? 

STRICT_ALIGNMENT affects us in compute_record_mode and 
default_return_in_memory.  (Note that m68k_return_in_memory emulates 
default_return_in_memory with STRICT_ALIGNMENT 1.)

Given a return type, compute_record_mode computes a return mode.  If 
STRICT_ALIGNMENT is 1, compute_record_mode chooses BLKmode for the type. 
(See the last "if" block in compute_record_mode.)  default_return_in_memory 
simply returns 1 if it sees BLKmode.

> 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?

When the return mode is BLKmode, we don't get to evaluate 
flag_pcc_struct_return in aggregate_value_p because 
targetm.calls.return_in_memory is called earlier.

Kazu Hirata