Re: m68k structure return register

[Vladimir Prus <vladimir@codesourcery.com>]

On Wednesday 20 June 2007 13:25:06 Vladimir Prus wrote:

Does anybody object to the course of action I've proposed
in the below email? If not, I'll send a patch.

- Volodya

> Hello,
> it appears that the way gdb expects code to
> return structure values on m68k is not exactly
> consistent with how gcc actually passes them.
> 
> Basically, for function returning structures,
> caller should pass an address of memory where
> structure should be returned, using either register a0,
> or register a1. Which register is used for what target
> is precisely where confusion lies.
> 
> In gcc/config/m68k there are three definitions:
> 
> m68k.h:#define M68K_STRUCT_VALUE_REGNUM A1_REG
> m68kelf.h:#define M68K_STRUCT_VALUE_REGNUM A0_REG
> netbsd-elf.h:#define M68K_STRUCT_VALUE_REGNUM A0_REG
> 
> This, together with config.gcc, gives me the following table
> of what register is used for what target.
> 
>         1. m68k-*-aout*                                 a1
>         2. m68k-*-coff*                                 a1
>         3. m68k-*-elf*                                  a0
>         4. m68k*-*-netbsdelf*                           a0
>         5. m68k*-*-openbsd*                             a1
>         6. m68k-*-uclinuxoldabi*                        a0
>         7. m68k-*-uclinux*                              a1
>         8. m68k-*-linux*                                a1
>         9. m68k-*-rtems*                                a0
> 
> Here's a breakdown of how those targets map to gdb's .mt files in
> config/m68k:
> 
> Group I -- monitor.mt -- just m68k-tdep.c
> 
>         1. m68k-*-aout*                                 a1
>         2. m68k-*-coff*                                 a1
>         3. m68k-*-elf*                                  a0
>         6. m68k-*-uclinuxoldabi*                        a0
>         7. m68k-*-uclinux*                              a1
>         9. m68k-*-rtems*                                a0
> 
> In gdb, all target here use the register set in m68k_gdbarch_init -- 
> currently A1, which register is wrong for half of targets. I'll get to this
> group shortly.
> 
> Group II -- nbsd.mt/obsd.mt -- m68k-tdep.c + m68kbsd-tdep.c
> 
>         4. m68k*-*-netbsdelf*                           a0
>         5. m68k*-*-openbsd*                             a1
>         
> In this group. netbsd has osabi sniffer that sets a0. openbsd
> just gets whatever  m68k_gdbarch_init sets -- currently a1.
> 
> Group III -- linux.mt -- m68k-tdep.c + m68klinux-tdep.c
> 
>         8. m68k-*-linux*                                a1
> 
> Here, there's osabi sniffer that causes a1 to be used.
> 
> The problem is therefore, group I. Notably, for m68k-elf gcc uses
> a0, whereas gdb assumes a1. And for m68k-elf, we cannot set
> any osabi sniffer. I think that nowdays m68k-elf is probably the most
> important target in that group.
> 
> I would suggest the following:
> 
> 1. Change gdb's default to use a0 register, so that bare-metal works.
> 2. If possible, add osabi sniffers to uclinux, openbsdb and rtems, that
> will cause gdb to use a1. 
> 
> Does this sound reasonable, and if so, anybody knows how I can
> implement osabi sniffers listed in (2)? I've being told osabi sniffer for
> uclinux might be impossible, and I don't know much about either openbsd
> or rtems.
> 
> Thanks,
> Volodya