BITS_PER_UNIT less than 8 (was: Re: BITS_PER_UNIT larger than 8 -- word addressing)

[Boris Boesler <baembel at gmx dot de>]

Am 05.12.2007 um 22:32 schrieb Ian Lance Taylor:

> Boris Boesler <baembel@gmx.de> writes:
>
> >   I assume that GCC internals assume that memory can be byte (8 bits)
> > addressed - for historical reasons.
>
> No.  gcc internals assume that memory can be addressed in units of
> size BITS_PER_UNIT.  The default for BITS_PER_UNIT is 8.  I have
> written backends for machines for which that is not true.
>
> It is unusual, and there is only one official target with
> BITS_PER_UNIT != 8 (c4x), so there is often some minor breakage.

 Ok, so what have I to do to write a back-end where all addresses  
are given in bits? Memory is addressed in bits, not bytes. So I set:

#define BITS_PER_UNIT 1
#define UNITS_PER_WORD 32

(As far as I can see, offsets are divided by BITS_PER_UNIT, so this  
seems to be a precondition for bit addressing.)

 All sizes and and boundary are set to 32. SImode is only four bits  
wide, so I added the integer modes OI and XI:
INT_MODE(OI, 32)
INT_MODE(XI, 64)

 In builtin_define_type_max I added the case "1", which will return  
without doing anything.

 Without these changes the compiler stops with internal error  
mesages. With these changes gcc/cc1 generates a bus error.

So, what can I do to get this running for my architecture?

Thanks in advance,
Boris