BITS_PER_UNIT issue in port....

BITS_PER_UNIT issue in port....

[Steve Silva]

Hi, 

I was not sure where to look but I thought I would try this.  I am in the middle of porting GCC 6.2 to an internal processor we are experimenting with.  I have about 2 months into it and I have made some decent progress.  The biggest issue I have right now is that our processor does not use 8 bit bytes but 16 bit bytes.  When I try and set BITS_PER_UNIT to 16, GCC will not compile because some of its internal structures become too big for the host platform.  Our processor only needs to support QI (16 bits) and HI (32 bits) modes; we don't really need anything SI or bigger.  I really don't want to hack on the GCC source code unless I am forced to, I would prefer to stay in the machine dependent realm.  Also, we are only using the "C" front end.  So I have the following questions: 

1.  Is it possible to set BITS_PER_UNIT to 16 at all?  If so, How? 
2.  Is there a minimum number of registers GCC has to have to work correctly?  What is that minimum? 

Thanks, 

Steve Silva (Broadcom Technologies)

Re: BITS_PER_UNIT issue in port....

[Jeff Law]

On 12/08/2016 01:37 PM, Steve Silva wrote:
> Hi,
>
> I was not sure where to look but I thought I would try this.  I am in
> the middle of porting GCC 6.2 to an internal processor we are
> experimenting with.  I have about 2 months into it and I have made
> some decent progress.  The biggest issue I have right now is that our
> processor does not use 8 bit bytes but 16 bit bytes.  When I try and
> set BITS_PER_UNIT to 16, GCC will not compile because some of its
> internal structures become too big for the host platform.  Our
> processor only needs to support QI (16 bits) and HI (32 bits) modes;
> we don't really need anything SI or bigger.  I really don't want to
> hack on the GCC source code unless I am forced to, I would prefer to
> stay in the machine dependent realm.  Also, we are only using the "C"
> front end.  So I have the following questions:
>
> 1.  Is it possible to set BITS_PER_UNIT to 16 at all?  If so, How?
I thought we had targets that did this in the past.  In theory it should 
be possible, but it's certainly not something that's done with any 
regularity or something that is tested.  It is highly likely that you'll 
run into various assumptions throughout GCC that BITS_PER_UNIT is 8.


  2.
> Is there a minimum number of registers GCC has to have to work
> correctly?  What is that minimum?
It depends on the structure of your register file.  If (for example) you 
have address and data registers, you'll likely need at least 4 address 
registers (stack pointer, frame pointer, 2 for the compiler to use in 
code generation).  You'll likely need 2-4 data registers.

If you have general purpose registers, then ~6 ought to work (frame 
pointer, stack pointer, 4 for code generation.

To some degree this will depend on things like your ABI, whether or not 
you have registers that have specialized uses, etc.

Folks have done ports on more constrained targets.  If you have zero 
page addressing, you could use zero-page memory locations as an extended 
register file.  Or you could look at the rl78 port which defines a set 
of virtual registers, then packs those down to the even more limited 
processor registers.

jeff