Re: Need advice on interface to non-gcc scheduler

["Damjan Lampret" <lampret at opencores dot org>]

Steven,

I am also surprised with similarities between both architectures. Our
approach would be similar to yours except in our case we were thinking to
use assembly output for some generic RISC as input to "optimizer" for OR2K.
BTW FUs in OR2K doesn't need to be equal since some instructions are more
common than others.

regards, Damjan

----- Original Message -----
From: Steven Roos <S.Roos@its.tudelft.nl>
To: <gcc@gcc.gnu.org>
Sent: Friday, March 31, 2000 3:59 PM
Subject: Re: Need advice on interface to non-gcc scheduler


> "Damjan Lampret" <lampret@opencores.org> wrote:
> > Hi Steven,
> >
> > is your architecture similar to something like
> > http://www.opencores.org/cores/or2k/ ?
>
> The philosophy behind the architecture is more or less the same, but
> on the implementation level there are some differences. The three most
> important differences are:
> 1. We use a one-dimensional array instead of a two-dimensional matrix
>    because we think that would map better onto hardware.
> 2. We allow only Ri (the only non-locally visible register on a unit)
>    as the destination of an operation.
> 3. Our FUs are not all equivalent because we think the hardware
>    required for some opcodes like multiply is too expensive to be
>    included in each unit.
>
> However, even with these differences the similarities are remarkable.
> Unfortunately I do not have a recent architecture description. The
> paper http://cardit.et.tudelft.nl/MOVE/papers/Roos99a.ps.gz gives
> a good introduction, but is in many ways outdated. For example, it
> does not yet include the concept of a local register file in each
> unit.
>
> > I've tried to model OpenRISC 2000 in
> > GCC but GCC doesn't understand very good concept of distributed register
> > file. I've tried this with register classes defining a new register
class
> > for each functional unit and its registers. Of course since GCC was
designed
> > for traditional CPUs where you have one central register file it does
not
> > generate efficient code for CPU similar to OR2K. I would be interested
if
> > anyone has better idea how to model OR2K in GCC so that GCC would
generate
> > efficient code?
> >
> > regards, Damjan
>
> I haven't found any compiler that can effectively handle the
> non-homogeneous and distributed register files as found in OR2K and my
> architecture. Most compilers (including gcc) try to assign a single
> register to the complete liverange of a variable and fail to
> distribute instructions and data evenly over the processor.
>
> Therefore I have put down a completely new scheduling algorithm that
> is capable of handling such register file topology. I think it can
> also schedule for OR2K with very limited modifications. It requires a
> completely new scheduler module, with its own internal representation.
> My idea is to use gcc with a RISC-like target as a front-end and
> switch my own module for the scheduling and register allocation.
>
> Regards,
> Steven
>