Re: A retargetable compiler

> > In LANCE documentation speaks about the tools OLIVE and IBURG for
> > make the synthesis, the code generators. I have found IBURG, but I
> > found nothing about OLIVE.
> > ..
> > And, more general, do you thing those are the right tools I need?
>
> I'm not familiar with OLIVE either, but IBURG (and its sibling BURG)
> might very well be the right tool for you. Iburg is good at finding
> the cheapest - you define the metrics - machine instruction sequence
> for a given tree of intermediate language instructions. This is
> especially useful if there are many alternative translations to choose
> from, as is often the case in CISC processors. If your processor is
> more of a RISC cpu then the problem is probably not so much choosing
> the right instructions, as well as scheduling them. In that case the
> gain from using a code generator generator is less spectacular but it
> will probably save you some time and make your code easier to
> maintain.
>
> If memory serves me well the lcc c-compiler was done by the authors of
> BURG and uses (I?)BURG in its backend. You may want to have a look at
> that compiler to see a real-world example.


LCC is a very good choice for quickly developing a working C compiler,
but it comprises only few code optimizations, only a local register
allocator and no instruction scheduler. So it's up to your
preferences: "time to market" or code quality. LCC uses LBURG, a
variant of IBURG, for the code selection part, but the difference is
not big.


What actually makes a difference from my experience with IBURG and
OLIVE is that OLIVE provides a much more convenient specification
formalism, since it allows to annotate action functions (similar to
yacc) to the tree grammar rules, and these allow for a very clean and
modular code selector implementation. Another plus of OLIVE is that
the instruction costs are not restricted to integer constants, but
arbitrary C code can be used. This is a nice feature in case of
several non-trivial target machines. In the LANCE system mentioned
above, we have implemented an interface to OLIVE and applied this to a
number of compiler development projects.


I am not aware of any dedicated information on OLIVE on the Web, but
it used to be included in the SPAM compiler package, which can be
downloaded from www.ee.princeton.edu/spam.


It should be noted, though, that IBURG/OLIVE just help to create a
rather small part of a C compiler, namely the code selector. A C
frontend, machine-independent optimizations, as well as register
allocation, scheduling, and peephole optimizations have to be
implemented as well. These things are all supported by the GNU C
compiler gcc, which might be another good choice. However,
retargeting gcc certainly gets difficult, when your target machine is
not as "clean" as a standard RISC or CISC processor.


In addition to the tools mentioned above there exist a number of
"point solutions" for retargetability, that refer to specific
processor classes like VLIW or DSP.


Hope this helps.


Rainer


--
Dr. Rainer Leupers
University of Dortmund, Computer Science 12
Otto-Hahn-Str. 16, D-44221 Dortmund, Germany
phone: +49 231 755-6151, fax: -6116, mobile: +49 177 2131146
email (office): leupers@icd.de, leupers@LS12.cs.uni-dortmund.de
email (home): rainer@leupers.de
http://ls12-www.cs.uni-dortmund.de/~leupers

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