[LLVMdev] Backend for the ZPU - a stack based / zero operand CPU

Owen Anderson resistor at mac.com
Fri Jun 20 13:01:36 PDT 2008

You might try looking at the recent work on PIC16.  If you look  
through the list archives, one of  the developers wrote about his  
experiences using LLVM for such a constrained architecture.  To  
summarize: while LLVM didn't offhand support the oddities of his  
architecture, it was easier to modifying to work with them than other  
compilers he had worked with.


On Jun 19, 2008, at 2:30 PM, Øyvind Harboe wrote:

> Hi all,
> Zylin has implemented the world smallest 32 bit CPU with
> a GCC backend. (I shall stand corrected if anyone claims
> & proves otherwise :-)
> Implementing a GCC backend for a zero operand/stack based
> architecture proved pretty tricky, but I'm quite pleased with
> the resulting code. I did make alterations to the architecture
> to make it fit GCC without sacrificing CPU size.
> I have been following llvm.org from a distance, since I finished
> the GCC backend a couple of years back and there has not
> really been a reason to build a new compiler for the ZPU.
> My llvm.org knowledge is ... shallow ... but I'm hoping that
> someone would find the time & pity to answer my questions:
> Q: Is a stack based / zero operand CPU and llvm a good match? (GCC
> wasn't)
> Q: Should I expect better code density / performance from llvm than  
> GCC for
> said architecture?
> Q: Can llvm help move global data into flash(i.e. determine that  
> global C
> variables are in fact constants)?
> The ZPU would probably be most effective in highly space constrained
> applications(kBytes of code/data), such that it would fit entirely  
> onto
> an FPGA/CPLD. It has very high code density (~80% of ARM Thumb)
> Overview of architecture(documentation is definitely the weak side of
> the ZPU):
> http://www.zylin.com/zpu_arch.html
> The ZPU is now hosted at:
> http://www.opencores.org/projects.cgi/web/zpu/overview
> -- 
> Øyvind Harboe
> http://www.zylin.com/zy1000.html
> ARM7 ARM9 XScale Cortex
> JTAG debugger and flash programmer
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