[LLVMdev] Re: LLVM to SUIF-MACH VM binary

[Chris Lattner]

On Tue, 18 Jan 2005, John Cortes wrote:
> Hi Chris,

Hi!  I'm CC'ing the llvmdev list for the benefit of others.

> Since I see you're very involved in LLVM, I need a little guidance on getting 
> from C to MACH-SUIF.
>
> I've been given the task of using LLVM to translate C code to another VM 
> architecture known as MACH-SUIF.  For this architecture, i don't think 
> there's an assembler (but there is a disassembler), so I'm going to have to 
> generate a binary file output.

Okay, that by itself shouldn't be a problem.  We do not currently support 
emitting .o files directly, but it has been planned for quite some time.

> Could I edit and add to the .../lib/Target directory and add in a new target 
> that outputs the binary that this new VM supports?  Do you think I would need 
> to add to any other directories withing the LLVM directories?

The bulk of your work will certainly be in your lib/Target directory.  I 
would start simple, and use the debugging output (enabled with 
-print-machineinstrs) to see if you're generating correct code when you 
start.  The code you have to add outside of lib/Target should be 
restricted to the work needed to get .o files writing.

For your project, the tricky part will be that you have to start emitting 
.o files before you can really test out the system.  This increases the 
number of dependencies that need to implemented before you can get 
"void main() {return;}" running, which is always the first milestone.

In order to get .o files writing, you'll want to implement the CodeEmitter 
interfaces, which are used to write binary machine code to memory 
(currently used by the JIT).  After that, you'll also want to add your new 
support for writing .o files (which is basically just adding file headers 
and packaging up relocations).  For this, you can choose to either add a 
new virtual method to the TargetMachine class "addPassesToEmitObjectFile", 
which will enable your functionality, or you can build it into the LLC 
tool directly.  If you chose to build it into LLC, you can use the results 
of the MachineCodeEmitter and addPassesToEmitMachineCode.  At this point, 
I'm not sure which way will work best for you.

If possible, it would be nice if you could make your .o file writing code 
as target-independent as possible (potentially defining a new 
TargetObjectFile interface for any hooks you need).  In particular, if 
MACH-SUIF is an ELF-based system, it would be nice to be able to use the 
ELF-specific routines to write X86-ELF or PPC-ELF files as well (assuming 
the hooks were implemented).  Clearly you can choose to worry about this 
or not at your discretion.

Can you say a little bit about MACH-SUIF?  With a brief google search, I 
didn't turn up anything that described the architecture.  Is it a 
RISC-like machine with 32-bit instruction words?

-Chris

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