[LLVMdev] Initial thoughts on an LLVM backend for N-address generic assembly

[nkavv at physics.auth.gr]

Hi all,

i'm just starting out with LLVM (although i've been observing its  
evolution since that first release some years ago :)

I would like to develop a backend for a generic assembly-like  
language, called NAC (N-Address Code). More info on NAC can be found  
here:
http://www.nkavvadias.com/hercules/nac-refman.html (HTML)
http://www.nkavvadias.com/hercules/nac-refman.pdf (PDF)

You can consider NAC similar to an LLVM subset for hardware synthesis.  
Although NAC was developed independently, certain decisions taken when  
designing it, may or may not defer from other textual IRs like LLVM or  
PTX.

I have a number of questions:

1) It seems to me that a template backend either the C backend or  
NVPTX are possible starting points. However, only NVPTX makes use of  
tblgen which removes some burder from the backend developer. I also  
think that a tblgen-based backend like NVPTX is easier to maintain in  
the long term. What do you think?

2) NAC uses the following statement form (for any given statement):

dst1, dst2, ..., dstm <= op src1, src2, ..., srcn;

which expresses an operation op with n source operands and m  
destination operands. Do you think that tblgen supports such form or  
should I sanitize it?

3) The NAC memory model uses separate address spaces per array. A  
general-use stack/heap might also be supported. Should I use dot  
directives to declare each address space in use for a given  
translation unit/module?

4) What is the maximum that I can get from tblgen? Which C++ source  
files cannot be generated by .td files and always have to be coded by  
hand?

In the end, source code for the LLVM->NAC backend and a NAC  
interpreter will be released, probably as third-party BSD-licensed  
tools.


Best regards
Nikolaos Kavvadias