[LLVMdev] Node definitions, Pseudo ops and lowering SELECT/COND_BRANCH to branch instructions

[Andy Nisbet]

Hello,
Im back trying to finish my backend to a simple RISC cpu SABRE now 
that most of the tedious process of examining undergraduate students 
is out of the way. I have managed to describe the registers and the 
instructions in the architecture and have added support for 32 bit 
immediates (thanks to Christopher Lamb) as the instruction set only 
supports 17 bit immediates directly.

Could anyone please describe the main features of how an llvm 
instruction such as SELECT and COND_BRANCH are lowered/expanded into 
branch instructions?
I have seen this thread
http://lists.cs.uiuc.edu/pipermail/llvmdev/2006-October/006892.html
but was wondering if anyone could comment on this process in a little 
more detail.

I presume I need to define various target independent nodes, like below ...
def SDT_SABREcondbr  : SDTypeProfile<0,3,  // no result, 3 operands, 
rega, regb, immediate
[ SDTCisVT<1,i32>, SDTCisVT<2,OtherVT> ]>;

def SABREcondbranch: SDNode <"SABREISD::COND_BRANCH" , SDT_SABREcondbr,
                         [SDNPHasChain]>;

def COND_BRANCH: Pseudo<(ops IntRegs:$a, IntRegs:$b, target:$dst),
         "${:comment}COND_BRANCH $a, $b, $dst",
         [(SABREcondbranch IntRegs:$a, IntRegs:$bm bb:$dst)]>;

and I need a setOperationAction(ISD::BRCOND, MVT::Other, Expand);

Im a little confused as to how BRCOND (or select for that matter) are 
then actually lowered to a particular branch instruction that is for 
the sake of argument ....
BLE %a, %b, newpc ie iff %a <= %b then branch to newpc.

I have been looking at the PPC and the sparc backend sources for 
information. I'd just like a little more (abstract) clarification 
before diving into this in some detail and gdb-ing through the 
specifics of the PPC/Sparc codegeneration for my helloworld bytecode.

Thanks,
         Andy

      Dr. Andy Nisbet: URL http://www.docm.mmu.ac.uk/STAFF/A.Nisbet
Department of Computing and Mathematics, John Dalton Building, Manchester
        Metropolitan University, Chester Street, Manchester M1 5GD, UK.
Email: A.Nisbet at mmu.ac.uk, Phone:(+44)-161-247-1556; Fax:(+44)-161-247-1483.

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