[LLVMdev] SelectionDAG

[Owen Anderson]

Joshua,

You can achieve this through the use of SDNodeXForm and Pat's.  Here's a snippet to get your started.

def imm_neg_XFORM : SDNodeXForm<imm, [{
  return CurDAG->getTargetConstant(-N->getSExtValue(), MVT::i8);
}]>;
def imm0_255_neg : PatLeaf<(i8 imm), [{
  unsigned Val = N->getZExtValue();
  return Val > 0 && Val < 256;
}], imm_neg_XFORM>;

def : Pat<(add GPR8:$src1, imm0_255_neg:$src2),
                 (SUB8ri GPR8:$src1, imm0_255_neg:$src3)>;

--Owen
             
On Jan 10, 2012, at 1:23 PM, Joshua Nedrud wrote:

> Hello,
> 	I am working on a AVR backend and have a version up and running that will convert LLVM IR code to assembly code for my target.  I have written a bunch of instructions from the AVR Instruction Set in AVRInstrInfo.td and not much else. In a simple test case I am attempting to compile (if that is the word you are supposed to use for this operation) test.ll:
> 
> define i8 @foo(i8 %a, i8 %b) {
> entry:
> 	%c = sub i8 %a, 5
> 	ret i8 %c
> }
> 
> with `build/Debug/bin/llc -march=avr llvm.ll -o -` and I receive:
> 
> 	.file	"test.ll"
> 	.text
> 	.global	foo
> 	.type	foo, at function
> foo:                                    # @foo
> # BB#0:                                 # %entry
> 	MOV	r22, r24
> 	LDI	r24, -5
> 	ADD	r24, r18
> 	RET
> .tmp0:
> 	.size	foo, .tmp0-foo
> 
> While I would like the output as such:
> 
> 	.file	"test.ll"
> 	.text
> 	.global	foo
> 	.type	foo, at function
> foo:                                    # @foo
> # BB#0:                                 # %entry
> 	SUB	r24, 5
> 	RET
> .tmp0:
> 	.size	foo, .tmp0-foo
> 
> With that my question is how to get LLVM to select SUB instead of the pattern above. I have tried to place SUB8ri (see below) before the ADD8rr instruction in the .td file, but that does not seem to make a difference. *things to note, the AVR instruction set does not contain a ADD8ri, so I would eventually like to convert all add register and immediate to subtract register and -immediate, but for now if I could just get LLVM to use SUB8ri in the most straightforward case that would work for me. Thanks,
> 	Joshua
> 
> // SUB8ri definition
> let Constraints = "$src = $dst" in
> def SUB8ri : Instruction {
>  let Namespace = "AVR";
>  dag OutOperandList = (outs GPR8:$dst);
>  dag InOperandList  = (ins GPR8:$src, i8imm:$src2);
>  let AsmString   = "SUB\t{$dst, $src2}";
>  let Pattern = [(set GPR8:$dst, (opnode GPR8:$src, imm:$src2))];
> }
> 
> Joshua Nedrud
> Master of Science, Biomedical Engineering
> 
> 
> 
> 
> 
> 
> 
> 
> 
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