[LLVMdev] Backend port: Adding negative immediates

Jan Tlatlik jtlatlik at techfak.uni-bielefeld.de
Sun Mar 24 17:16:30 PDT 2013


I'm doing a backend port and I'm having trouble with adds that have
negative immediates.
My architecture only has instructions for subtracting and adding 8bit
immediate values (they will be zero-extended, thus unsigned).
Bigger immediates have to be moved in a register first.

The problem is:
Expressions like "b - 1" result in "add nsw i32 %b, -1" in LLVM IR.
They cannot be matched to the 8bit unsigned immediate add during
Instruction Selection and will eventually be moved in a register.
It would be better to transform these adds with small negative
immediates into subs during Target Lowering.

So I tried to custom lower ISD::ADD in IselLowering:

MxmTargetLowering::MxmTargetLowering(TargetMachine &TM)
  : TargetLowering(TM, new TargetLoweringObjectFileELF()) {
setOperationAction(ISD::ADD, MVT::i32, Custom);

//check for immediate adds with constant values -256 < x < 0 and
transform them
//to sub
SDValue MxmTargetLowering::LowerADD(SDValue Op, SelectionDAG &DAG) const {
    SDValue Op0 = Op.getOperand(0);
    SDValue Op1 = Op.getOperand(1);

    if(ConstantSDNode *CN = dyn_cast<ConstantSDNode>(Op1)) {
        int val = CN->getSExtValue();
        if(val < 0 && val > -256) {
            return DAG.getNode(ISD::SUB, dl, MVT::i32,
                Op0,DAG.getConstant(-val, MVT::i32));
    return Op;

SDValue MxmTargetLowering::
LowerOperation(SDValue Op, SelectionDAG &DAG) const {

    switch(Op.getOpcode()) {
        case ISD::GlobalAddress:
            return LowerGlobalAddress(Op, DAG);
        case ISD::ConstantPool:
            return LowerConstantPool(Op,DAG);
        case ISD::ADD:
            return LowerADD(Op,DAG);
            llvm_unreachable("illegal custom lowering operation");

Unfortunately, this has absolutely no effect. The resultig ISel-DAG has
still the adds with negative immediates.
Maybe I'm doing it completely wrong. Could anyone give me some advice
with that?

Jan Tlatlik

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