Add DAGNode After Load

Code Generation
1

I need to add a custom DAGNode after a extloadf32. Currently trying to add node within customLowering LowerLoad(possibly not the right approach).

But I keep getting errors and not sure how to accomplish this . What I’m asking is there a correct way to do this or just not possible?

Using this to catch the extloadf32 to a lower operation to lower load:
setLoadExtAction(ISD::EXTLOAD, MVT::f64, MVT::f32, Custom);

Lower Load :
SDValue XXXTargetLowering::LowerLOAD(SDValue Op,SelectionDAG &DAG) const {
SDLoc DL(Op);
LoadSDNode *LoadNode = cast(Op);
SDValue SFTD = DAG.getNode(XXXISD:SFTD, DL, MVT::f64, Op);
DAG.ReplaceAllUsesOfValueWith(Op, SFTD);
return Op;
}

The problem is I’m trying to replace all the uses of the result of the load using ReplaceAllUsesofValue() but then the DAG node for SFTD gets messed up: t17: f64 XXXISD:SFTD t17 as SFTD takes the input “Op”. What’s the correct way to fix the uses but the DAG would be correct.

What I want to accomplish:
t16 : f64,ch = load…
t17 : f64 = XXXISD:SFTD t16
t18 : ch = store… t16:1, t17

What happens:
t16 : f64,ch = load…
t17 : f64 = XXXISD:SFTD t17
t18 : ch = store… t16:1, t17

2

That is a problem indeed. Node replacement is handled by the lowering algorithm, you just need to return the legalized code.

Edit: Be aware that if you return identical load, just wrapped in the extra SFTD operation, the lowering code will call your function again on the load.

3

How do I return just the legalized code when the result type of the custom instruction is only one value when it needs two as it’s checking for the chain of the load.

4

Use SelectionDAG::getMergeValues.

1 Like