[PATCH] D41924: dagcombine: Transfer debug information when folding (zext (truncate x)) -> (zext (truncate x))

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon added inline comments.


================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:7803
+        // Transfer the debug info; the new node is equivalent to N0.
+        DAG.transferDbgValues(N0, ZExtOrTrunc);
+        return ZExtOrTrunc;
----------------
aprantl wrote:
> RKSimon wrote:
> > I'm surprised we need to do this - can we achieve the same by using SDLoc(N0) instead of SDLoc(N) in both the above?
> Just to make sure we are talking about the same thing, I changed the code to be
> 
> ```
>     if (SrcVT.bitsLT(VT)) {
>       if (!LegalOperations || (TLI.isOperationLegal(ISD::AND, SrcVT) &&
>                                TLI.isOperationLegal(ISD::ZERO_EXTEND, VT))) {
>         SDValue Op = N0.getOperand(0);
>         Op = DAG.getZeroExtendInReg(Op, SDLoc(N0), MinVT.getScalarType());
>         AddToWorklist(Op.getNode());
>         SDValue ZExtOrTrunc = DAG.getZExtOrTrunc(Op, SDLoc(N0), VT);
>         // Transfer the debug info; the new node is equivalent to N0.
>         //DAG.transferDbgValues(N0, ZExtOrTrunc);
>         return ZExtOrTrunc;
>       }
>     }
> ```
> 
> That does *not* preserve the DBG_VALUE.
OK - but its very rare to have to call transferDbgValues directly like this - I just have the feeling that there is a better way......


Repository:
  rL LLVM

https://reviews.llvm.org/D41924