[llvm] r317926 - [DAGcombine] Do not replace truncate node by itself when doing constant folding, this trigger needless extra rounds of combine for nothing. NFC

[Friedman, Eli via llvm-commits]

On 11/10/2017 12:59 PM, Amaury Sechet via llvm-commits wrote:
> Author: deadalnix
> Date: Fri Nov 10 12:59:53 2017
> New Revision: 317926
>
> URL: http://llvm.org/viewvc/llvm-project?rev=317926&view=rev
> Log:
> [DAGcombine] Do not replace truncate node by itself when doing constant folding, this trigger needless extra rounds of combine for nothing. NFC
>
> Modified:
>      llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
>
> Modified: llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp?rev=317926&r1=317925&r2=317926&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
> +++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Fri Nov 10 12:59:53 2017
> @@ -8360,12 +8360,18 @@ SDValue DAGCombiner::visitTRUNCATE(SDNod
>     // noop truncate
>     if (N0.getValueType() == N->getValueType(0))
>       return N0;
> -  // fold (truncate c1) -> c1
> -  if (DAG.isConstantIntBuildVectorOrConstantInt(N0))
> -    return DAG.getNode(ISD::TRUNCATE, SDLoc(N), VT, N0);
> +
>     // fold (truncate (truncate x)) -> (truncate x)
>     if (N0.getOpcode() == ISD::TRUNCATE)
>       return DAG.getNode(ISD::TRUNCATE, SDLoc(N), VT, N0.getOperand(0));
> +
> +  // fold (truncate c1) -> c1
> +  if (DAG.isConstantIntBuildVectorOrConstantInt(N0)) {
> +    SDValue C = DAG.getNode(ISD::TRUNCATE, SDLoc(N), VT, N0);
> +    if (C.getNode() != N)

C.getNode() != N will always be true, I think, due to constant-folding 
in getNode().  Or am I missing something?

-Eli

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