[llvm] r353428 - [DAG] Cleanup of unused node in SimplifySelectCC.

[Sanjay Patel via llvm-commits]

Couldn't we simplify this before it's ever created instead of deleting
later?
Ie, we have things like "FoldBUILD_VECTOR()" and "FoldCONCAT_VECTORS()" in
SelectionDAG::getNode(), but no similar simplifications for ISD::SELECT_CC.

On Thu, Feb 7, 2019 at 10:13 AM Nirav Dave via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> Author: niravd
> Date: Thu Feb  7 09:13:55 2019
> New Revision: 353428
>
> URL: http://llvm.org/viewvc/llvm-project?rev=353428&view=rev
> Log:
> [DAG] Cleanup of unused node in SimplifySelectCC.
>
> 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=353428&r1=353427&r2=353428&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (original)
> +++ llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp Thu Feb  7
> 09:13:55 2019
> @@ -18660,14 +18660,21 @@ SDValue DAGCombiner::SimplifySelectCC(co
>    auto *N3C = dyn_cast<ConstantSDNode>(N3.getNode());
>
>    // Determine if the condition we're dealing with is constant.
> -  SDValue SCC = SimplifySetCC(getSetCCResultType(CmpOpVT), N0, N1, CC, DL,
> -                              false);
> -  if (SCC.getNode()) AddToWorklist(SCC.getNode());
> -
> -  if (auto *SCCC = dyn_cast_or_null<ConstantSDNode>(SCC.getNode())) {
> -    // fold select_cc true, x, y -> x
> -    // fold select_cc false, x, y -> y
> -    return !SCCC->isNullValue() ? N2 : N3;
> +  if (SDValue SCC =
> +          SimplifySetCC(getSetCCResultType(CmpOpVT), N0, N1, CC, DL,
> false)) {
> +    AddToWorklist(SCC.getNode());
> +    if (auto *SCCC = dyn_cast<ConstantSDNode>(SCC.getNode())) {
> +      // fold select_cc true, x, y -> x
> +      // fold select_cc false, x, y -> y
> +      bool isNull = SCCC->isNullValue();
> +      SDValue RV = isNull ? N3 : N2;
> +      // Delete SCC if we don't use it.
> +      if (SCCC != RV.getNode())
> +        recursivelyDeleteUnusedNodes(SCCC);
> +      return RV;
> +    }
> +    // Don't combine. Cleanup SCC.
> +    recursivelyDeleteUnusedNodes(SCC.getNode());
>    }
>
>    if (SDValue V =
>
>
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