[llvm] [SelectionDAG] Clean up SCALAR_TO_VECTOR handling in SimplifyDemanded… (PR #157027)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: Björn Pettersson (bjope)

<details>
<summary>Changes</summary>

…VectorElts

This patch reverts changes from commit 585e65d3307f5f0 (https://reviews.llvm.org/D104250), as it doesn't seem to be needed nowadays.

The removed code was doing a recursive call to
SimplifyDemandedVectorElts trying to simplify the vector %vec when finding things like
  (SCALAR_TO_VECTOR (EXTRACT_VECTOR_ELT %vec, 0))

I figure that (EXTRACT_VECTOR_ELT %vec, 0) would be simplified based on only demanding element zero regardless of being used in a SCALAR_TO_VECTOR operation or not.

It had been different if the code tried to simplify the whole expression as %vec. That could also have motivate why to make element zero a special case. But it only simplified %vec without folding away the SCALAR_TO_VECTOR.

---
Full diff: https://github.com/llvm/llvm-project/pull/157027.diff


1 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp (-21) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index a8c7c16e2fa22..fd6d20e146bb2 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -3227,27 +3227,6 @@ bool TargetLowering::SimplifyDemandedVectorElts(
       KnownUndef.setAllBits();
       return TLO.CombineTo(Op, TLO.DAG.getUNDEF(VT));
     }
-    SDValue ScalarSrc = Op.getOperand(0);
-    if (ScalarSrc.getOpcode() == ISD::EXTRACT_VECTOR_ELT) {
-      SDValue Src = ScalarSrc.getOperand(0);
-      SDValue Idx = ScalarSrc.getOperand(1);
-      EVT SrcVT = Src.getValueType();
-
-      ElementCount SrcEltCnt = SrcVT.getVectorElementCount();
-
-      if (SrcEltCnt.isScalable())
-        return false;
-
-      unsigned NumSrcElts = SrcEltCnt.getFixedValue();
-      if (isNullConstant(Idx)) {
-        APInt SrcDemandedElts = APInt::getOneBitSet(NumSrcElts, 0);
-        APInt SrcUndef = KnownUndef.zextOrTrunc(NumSrcElts);
-        APInt SrcZero = KnownZero.zextOrTrunc(NumSrcElts);
-        if (SimplifyDemandedVectorElts(Src, SrcDemandedElts, SrcUndef, SrcZero,
-                                       TLO, Depth + 1))
-          return true;
-      }
-    }
     KnownUndef.setHighBits(NumElts - 1);
     break;
   }

``````````

</details>


https://github.com/llvm/llvm-project/pull/157027