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

[Björn Pettersson via llvm-commits]

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

…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.

>From defab1bcd6eb9bfc13386fa9da3e6f42c940d3dd Mon Sep 17 00:00:00 2001
From: Bjorn Pettersson <bjorn.a.pettersson at ericsson.com>
Date: Thu, 4 Sep 2025 18:35:10 +0200
Subject: [PATCH] [SelectionDAG] Clean up SCALAR_TO_VECTOR handling in
 SimplifyDemandedVectorElts

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.
---
 .../CodeGen/SelectionDAG/TargetLowering.cpp   | 21 -------------------
 1 file changed, 21 deletions(-)

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;
   }