[llvm] [AArch64] Fix #94909: Optimize vector fmul(sitofp(x), 0.5) -> scvtf(x, 2) (PR #141480)

[JP Hafer via llvm-commits]

================
@@ -3952,6 +3960,129 @@ static bool checkCVTFixedPointOperandWithFBits(SelectionDAG *CurDAG, SDValue N,
   return true;
 }
 
+static bool checkCVTFixedPointOperandWithFBitsForVectors(SelectionDAG *CurDAG,
+                                                         SDValue N,
+                                                         SDValue &FixedPos,
+                                                         unsigned FloatWidth,
+                                                         bool isReciprocal) {
+
+  // N must be a bitcast/nvcast of a vector float type.
+  if (!((N.getOpcode() == ISD::BITCAST ||
+         N.getOpcode() == AArch64ISD::NVCAST) &&
+        N.getValueType().isVector() && N.getValueType().isFloatingPoint())) {
+    return false;
+  }
+
+  if (N.getNumOperands() == 0)
+    return false;
+  SDValue ImmediateNode = N.getOperand(0);
+
+  bool isSplatConfirmed = false;
+
+  if (ImmediateNode.getOpcode() == AArch64ISD::DUP ||
+      ImmediateNode.getOpcode() == ISD::SPLAT_VECTOR) {
+    // These opcodes inherently mean a splat.
+    isSplatConfirmed = true;
+  } else if (ImmediateNode.getOpcode() == ISD::BUILD_VECTOR) {
+    // For BUILD_VECTOR, we must explicitly check if it's a constant splat.
+    BuildVectorSDNode *BVN = cast<BuildVectorSDNode>(ImmediateNode.getNode());
+    APInt SplatValue;
+    APInt SplatUndef;
+    unsigned SplatBitSize;
+    bool HasAnyUndefs;
+    if (BVN->isConstantSplat(SplatValue, SplatUndef, SplatBitSize,
+                             HasAnyUndefs)) {
+      isSplatConfirmed = true;
+    } else {
+      return false;
+    }
+  } else if (ImmediateNode.getOpcode() == AArch64ISD::MOVIshift) {
+    // This implies that the DAG structure was (DUP (MOVIshift C)) or
+    // (BUILD_VECTOR (MOVIshift C)).
+    isSplatConfirmed = true;
+  } else {
+    return false;
+  }
+
+  // If we reached here, isSplatConfirmed should be true and ScalarSourceNode
+  // should be set. But just in case ...
+  if (!isSplatConfirmed)
----------------
jph-13 wrote:

I left this and removed the returns from above. I thought the code is more descriptive that way.

https://github.com/llvm/llvm-project/pull/141480