[llvm] [AArch64][Codegen]Transform saturating smull to sqdmulh (PR #143671)

[David Green via llvm-commits]

================
@@ -20717,6 +20717,83 @@ static SDValue performBuildVectorCombine(SDNode *N,
   return SDValue();
 }
 
+// A special combine for the vqdmulh family of instructions.
+// smin( sra ( mul( sext v0, sext v1 ) ), SHIFT_AMOUNT ),
+// SATURATING_VAL ) can be reduced to sext(sqdmulh(...))
+static SDValue trySQDMULHCombine(SDNode *N, SelectionDAG &DAG) {
+
+  if (N->getOpcode() != ISD::TRUNCATE)
+    return SDValue();
+
+  EVT VT = N->getValueType(0);
+
+  if (!VT.isVector() || VT.getScalarSizeInBits() > 64)
+    return SDValue();
+
+  SDValue SMin = N->getOperand(0);
+
+  if (SMin.getOpcode() != ISD::SMIN)
+    return SDValue();
+
+  ConstantSDNode *Clamp = isConstOrConstSplat(SMin.getOperand(1));
+
+  if (!Clamp)
+    return SDValue();
+
+  MVT ScalarType;
+  unsigned ShiftAmt = 0;
+  switch (Clamp->getSExtValue()) {
+  case (1ULL << 15) - 1:
+    ScalarType = MVT::i16;
+    ShiftAmt = 16;
+    break;
+  case (1ULL << 31) - 1:
+    ScalarType = MVT::i32;
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davemgreen wrote:

ScalarType isn't used. It could check is matches the SExt scalar type.
I might be good to have test cases for edge conditions like that too.

https://github.com/llvm/llvm-project/pull/143671