[llvm] [X86] Lower vXi8 multiplies by constant using PMADDUBSW on SSSE3+ targets (PR #95403)

[Phoebe Wang via llvm-commits]

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@@ -28506,6 +28508,31 @@ static SDValue LowerMUL(SDValue Op, const X86Subtarget &Subtarget,
 
     MVT ExVT = MVT::getVectorVT(MVT::i16, NumElts / 2);
 
+    // For vXi8 mul-by-constant, try PMADDUBSW to avoid the need for extension.
+    // Don't do this if we only need to unpack one half.
+    if (Subtarget.hasSSSE3() &&
+        ISD::isBuildVectorOfConstantSDNodes(B.getNode())) {
+      bool IsLoLaneZeroOrUndef = true;
+      bool IsHiLaneZeroOrUndef = true;
+      for (auto [Idx, Val] : enumerate(B->ops())) {
+        if ((Idx % NumEltsPerLane) >= (NumEltsPerLane / 2))
+          IsHiLaneZeroOrUndef &= isNullConstantOrUndef(Val);
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phoebewang wrote:

IsHiLaneAllZeroOrUndef?

https://github.com/llvm/llvm-project/pull/95403