[llvm] [PowerPC] Enable custom lowering for bswap64 builtin on Power8 64 bits with improved parallelism (PR #187259)

[Aditi Medhane via llvm-commits]

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@@ -11622,6 +11623,46 @@ SDValue PPCTargetLowering::LowerBSWAP(SDValue Op, SelectionDAG &DAG) const {
   SDLoc dl(Op);
   if (!Subtarget.isPPC64())
     return Op;
+
+  // Apply the optimization to Power8 and 64-bits which allows parallelism for
+  // rotate instructions which should make the bswap64 builtin faster.
+  if (!Subtarget.hasP9Vector()) {
+    SDValue Input = Op.getOperand(0);
+
+    auto Swap32 = [&](SDValue Val32) -> SDValue {
+      SDValue Rot = DAG.getNode(ISD::ROTL, dl, MVT::i32, Val32,
+                                DAG.getConstant(8, dl, MVT::i32));
+      SDValue Swap =
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AditiRM wrote:

write lambda function for this specific utility

https://github.com/llvm/llvm-project/pull/187259