[llvm] [SelectionDAG] Add PARTIAL_REDUCE_U/SMLA ISD Nodes (PR #125207)

[Paul Walker via llvm-commits]

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@@ -135,6 +135,13 @@ static cl::opt<unsigned> SwitchPeelThreshold(
              "switch statement. A value greater than 100 will void this "
              "optimization"));
 
+// FIXME : This is a temporary flag, and is used to help transition to
+// performing lowering the proper way using the new PARTIAL_REDUCE_MLA ISD
+// nodes.
+static cl::opt<bool> NewPartialReduceLowering(
+    "new-partial-reduce-lowering", cl::init(false), cl::ReallyHidden,
+    cl::desc("Use the new method of lowering partial reductions."));
+
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paulwalker-arm wrote:

Is this really necessary?  To me it looks like we always end up using the same code the only difference being the flag trigger expansion earlier.  Is there a genuine problem the new command line option stops us from hitting?

I'm not strictly against having such a temporary flag but given the intrinsic is already tightly controlled it just feels overly paranoid to me.

https://github.com/llvm/llvm-project/pull/125207