[llvm] [RISCV] Form vredsum from explode_vector + scalar (left) reduce (PR #67821)

[Craig Topper via llvm-commits]

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@@ -11122,6 +11122,85 @@ void RISCVTargetLowering::ReplaceNodeResults(SDNode *N,
   }
 }
 
+/// Perform two related transforms whose purpose is to incrementally recognize
+/// an explode_vector followed by scalar reduction as a vector reduction node.
+/// This exists to recover from a deficiency in SLP which can't handle
+/// forrests with multiple roots sharing common nodes.  In some cases, one
+/// of the trees will be vectorized, and the other will remain (unprofitably)
+/// scalarized.
+static SDValue combineBinOpOfExtractToReduceTree(SDNode *N, SelectionDAG &DAG,
+                                                 const RISCVSubtarget &Subtarget) {
+
+  // This transforms need to run before all integer types have been legalized
+  // to i64 (so that the vector element type matches the add type), and while
+  // it's safe to introduce odd sized vector types.
+  if (DAG.NewNodesMustHaveLegalTypes)
+    return SDValue();
+
+  const SDLoc DL(N);
+  const EVT VT = N->getValueType(0);
+  const unsigned Opc = N->getOpcode();
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topperc wrote:

Unused var in release builds I think?

https://github.com/llvm/llvm-project/pull/67821