[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
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topperc wrote:

forrests -> forests

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