[llvm] [DAGCombiner, NVPTX] Port 'rem' custom combine from NVPTX to generic combiner (PR #167147)

[Simon Pilgrim via llvm-commits]

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@@ -900,6 +900,41 @@ namespace {
                          ISD::NodeType ExtType);
   };
 
+/// Generic remainder optimization : Folds a remainder operation (A % B) by reusing the computed quotient (A / B).
+static SDValue PerformREMCombineGeneric(SDNode *N, DAGCombiner &DC,
+                                        CodeGenOptLevel OptLevel) {
+  assert(N->getOpcode() == ISD::SREM || N->getOpcode() == ISD::UREM);
+
+  // Don't do anything at less than -O2.
+  if (OptLevel < CodeGenOptLevel::Default)
+    return SDValue();
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RKSimon wrote:

why is this necessary?

https://github.com/llvm/llvm-project/pull/167147