[llvm] [SelectionDAG] Faster generic expansion of llvm.pext and llvm.pdep (PR #227292)

[Simon Pilgrim via llvm-commits]

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@@ -5736,6 +5736,17 @@ class LLVM_ABI TargetLowering : public TargetLoweringBase {
   /// \returns The expansion if successful, SDValue() otherwise
   SDValue expandCLMUL(SDNode *N, SelectionDAG &DAG) const;
 
+  /// How expandPEXT and expandPDEP compute the result: Hacker's Delight 7-4
+  /// and 7-5 with carry-less multiplies, one shift-and-XOR network over the
+  /// whole word, or such networks within each byte followed by a variable
+  /// shift per byte.
+  enum class PEXTExpansion { CarryLessMultiply, WholeWord, Bytewise };
+
+  /// Return how to expand PEXT and PDEP of type \p VT. By default, carry-less
+  /// multiplies where CLMUL is legal (or can be lowered, for vectors), bytewise
+  /// for scalars of 16 to 64 bits, and the whole-word network otherwise.
+  virtual PEXTExpansion preferredPEXTExpansion(SelectionDAG &DAG, EVT VT) const;
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RKSimon wrote:

Please don't add TLI callbacks - use isOperationLegal style checks if you can, or have targets use custom lowering that calls specific TargetLowering::expandPDEP* variants.

https://github.com/llvm/llvm-project/pull/227292