[llvm] [GlobalISel] Add multi-way splitting support for wide scalar shifts. (PR #155353)

[David Green via llvm-commits]

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@@ -364,6 +364,42 @@ class LegalizerHelper {
                                                       LLT HalfTy,
                                                       LLT ShiftAmtTy);
 
+  /// Multi-way shift legalization: directly split wide shifts into target-sized
+  /// parts in a single step, avoiding recursive binary splitting.
+  LLVM_ABI LegalizeResult narrowScalarShiftMultiway(MachineInstr &MI,
+                                                    LLT TargetTy);
+
+  /// Optimized path for constant shift amounts using static indexing.
+  /// Directly calculates which source parts contribute to each output part
+  /// without generating runtime select chains.
+  LLVM_ABI LegalizeResult narrowScalarShiftByConstantMultiway(MachineInstr &MI,
+                                                              const APInt &Amt,
+                                                              LLT TargetTy,
+                                                              LLT ShiftAmtTy);
+
+  struct ShiftParams {
+    Register WordShift;   // Number of complete words to shift
+    Register BitShift;    // Number of bits to shift within words
+    Register InvBitShift; // Complement bit shift (TargetBits - BitShift)
+    Register Zero;        // Zero constant for SHL/LSHR fill
+    Register SignBit;     // Sign extension value for ASHR fill
+  };
+
+  /// Generates a single output part for constant shifts using direct indexing.
+  /// Calculates which source parts contribute and how they're combined.
+  Register buildConstantShiftPart(unsigned Opcode, unsigned PartIdx,
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davemgreen wrote:

+LLVM_ABI maybe? I'm not sure when that is needed and when it isn't.

https://github.com/llvm/llvm-project/pull/155353