[llvm] [CodeGen][TII] Allow reassociation on custom operand indices (PR #88306)

[Min-Yih Hsu via llvm-commits]

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@@ -1051,43 +1051,45 @@ static std::pair<bool, bool> mustSwapOperands(MachineCombinerPattern Pattern) {
   }
 }
 
+void TargetInstrInfo::getReassociateOperandIdx(
+    const MachineInstr &Root, MachineCombinerPattern Pattern,
+    std::array<unsigned, 5> &OperandIndices) const {
+  switch (Pattern) {
+  case MachineCombinerPattern::REASSOC_AX_BY:
+    OperandIndices = {1, 1, 1, 2, 2};
+    break;
+  case MachineCombinerPattern::REASSOC_AX_YB:
+    OperandIndices = {2, 1, 2, 2, 1};
+    break;
+  case MachineCombinerPattern::REASSOC_XA_BY:
+    OperandIndices = {1, 2, 1, 1, 2};
+    break;
+  case MachineCombinerPattern::REASSOC_XA_YB:
+    OperandIndices = {2, 2, 2, 1, 1};
+    break;
+  default:
+    llvm_unreachable("unexpected MachineCombinerPattern");
+  }
+}
+
 /// Attempt the reassociation transformation to reduce critical path length.
 /// See the above comments before getMachineCombinerPatterns().
 void TargetInstrInfo::reassociateOps(
-    MachineInstr &Root, MachineInstr &Prev,
-    MachineCombinerPattern Pattern,
+    MachineInstr &Root, MachineInstr &Prev, MachineCombinerPattern Pattern,
     SmallVectorImpl<MachineInstr *> &InsInstrs,
     SmallVectorImpl<MachineInstr *> &DelInstrs,
+    ArrayRef<unsigned> OperandIndices,
     DenseMap<unsigned, unsigned> &InstrIdxForVirtReg) const {
   MachineFunction *MF = Root.getMF();
   MachineRegisterInfo &MRI = MF->getRegInfo();
   const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
   const TargetRegisterInfo *TRI = MF->getSubtarget().getRegisterInfo();
   const TargetRegisterClass *RC = Root.getRegClassConstraint(0, TII, TRI);
 
-  // This array encodes the operand index for each parameter because the
-  // operands may be commuted. Each row corresponds to a pattern value,
-  // and each column specifies the index of A, B, X, Y.
-  unsigned OpIdx[4][4] = {
-    { 1, 1, 2, 2 },
-    { 1, 2, 2, 1 },
-    { 2, 1, 1, 2 },
-    { 2, 2, 1, 1 }
-  };
-
-  int Row;
-  switch (Pattern) {
-  case MachineCombinerPattern::REASSOC_AX_BY: Row = 0; break;
-  case MachineCombinerPattern::REASSOC_AX_YB: Row = 1; break;
-  case MachineCombinerPattern::REASSOC_XA_BY: Row = 2; break;
-  case MachineCombinerPattern::REASSOC_XA_YB: Row = 3; break;
-  default: llvm_unreachable("unexpected MachineCombinerPattern");
-  }
-
-  MachineOperand &OpA = Prev.getOperand(OpIdx[Row][0]);
-  MachineOperand &OpB = Root.getOperand(OpIdx[Row][1]);
-  MachineOperand &OpX = Prev.getOperand(OpIdx[Row][2]);
-  MachineOperand &OpY = Root.getOperand(OpIdx[Row][3]);
+  MachineOperand &OpA = Prev.getOperand(OperandIndices[1]);
----------------
mshockwave wrote:

It's actually used in `TargetInstrInfo::genAlternativeCodeSequence`, which is `reassociateOps`'s caller, to get the Prev instruction from Root.

https://github.com/llvm/llvm-project/pull/88306