[llvm] [Xtensa] Implement lowering SELECT_CC, SETCC. (PR #97017)

[Sergei Barannikov via llvm-commits]

================
@@ -697,6 +752,86 @@ const char *XtensaTargetLowering::getTargetNodeName(unsigned Opcode) const {
     return "XtensaISD::PCREL_WRAPPER";
   case XtensaISD::RET:
     return "XtensaISD::RET";
+  case XtensaISD::SELECT_CC:
+    return "XtensaISD::SELECT_CC";
   }
   return nullptr;
 }
+
+//===----------------------------------------------------------------------===//
+// Custom insertion
+//===----------------------------------------------------------------------===//
+
+MachineBasicBlock *
+XtensaTargetLowering::emitSelectCC(MachineInstr &MI,
+                                   MachineBasicBlock *MBB) const {
+  const TargetInstrInfo &TII = *Subtarget.getInstrInfo();
+  DebugLoc DL = MI.getDebugLoc();
+
+  MachineOperand &LHS = MI.getOperand(1);
+  MachineOperand &RHS = MI.getOperand(2);
+  MachineOperand &TrueValue = MI.getOperand(3);
+  MachineOperand &FalseValue = MI.getOperand(4);
+  unsigned BrKind = MI.getOperand(5).getImm();
+
+  // To "insert" a SELECT_CC instruction, we actually have to insert
+  // CopyMBB and SinkMBB  blocks and add branch to MBB. We build phi
+  // operation in SinkMBB like phi (TrueVakue,FalseValue), where TrueValue
+  // is passed from MMB and FalseValue is passed from CopyMBB.
+  //   MBB
+  //   |   \
+  //   |   CopyMBB
+  //   |   /
+  //   SinkMBB
+  // The incoming instruction knows the
+  // destination vreg to set, the condition code register to branch on, the
+  // true/false values to select between, and a branch opcode to use.
+  const BasicBlock *LLVM_BB = MBB->getBasicBlock();
+  MachineFunction::iterator It = ++MBB->getIterator();
+
+  MachineFunction *F = MBB->getParent();
+  MachineBasicBlock *CopyMBB = F->CreateMachineBasicBlock(LLVM_BB);
+  MachineBasicBlock *SinkMBB = F->CreateMachineBasicBlock(LLVM_BB);
----------------
s-barannikov wrote:

You will need a code like this here (for a future patch that adds related tests), see 2dcf0512:
```
  // Set the call frame size on entry to the new basic blocks.
  uint64_t CallFrameSize = TII.getCallFrameSizeAt(MI);
  CopyMBB->setCallFrameSize(CallFrameSize);
  SinkMBB->setCallFrameSize(CallFrameSize);
```


https://github.com/llvm/llvm-project/pull/97017