[llvm] [llvm][CodeGen] Fix the empty interval issue in Window Scheduler(#128714) (PR #129204)

[Matt Arsenault via llvm-commits]

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@@ -344,13 +350,16 @@ void ModuloScheduleExpander::generateEpilog(
 static void replaceRegUsesAfterLoop(unsigned FromReg, unsigned ToReg,
                                     MachineBasicBlock *MBB,
                                     MachineRegisterInfo &MRI,
-                                    LiveIntervals &LIS) {
+                                    LiveIntervals &LIS,
+                                    SmallVector<Register> &EmptyIntervalRegs) {
   for (MachineOperand &O :
        llvm::make_early_inc_range(MRI.use_operands(FromReg)))
     if (O.getParent()->getParent() != MBB)
       O.setReg(ToReg);
-  if (!LIS.hasInterval(ToReg))
+  if (!LIS.hasInterval(ToReg)) {
     LIS.createEmptyInterval(ToReg);
+    EmptyIntervalRegs.push_back(ToReg);
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arsenm wrote:

This seems like a hazardous update strategy. You're creating temporarily wrong liveness and recomputing it from scratch later. If you're going to manage the interval here, do it completely or not at all 

https://github.com/llvm/llvm-project/pull/129204