[llvm] [CodeGen] Fix stale LiveIntervals regmask tables after MachineBasicBl… (PR #209610)

[Nathan Corbyn via llvm-commits]

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@@ -294,6 +294,48 @@ void LiveIntervals::computeRegMasks() {
   }
 }
 
+void LiveIntervals::reassignRegMaskSlots(MachineBasicBlock &Orig,
+                                         MachineBasicBlock &SplitBB) {
+  assert(&Orig != &SplitBB && "expected distinct blocks");
+  std::pair<unsigned, unsigned> &OrigRMB = RegMaskBlocks[Orig.getNumber()];
+  std::pair<unsigned, unsigned> &SplitRMB = RegMaskBlocks[SplitBB.getNumber()];
+
+  // splitAt moved the tail of Orig into SplitBB. RegMaskSlots is sorted, so the
+  // slots now belonging to SplitBB are those in Orig's slice at/after SplitBB's
+  // start index.
+  ArrayRef<SlotIndex> OrigSlots =
+      getRegMaskSlots().slice(OrigRMB.first, OrigRMB.second);
+  unsigned KeptCount = llvm::lower_bound(OrigSlots, getMBBStartIdx(&SplitBB)) -
+                       OrigSlots.begin();
+  if (KeptCount == OrigRMB.second)
+    return; // No regmask slots moved into SplitBB.
+
+  SplitRMB.first = OrigRMB.first + KeptCount;
+  SplitRMB.second = OrigRMB.second - KeptCount;
+  OrigRMB.second = KeptCount;
+}
+
+void LiveIntervals::insertMBBInMapsImpl(
+    MachineBasicBlock *MBB, [[maybe_unused]] bool AssumeRegMaskEmpty) {
+#ifdef EXPENSIVE_CHECKS
+  // Outside of splitAt(), the block is recorded as having no regmask slots, so
+  // it must not contain any.
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cofibrant wrote:

This can be dropped

https://github.com/llvm/llvm-project/pull/209610