[llvm] [TwoAddressInstruction] Update LiveIntervals after INSERT_SUBREG with undef read (PR #66211)

[Jay Foad via llvm-commits]

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@@ -1868,12 +1868,16 @@ bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &Func) {
             // %reg.subidx.
             LaneBitmask LaneMask =
                 TRI->getSubRegIndexLaneMask(mi->getOperand(0).getSubReg());
-            SlotIndex Idx = LIS->getInstructionIndex(*mi);
+            SlotIndex Idx = LIS->getInstructionIndex(*mi).getRegSlot();
             for (auto &S : LI.subranges()) {
               if ((S.LaneMask & LaneMask).none()) {
-                LiveRange::iterator UseSeg = S.FindSegmentContaining(Idx);
-                LiveRange::iterator DefSeg = std::next(UseSeg);
-                S.MergeValueNumberInto(DefSeg->valno, UseSeg->valno);
+                LiveRange::iterator DefSeg = S.FindSegmentContaining(Idx);
+                if (mi->getOperand(0).isUndef()) {
+                  S.removeValNo(DefSeg->valno);
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jayfoad wrote:

TBH I am pretty nervous about all this low level fiddling around with live ranges. I would be much happier if there was some higher level API I could use to do this kind of update.

https://github.com/llvm/llvm-project/pull/66211