[llvm] [LiveVariables] Mark use as implicit-def if defined at instr (PR #119446)

[Matt Arsenault via llvm-commits]

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@@ -277,11 +277,13 @@ void LiveVariables::HandlePhysRegUse(Register Reg, MachineInstr &MI) {
           continue;
         if (PartDefRegs.count(SubReg))
           continue;
+
+        // Check if SubReg is defined at LastPartialDef.
+        bool IsDefinedHere = LastPartialDef->modifiesRegister(SubReg, TRI);
         // This part of Reg was defined before the last partial def. It's killed
         // here.
-        LastPartialDef->addOperand(MachineOperand::CreateReg(SubReg,
-                                                             false/*IsDef*/,
-                                                             true/*IsImp*/));
+        LastPartialDef->addOperand(
+            MachineOperand::CreateReg(SubReg, IsDefinedHere, true /*IsImp*/));
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arsenm wrote:

Swapping what used to be a use with a def sounds wrong. Do we need multiple implicit operands to model this liveness?

https://github.com/llvm/llvm-project/pull/119446