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

[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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jofrn wrote:

Yes, it can be skipped. However, `llvm/test/CodeGen/AArch64/ldrpre-ldr-merge.mir` would need to be updated to not mark `implicit-def $w2_hi`, which may be incorrect, so I believe it is better to leave the `implicit-def` in.

I'd argue that we are not really replacing what used to be a use with a def. They should have been defs all along because the sub or super register has a definition at this line.

If we do not fix it here, the logic to fix it in LiveRangeCalc.cpp needs to update `LiveRangeCalc::findReachingDefs` to take into consideration definitions that are found at the current instruction. And it propagates a change to be made in RegisterPressure.cpp, so I think it is cleaner to have it here.

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