[llvm] [MachineScheduler] Fix physreg dependencies of ExitSU (PR #123541)

[Sergei Barannikov via llvm-commits]

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@@ -263,6 +275,9 @@ void ScheduleDAGInstrs::addPhysRegDataDeps(SUnit *SU, unsigned OperIdx) {
       bool ImplicitPseudoUse = false;
       SDep Dep;
       if (UseOpIdx < 0) {
+        // FIXME: UseOpIdx can be passed to computeOperandLatency, which can
+        //   pass it to findUseIdx, which treats it as unsigned. If this is
+        //   the expected behavior, it should be commented.
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s-barannikov wrote:

> Operand indexes are unsigned. Where is the signed usage?

Here: https://github.com/llvm/llvm-project/blob/8b87e2ffcf597d7eba1cf3ff12bea1fae1f999e6/llvm/lib/CodeGen/ScheduleDAGInstrs.cpp#L277

https://github.com/llvm/llvm-project/pull/123541