[MachineSink] Drop debug info for instructions deleted by sink-and-fold

llvm/lib/CodeGen/MachineSink.cpp

@@ -510,30 +510,23 @@ bool MachineSinking::PerformSinkAndFold(MachineInstr &MI,
     LLVM_DEBUG(dbgs() << "Sinking copy of"; MI.dump(); dbgs() << "into";
                SinkDst->dump());
     if (SinkDst->isCopy()) {
+      // TODO: After performing the sink-and-fold, the original instruction is
+      // deleted. Its value is still available (in a hard register), so if there
+      // are debug instructions which refer to the (now deleted) virtual
+      // register they could be updated to refer to the hard register, in
+      // principle. However, it's not clear how to do that, moreover in some
+      // cases the debug instructions may need to be replicated proportionally
+      // to the number of the COPY instructions replaced and in some extreme
+      // cases we can end up with quadratic increase in the number of debug
+      // instructions.
+
       // Sink a copy of the instruction, replacing a COPY instruction.
       MachineBasicBlock::iterator InsertPt = SinkDst->getIterator();
       Register DstReg = SinkDst->getOperand(0).getReg();
       TII->reMaterialize(*SinkDst->getParent(), InsertPt, DstReg, 0, MI, *TRI);
-      // If the original instruction did not have source location, reuse a one
-      // from the COPY.
+      // Reuse the source location from the COPY.
       New = &*std::prev(InsertPt);
-      if (const DebugLoc &NewLoc = New->getDebugLoc(); !NewLoc)
-        New->setDebugLoc(SinkDst->getDebugLoc());
-      // Sink DBG_VALUEs, which refer to the original instruction's destination
-      // (DefReg).
-      MachineBasicBlock &SinkMBB = *SinkDst->getParent();
-      auto &DbgUsers = SeenDbgUsers[DefReg];
-      for (auto &U : DbgUsers) {
-        MachineInstr *DbgMI = U.getPointer();
-        if (U.getInt())
-          continue;
-        MachineInstr *NewDbgMI = SinkDst->getMF()->CloneMachineInstr(DbgMI);
-        SinkMBB.insertAfter(InsertPt, NewDbgMI);
-        for (auto &SrcMO : DbgMI->getDebugOperandsForReg(DefReg)) {
-          auto &DstMO = NewDbgMI->getOperand(SrcMO.getOperandNo());
-          DstMO.setReg(DstReg);
-        }
-      }
+      New->setDebugLoc(SinkDst->getDebugLoc());
     } else {
       // Fold instruction into the addressing mode of a memory instruction.
       New = TII->emitLdStWithAddr(*SinkDst, MaybeAM);