[llvm] [DebugInfo][RegisterCoalescer] Drop DBG_VALUEs with unsupported register class (PR #143132)

[Matt Arsenault via llvm-commits]

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@@ -118,28 +118,44 @@ MachineRegisterInfo::constrainRegAttrs(Register Reg,
   return true;
 }
 
-bool
+std::pair<bool, SmallVector<MachineInstr *, 8>>
 MachineRegisterInfo::recomputeRegClass(Register Reg) {
   const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
   const TargetRegisterClass *OldRC = getRegClass(Reg);
   const TargetRegisterInfo *TRI = getTargetRegisterInfo();
+  // Largest legal super-class for non-debug uses.
   const TargetRegisterClass *NewRC = TRI->getLargestLegalSuperClass(OldRC, *MF);
+  // Largest legal super-class for all uses.
+  const TargetRegisterClass *NewRCWithDebug = NewRC;
+  SmallVector<MachineInstr *, 8> DebugInsns;
 
   // Stop early if there is no room to grow.
   if (NewRC == OldRC)
-    return false;
+    return {false, {}};
 
   // Accumulate constraints from all uses.
-  for (MachineOperand &MO : reg_nodbg_operands(Reg)) {
-    // Apply the effect of the given operand to NewRC.
+  for (MachineOperand &MO : reg_operands(Reg)) {
     MachineInstr *MI = MO.getParent();
     unsigned OpNo = &MO - &MI->getOperand(0);
-    NewRC = MI->getRegClassConstraintEffect(OpNo, NewRC, TII, TRI);
+    if (MO.isDebug()) {
+      DebugInsns.push_back(MI);
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arsenm wrote:

This is much too aggressive, not every debug instruction needs to be dropped 

https://github.com/llvm/llvm-project/pull/143132