[WIP][DebugInfo] Allow MachineCopyPropagate to update DebugInfo RegUses

[jmorse]

Summary

This is a work-in-progress related to PR38754 [0], uploaded for visibility rather than review.

The purpose of the patch is to kill one use of changeDebugValuesDefReg and thus avoid using the related MachineInstr::collectDebugValues method. It constitutes an alternate fix for PR38773: rather than manually updating the registers that DBG_VALUEs refer to after MCP fires, instead mark DBG_VALUE register uses as being renamable. My understanding is that there's no functional (i.e. real instructions) reason why DBG_VALUEs can't be renamable, and it'll reduce the probability that less-well-written passes will behave differently when DBG_VALUEs are present. DBG_VALUE reg uses of the MCP'd register are then updated just like any other insn.

Either way seems fine to me; as mentioned though I'll upload a patch in a moment that messes with collectDebugValues, hence this option may be preferable.

[0] https://bugs.llvm.org/show_bug.cgi?id=38754

[jmorse]

This was superceeded by D56265.

lib/CodeGen/LiveDebugVariables.cpp

if (Loc.isReg() && Loc.getReg() &&
         TargetRegisterInfo::isVirtualRegister(Loc.getReg())) {
       unsigned VirtReg = Loc.getReg();
       if (VRM.isAssignedReg(VirtReg) &&
           TargetRegisterInfo::isPhysicalRegister(VRM.getPhys(VirtReg))) {
         // This can create a %noreg operand in rare cases when the sub-register
         // index is no longer available. That means the user value is in a
         // non-existent sub-register, and %noreg is exactly what we want.
         Loc.substPhysReg(VRM.getPhys(VirtReg), TRI);
+        Loc.setIsRenamable();
       } else if (VRM.getStackSlot(VirtReg) != VirtRegMap::NO_STACK_SLOT) {
         // Retrieve the stack slot offset.
         unsigned SpillSize;
         const MachineRegisterInfo &MRI = MF.getRegInfo();
         const TargetRegisterClass *TRC = MRI.getRegClass(VirtReg);
         bool Success = TII.getStackSlotRange(TRC, Loc.getSubReg(), SpillSize,
                                              SpillOffset, MF);
 

lib/CodeGen/MachineCopyPropagation.cpp

 }
 
 /// Decide whether we should forward the source of \param Copy to its use in
 /// \param UseI based on the physical register class constraints of the opcode
 /// and avoiding introducing more cross-class COPYs.
 bool MachineCopyPropagation::isForwardableRegClassCopy(const MachineInstr &Copy,
                                                        const MachineInstr &UseI,
                                                        unsigned UseIdx) {
+  // Debug values have no register classes to be examined
+  if (UseI.isDebugValue()) {
+    assert(UseI.getOperand(UseIdx).isReg());
+    return true;
+  }
 
   unsigned CopySrcReg = Copy.getOperand(1).getReg();
 
   // If the new register meets the opcode register constraints, then allow
   // forwarding.
   if (const TargetRegisterClass *URC =
           UseI.getRegClassConstraint(UseIdx, TII, TRI))
     return URC->contains(CopySrcReg);
void MachineCopyPropagation::CopyPropagateBlock(MachineBasicBlock &MBB) {
   if (MBB.succ_empty()) {
     for (MachineInstr *MaybeDead : MaybeDeadCopies) {
       LLVM_DEBUG(dbgs() << "MCP: Removing copy due to no live-out succ: ";
                  MaybeDead->dump());
       assert(!MRI->isReserved(MaybeDead->getOperand(0).getReg()));
 
       // Update matching debug values.
       assert(MaybeDead->isCopy());
-      MaybeDead->changeDebugValuesDefReg(MaybeDead->getOperand(1).getReg());
+      //MaybeDead->changeDebugValuesDefReg(MaybeDead->getOperand(1).getReg());
 
       MaybeDead->eraseFromParent();
       Changed = true;
       ++NumDeletes;
     }
   }
 
   MaybeDeadCopies.clear();

test/CodeGen/MIR/X86/pr38773.mir

bb.2.sw.bb1:
     liveins: $ecx
   
     $eax = MOV32ri 4
     $edx = MOV32r0 implicit-def dead $eflags
     IDIV32r killed renamable $ecx, implicit-def $eax, implicit-def dead $edx, implicit-def dead $eflags, implicit $eax, implicit killed $edx
     renamable $ecx = COPY $eax
     ; CHECK:        IDIV32r killed renamable $ecx
     ; CHECK-NEXT:   DBG_VALUE $eax, $noreg, !12, !DIExpression(), debug-location !13
-    DBG_VALUE $ecx, $noreg, !12, !DIExpression(), debug-location !13
+    DBG_VALUE renamable $ecx, $noreg, !12, !DIExpression(), debug-location !13
     $eax = COPY killed renamable $ecx
     RET 0, $eax
 
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