[InlineSpiller] Re-tie operands if folding failed

InlineSpiller::foldMemoryOperand unties registers before an attempt to fold and
does not restore tied-ness in case of failure.

I do not have a particular test for demo of invalid behavior.
This is something of clean-up.
It is better to keep the behavior correct in case some time in future it happens.

Reviewers: reames, dantrushin
Reviewed By: dantrushin, reames
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D94389

Author: Serguei Katkov

llvm/lib/CodeGen/InlineSpiller.cpp

@@ -853,16 +853,13 @@ foldMemoryOperand(ArrayRef<std::pair<MachineInstr *, unsigned>> Ops,
       continue;
     }
 
-    if (UntieRegs && MO.isTied())
-      MI->untieRegOperand(Idx);
-
     if (!SpillSubRegs && MO.getSubReg())
       return false;
     // We cannot fold a load instruction into a def.
     if (LoadMI && MO.isDef())
       return false;
     // Tied use operands should not be passed to foldMemoryOperand.
-    if (!MI->isRegTiedToDefOperand(Idx))
+    if (UntieRegs || !MI->isRegTiedToDefOperand(Idx))
       FoldOps.push_back(Idx);
   }
 
@@ -873,11 +870,31 @@ foldMemoryOperand(ArrayRef<std::pair<MachineInstr *, unsigned>> Ops,
 
   MachineInstrSpan MIS(MI, MI->getParent());
 
+  SmallVector<std::pair<unsigned, unsigned> > TiedOps;
+  if (UntieRegs)
+    for (unsigned Idx : FoldOps) {
+      MachineOperand &MO = MI->getOperand(Idx);
+      if (!MO.isTied())
+        continue;
+      unsigned Tied = MI->findTiedOperandIdx(Idx);
+      if (MO.isUse())
+        TiedOps.emplace_back(Tied, Idx);
+      else {
+        assert(MO.isDef() && "Tied to not use and def?");
+        TiedOps.emplace_back(Idx, Tied);
+      }
+      MI->untieRegOperand(Idx);
+    }
+
   MachineInstr *FoldMI =
       LoadMI ? TII.foldMemoryOperand(*MI, FoldOps, *LoadMI, &LIS)
              : TII.foldMemoryOperand(*MI, FoldOps, StackSlot, &LIS, &VRM);
-  if (!FoldMI)
+  if (!FoldMI) {
+    // Re-tie operands.
+    for (auto Tied : TiedOps)
+      MI->tieOperands(Tied.first, Tied.second);
     return false;
+  }
 
   // Remove LIS for any dead defs in the original MI not in FoldMI.
   for (MIBundleOperands MO(*MI); MO.isValid(); ++MO) {