[SafeStack] Insert the deref before remaining elements

[phosek]

Summary

This is a follow up to D64971. While we need to insert the deref after
the offset, it needs to come before the remaining elements in the
original expression since the deref needs to happen before the LLVM
fragment if present.

[vsk]

Mind extending debug-loc2.ll with, say, a '!DIExpression(DW_OP_deref, DW_OP_LLVM_fragment, 0, 32)' example?

llvm/lib/Transforms/Utils/Local.cpp

1606

Can this whole block be replaced with: 'DIExpr = DIExpression::prepend(DIExpr, 0, Offset)'? It should handle fragments correctly and avoid inserting any extra derefs.

[phosek]

Done

[vsk]

Thanks a lot, lgtm.

llvm/lib/Transforms/Utils/Local.cpp

if (!DIExpr || DIExpr->getNumElements() < 1 ||
       DIExpr->getElement(0) != dwarf::DW_OP_deref)
     return;
 
   // Insert the offset before the first deref.
   // We could just change the offset argument of dbg.value, but it's unsigned...
   if (Offset) {
     SmallVector<uint64_t, 4> Ops;
     DIExpression::appendOffset(Ops, Offset);
-    Ops.append(DIExpr->elements_begin() + 1, DIExpr->elements_end());
-    Ops.push_back(dwarf::DW_OP_deref);
+    Ops.append(DIExpr->elements_begin(), DIExpr->elements_end());
     DIExpr = Builder.createExpression(Ops);
   }
 
   Builder.insertDbgValueIntrinsic(NewAddress, DIVar, DIExpr, Loc, DVI);
   DVI->eraseFromParent();
 }
 
 void llvm::replaceDbgValueForAlloca(AllocaInst *AI, Value *NewAllocaAddress,
                                     DIBuilder &Builder, int Offset) {