[DebugInfoMetadata] return early on nullptr arg

[nickdesaulniers]

Summary

This was flagged in https://www.viva64.com/en/b/0629/ under "Snippet No.
40".

It seems there's a nullptr check, but then Expr is later dereferenced
outside of the check. Since we can return an llvm::Optional, assume the
caller can handle llvm::None.

[dblaikie]

Any chance of a test case for this? (if you replace the null check with an assert for non-null, that might help identify a test case that hits this code?

llvm/lib/IR/DebugInfoMetadata.cpp

DIExpression *DIExpression::appendToStack(const DIExpression *Expr,
   NewOps.append(Ops.begin(), Ops.end());
   if (NeedsStackValue)
     NewOps.push_back(dwarf::DW_OP_stack_value);
   return DIExpression::append(Expr, NewOps);
 }
 
 Optional<DIExpression *> DIExpression::createFragmentExpression(
     const DIExpression *Expr, unsigned OffsetInBits, unsigned SizeInBits) {
+  if (!Expr)
+    return None;
   SmallVector<uint64_t, 8> Ops;
   // Copy over the expression, but leave off any trailing DW_OP_LLVM_fragment.
-  if (Expr) {
-    for (auto Op : Expr->expr_ops()) {
-      switch (Op.getOp()) {
-      default: break;
-      case dwarf::DW_OP_plus:
-      case dwarf::DW_OP_minus:
-        // We can't safely split arithmetic into multiple fragments because we
-        // can't express carry-over between fragments.
-        //
-        // FIXME: We *could* preserve the lowest fragment of a constant offset
-        // operation if the offset fits into SizeInBits.
-        return None;
-      case dwarf::DW_OP_LLVM_fragment: {
-        // Make the new offset point into the existing fragment.
-        uint64_t FragmentOffsetInBits = Op.getArg(0);
-        uint64_t FragmentSizeInBits = Op.getArg(1);
-        (void)FragmentSizeInBits;
-        assert((OffsetInBits + SizeInBits <= FragmentSizeInBits) &&
-               "new fragment outside of original fragment");
-        OffsetInBits += FragmentOffsetInBits;
-        continue;
-      }
-      }
-      Op.appendToVector(Ops);
-    }
-  }
   Ops.push_back(dwarf::DW_OP_LLVM_fragment);
   Ops.push_back(OffsetInBits);
   Ops.push_back(SizeInBits);
   return DIExpression::get(Expr->getContext(), Ops);
 }
 
 bool DIExpression::isConstant() const {
   // Recognize DW_OP_constu C DW_OP_stack_value (DW_OP_LLVM_fragment Len Ofs)?.